diff --git a/packages/react-dom/src/__tests__/CSSPropertyOperations-test.js b/packages/react-dom/src/__tests__/CSSPropertyOperations-test.js
index 10190db51ef9e..d1f97800dd993 100644
--- a/packages/react-dom/src/__tests__/CSSPropertyOperations-test.js
+++ b/packages/react-dom/src/__tests__/CSSPropertyOperations-test.js
@@ -9,9 +9,9 @@
 
 'use strict';
 
-var React = require('react');
-var ReactDOM = require('react-dom');
-var ReactDOMServer = require('react-dom/server');
+const React = require('react');
+const ReactDOM = require('react-dom');
+const ReactDOMServer = require('react-dom/server');
 
 function normalizeCodeLocInfo(str) {
   return str && str.replace(/at .+?:\d+/g, 'at **');
@@ -19,66 +19,66 @@ function normalizeCodeLocInfo(str) {
 
 describe('CSSPropertyOperations', () => {
   it('should automatically append `px` to relevant styles', () => {
-    var styles = {
+    const styles = {
       left: 0,
       margin: 16,
       opacity: 0.5,
       padding: '4px',
     };
-    var div = <div style={styles} />;
-    var html = ReactDOMServer.renderToString(div);
+    const div = <div style={styles} />;
+    const html = ReactDOMServer.renderToString(div);
     expect(html).toContain('"left:0;margin:16px;opacity:0.5;padding:4px"');
   });
 
   it('should trim values', () => {
-    var styles = {
+    const styles = {
       left: '16 ',
       opacity: 0.5,
       right: ' 4 ',
     };
-    var div = <div style={styles} />;
-    var html = ReactDOMServer.renderToString(div);
+    const div = <div style={styles} />;
+    const html = ReactDOMServer.renderToString(div);
     expect(html).toContain('"left:16;opacity:0.5;right:4"');
   });
 
   it('should not append `px` to styles that might need a number', () => {
-    var styles = {
+    const styles = {
       flex: 0,
       opacity: 0.5,
     };
-    var div = <div style={styles} />;
-    var html = ReactDOMServer.renderToString(div);
+    const div = <div style={styles} />;
+    const html = ReactDOMServer.renderToString(div);
     expect(html).toContain('"flex:0;opacity:0.5"');
   });
 
   it('should create vendor-prefixed markup correctly', () => {
-    var styles = {
+    const styles = {
       msTransition: 'none',
       MozTransition: 'none',
     };
-    var div = <div style={styles} />;
-    var html = ReactDOMServer.renderToString(div);
+    const div = <div style={styles} />;
+    const html = ReactDOMServer.renderToString(div);
     expect(html).toContain('"-ms-transition:none;-moz-transition:none"');
   });
 
   it('should set style attribute when styles exist', () => {
-    var styles = {
+    const styles = {
       backgroundColor: '#000',
       display: 'none',
     };
-    var div = <div style={styles} />;
-    var root = document.createElement('div');
+    let div = <div style={styles} />;
+    const root = document.createElement('div');
     div = ReactDOM.render(div, root);
     expect(/style=".*"/.test(root.innerHTML)).toBe(true);
   });
 
   it('should not set style attribute when no styles exist', () => {
-    var styles = {
+    const styles = {
       backgroundColor: null,
       display: null,
     };
-    var div = <div style={styles} />;
-    var html = ReactDOMServer.renderToString(div);
+    const div = <div style={styles} />;
+    const html = ReactDOMServer.renderToString(div);
     expect(/style=/.test(html)).toBe(false);
   });
 
@@ -92,7 +92,7 @@ describe('CSSPropertyOperations', () => {
     }
 
     spyOnDev(console, 'error');
-    var root = document.createElement('div');
+    const root = document.createElement('div');
     ReactDOM.render(<Comp />, root);
     if (__DEV__) {
       expect(console.error.calls.count()).toBe(1);
@@ -114,11 +114,11 @@ describe('CSSPropertyOperations', () => {
     }
 
     spyOnDev(console, 'error');
-    var styles = {
+    const styles = {
       '-ms-transform': 'translate3d(0, 0, 0)',
       '-webkit-transform': 'translate3d(0, 0, 0)',
     };
-    var root = document.createElement('div');
+    const root = document.createElement('div');
     ReactDOM.render(<Comp />, root);
     ReactDOM.render(<Comp style={styles} />, root);
 
@@ -155,7 +155,7 @@ describe('CSSPropertyOperations', () => {
     }
 
     spyOnDev(console, 'error');
-    var root = document.createElement('div');
+    const root = document.createElement('div');
     ReactDOM.render(<Comp />, root);
     if (__DEV__) {
       // msTransform is correct already and shouldn't warn
@@ -194,7 +194,7 @@ describe('CSSPropertyOperations', () => {
     }
 
     spyOnDev(console, 'error');
-    var root = document.createElement('div');
+    const root = document.createElement('div');
     ReactDOM.render(<Comp />, root);
     if (__DEV__) {
       expect(console.error.calls.count()).toBe(2);
@@ -223,7 +223,7 @@ describe('CSSPropertyOperations', () => {
     }
 
     spyOnDev(console, 'error');
-    var root = document.createElement('div');
+    const root = document.createElement('div');
     ReactDOM.render(<Comp />, root);
 
     if (__DEV__) {
@@ -243,7 +243,7 @@ describe('CSSPropertyOperations', () => {
       }
     }
 
-    var root = document.createElement('div');
+    const root = document.createElement('div');
     ReactDOM.render(<Comp />, root);
   });
 
@@ -257,7 +257,7 @@ describe('CSSPropertyOperations', () => {
     }
 
     spyOnDev(console, 'error');
-    var root = document.createElement('div');
+    const root = document.createElement('div');
     ReactDOM.render(<Comp />, root);
 
     if (__DEV__) {
@@ -277,7 +277,7 @@ describe('CSSPropertyOperations', () => {
       }
     }
 
-    var root = document.createElement('div');
+    const root = document.createElement('div');
     ReactDOM.render(<Comp />, root);
 
     expect(root.children[0].style.Foo).toEqual('5');
diff --git a/packages/react-dom/src/__tests__/DOMPropertyOperations-test.js b/packages/react-dom/src/__tests__/DOMPropertyOperations-test.js
index 7c5358831446a..8cdf53f1c4ba8 100644
--- a/packages/react-dom/src/__tests__/DOMPropertyOperations-test.js
+++ b/packages/react-dom/src/__tests__/DOMPropertyOperations-test.js
@@ -10,8 +10,8 @@
 'use strict';
 
 describe('DOMPropertyOperations', () => {
-  var React;
-  var ReactDOM;
+  let React;
+  let ReactDOM;
 
   beforeEach(() => {
     jest.resetModules();
@@ -21,20 +21,20 @@ describe('DOMPropertyOperations', () => {
 
   describe('setValueForProperty', () => {
     it('should set values as properties by default', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div title="Tip!" />, container);
       expect(container.firstChild.title).toBe('Tip!');
     });
 
     it('should set values as attributes if necessary', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div role="#" />, container);
       expect(container.firstChild.getAttribute('role')).toBe('#');
       expect(container.firstChild.role).toBeUndefined();
     });
 
     it('should set values as namespace attributes if necessary', () => {
-      var container = document.createElement('svg');
+      const container = document.createElement('svg');
       ReactDOM.render(<image xlinkHref="about:blank" />, container);
       expect(
         container.firstChild.getAttributeNS(
@@ -45,7 +45,7 @@ describe('DOMPropertyOperations', () => {
     });
 
     it('should set values as boolean properties', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div disabled="disabled" />, container);
       expect(container.firstChild.getAttribute('disabled')).toBe('');
       ReactDOM.render(<div disabled={true} />, container);
@@ -63,32 +63,32 @@ describe('DOMPropertyOperations', () => {
     it('should convert attribute values to string first', () => {
       // Browsers default to this behavior, but some test environments do not.
       // This ensures that we have consistent behavior.
-      var obj = {
+      const obj = {
         toString: function() {
           return 'css-class';
         },
       };
 
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div className={obj} />, container);
       expect(container.firstChild.getAttribute('class')).toBe('css-class');
     });
 
     it('should not remove empty attributes for special properties', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<input value="" />, container);
       expect(container.firstChild.getAttribute('value')).toBe('');
       expect(container.firstChild.value).toBe('');
     });
 
     it('should remove for falsey boolean properties', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div allowFullScreen={false} />, container);
       expect(container.firstChild.hasAttribute('allowFullScreen')).toBe(false);
     });
 
     it('should remove when setting custom attr to null', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div data-foo="bar" />, container);
       expect(container.firstChild.hasAttribute('data-foo')).toBe(true);
       ReactDOM.render(<div data-foo={null} />, container);
@@ -96,7 +96,7 @@ describe('DOMPropertyOperations', () => {
     });
 
     it('should set className to empty string instead of null', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div className="selected" />, container);
       expect(container.firstChild.className).toBe('selected');
       ReactDOM.render(<div className={null} />, container);
@@ -107,7 +107,7 @@ describe('DOMPropertyOperations', () => {
     });
 
     it('should remove property properly for boolean properties', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div hidden={true} />, container);
       expect(container.firstChild.hasAttribute('hidden')).toBe(true);
       ReactDOM.render(<div hidden={false} />, container);
@@ -117,7 +117,7 @@ describe('DOMPropertyOperations', () => {
 
   describe('value mutation method', function() {
     it('should update an empty attribute to zero', function() {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(
         <input type="radio" value="" onChange={function() {}} />,
         container,
@@ -131,7 +131,7 @@ describe('DOMPropertyOperations', () => {
     });
 
     it('should always assign the value attribute for non-inputs', function() {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<progress />, container);
       spyOnDevAndProd(container.firstChild, 'setAttribute');
       ReactDOM.render(<progress value={30} />, container);
@@ -142,7 +142,7 @@ describe('DOMPropertyOperations', () => {
 
   describe('deleteValueForProperty', () => {
     it('should remove attributes for normal properties', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div title="foo" />, container);
       expect(container.firstChild.getAttribute('title')).toBe('foo');
       ReactDOM.render(<div />, container);
@@ -150,7 +150,7 @@ describe('DOMPropertyOperations', () => {
     });
 
     it('should not remove attributes for special properties', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       spyOnDev(console, 'error');
       ReactDOM.render(
         <input type="text" value="foo" onChange={function() {}} />,
diff --git a/packages/react-dom/src/__tests__/EventPluginHub-test.js b/packages/react-dom/src/__tests__/EventPluginHub-test.js
index ec1f83d0ee821..3fb1aa72c4501 100644
--- a/packages/react-dom/src/__tests__/EventPluginHub-test.js
+++ b/packages/react-dom/src/__tests__/EventPluginHub-test.js
@@ -12,8 +12,8 @@
 jest.mock('../events/isEventSupported');
 
 describe('EventPluginHub', () => {
-  var React;
-  var ReactTestUtils;
+  let React;
+  let ReactTestUtils;
 
   beforeEach(() => {
     jest.resetModules();
@@ -23,7 +23,7 @@ describe('EventPluginHub', () => {
 
   it('should prevent non-function listeners, at dispatch', () => {
     spyOnDev(console, 'error');
-    var node = ReactTestUtils.renderIntoDocument(
+    const node = ReactTestUtils.renderIntoDocument(
       <div onClick="not a function" />,
     );
     expect(function() {
@@ -40,7 +40,7 @@ describe('EventPluginHub', () => {
   });
 
   it('should not prevent null listeners, at dispatch', () => {
-    var node = ReactTestUtils.renderIntoDocument(<div onClick={null} />);
+    const node = ReactTestUtils.renderIntoDocument(<div onClick={null} />);
     expect(function() {
       ReactTestUtils.SimulateNative.click(node);
     }).not.toThrow();
diff --git a/packages/react-dom/src/__tests__/ReactBrowserEventEmitter-test.internal.js b/packages/react-dom/src/__tests__/ReactBrowserEventEmitter-test.internal.js
index ddbd8fa95ef55..e590851146416 100644
--- a/packages/react-dom/src/__tests__/ReactBrowserEventEmitter-test.internal.js
+++ b/packages/react-dom/src/__tests__/ReactBrowserEventEmitter-test.internal.js
@@ -9,42 +9,42 @@
 
 'use strict';
 
-var EventPluginHub;
-var EventPluginRegistry;
-var React;
-var ReactDOM;
-var ReactDOMComponentTree;
-var ReactBrowserEventEmitter;
-var ReactTestUtils;
-
-var idCallOrder;
-var recordID = function(id) {
+let EventPluginHub;
+let EventPluginRegistry;
+let React;
+let ReactDOM;
+let ReactDOMComponentTree;
+let ReactBrowserEventEmitter;
+let ReactTestUtils;
+
+let idCallOrder;
+const recordID = function(id) {
   idCallOrder.push(id);
 };
-var recordIDAndStopPropagation = function(id, event) {
+const recordIDAndStopPropagation = function(id, event) {
   recordID(id);
   event.stopPropagation();
 };
-var recordIDAndReturnFalse = function(id, event) {
+const recordIDAndReturnFalse = function(id, event) {
   recordID(id);
   return false;
 };
-var LISTENER = jest.fn();
-var ON_CLICK_KEY = 'onClick';
-var ON_CHANGE_KEY = 'onChange';
-var ON_MOUSE_ENTER_KEY = 'onMouseEnter';
+const LISTENER = jest.fn();
+const ON_CLICK_KEY = 'onClick';
+const ON_CHANGE_KEY = 'onChange';
+const ON_MOUSE_ENTER_KEY = 'onMouseEnter';
 
-var GRANDPARENT;
-var PARENT;
-var CHILD;
+let GRANDPARENT;
+let PARENT;
+let CHILD;
 
-var getListener;
-var putListener;
-var deleteAllListeners;
+let getListener;
+let putListener;
+let deleteAllListeners;
 
 function registerSimpleTestHandler() {
   putListener(CHILD, ON_CLICK_KEY, LISTENER);
-  var listener = getListener(CHILD, ON_CLICK_KEY);
+  const listener = getListener(CHILD, ON_CLICK_KEY);
   expect(listener).toEqual(LISTENER);
   return getListener(CHILD, ON_CLICK_KEY);
 }
@@ -63,11 +63,11 @@ describe('ReactBrowserEventEmitter', () => {
     ReactBrowserEventEmitter = require('../events/ReactBrowserEventEmitter');
     ReactTestUtils = require('react-dom/test-utils');
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var GRANDPARENT_PROPS = {};
-    var PARENT_PROPS = {};
-    var CHILD_PROPS = {};
+    let GRANDPARENT_PROPS = {};
+    let PARENT_PROPS = {};
+    let CHILD_PROPS = {};
 
     function Child(props) {
       return <div ref={c => (CHILD = c)} {...props} />;
@@ -93,7 +93,7 @@ describe('ReactBrowserEventEmitter', () => {
     renderTree();
 
     getListener = function(node, eventName) {
-      var inst = ReactDOMComponentTree.getInstanceFromNode(node);
+      const inst = ReactDOMComponentTree.getInstanceFromNode(node);
       return EventPluginHub.getListener(inst, eventName);
     };
     putListener = function(node, eventName, listener) {
@@ -131,20 +131,20 @@ describe('ReactBrowserEventEmitter', () => {
 
   it('should store a listener correctly', () => {
     registerSimpleTestHandler();
-    var listener = getListener(CHILD, ON_CLICK_KEY);
+    const listener = getListener(CHILD, ON_CLICK_KEY);
     expect(listener).toBe(LISTENER);
   });
 
   it('should retrieve a listener correctly', () => {
     registerSimpleTestHandler();
-    var listener = getListener(CHILD, ON_CLICK_KEY);
+    const listener = getListener(CHILD, ON_CLICK_KEY);
     expect(listener).toEqual(LISTENER);
   });
 
   it('should clear all handlers when asked to', () => {
     registerSimpleTestHandler();
     deleteAllListeners(CHILD);
-    var listener = getListener(CHILD, ON_CLICK_KEY);
+    const listener = getListener(CHILD, ON_CLICK_KEY);
     expect(listener).toBe(undefined);
   });
 
@@ -298,8 +298,8 @@ describe('ReactBrowserEventEmitter', () => {
    */
 
   it('should invoke handlers that were removed while bubbling', () => {
-    var handleParentClick = jest.fn();
-    var handleChildClick = function(event) {
+    const handleParentClick = jest.fn();
+    const handleChildClick = function(event) {
       deleteAllListeners(PARENT);
     };
     putListener(CHILD, ON_CLICK_KEY, handleChildClick);
@@ -309,8 +309,8 @@ describe('ReactBrowserEventEmitter', () => {
   });
 
   it('should not invoke newly inserted handlers while bubbling', () => {
-    var handleParentClick = jest.fn();
-    var handleChildClick = function(event) {
+    const handleParentClick = jest.fn();
+    const handleChildClick = function(event) {
       putListener(PARENT, ON_CLICK_KEY, handleParentClick);
     };
     putListener(CHILD, ON_CLICK_KEY, handleChildClick);
@@ -347,17 +347,17 @@ describe('ReactBrowserEventEmitter', () => {
 
     ReactBrowserEventEmitter.listenTo(ON_CHANGE_KEY, document);
 
-    var setEventListeners = [];
-    var listenCalls = EventTarget.prototype.addEventListener.calls.allArgs();
-    for (var i = 0; i < listenCalls.length; i++) {
+    const setEventListeners = [];
+    const listenCalls = EventTarget.prototype.addEventListener.calls.allArgs();
+    for (let i = 0; i < listenCalls.length; i++) {
       setEventListeners.push(listenCalls[i][1]);
     }
 
-    var module = EventPluginRegistry.registrationNameModules[ON_CHANGE_KEY];
-    var dependencies = module.eventTypes.change.dependencies;
+    const module = EventPluginRegistry.registrationNameModules[ON_CHANGE_KEY];
+    const dependencies = module.eventTypes.change.dependencies;
     expect(setEventListeners.length).toEqual(dependencies.length);
 
-    for (i = 0; i < setEventListeners.length; i++) {
+    for (let i = 0; i < setEventListeners.length; i++) {
       expect(dependencies.indexOf(setEventListeners[i])).toBeTruthy();
     }
   });
diff --git a/packages/react-dom/src/__tests__/ReactChildReconciler-test.js b/packages/react-dom/src/__tests__/ReactChildReconciler-test.js
index 9bbf9176cf69a..ebc03fa0ff99a 100644
--- a/packages/react-dom/src/__tests__/ReactChildReconciler-test.js
+++ b/packages/react-dom/src/__tests__/ReactChildReconciler-test.js
@@ -12,8 +12,8 @@
 
 'use strict';
 
-var React;
-var ReactTestUtils;
+let React;
+let ReactTestUtils;
 
 describe('ReactChildReconciler', () => {
   function normalizeCodeLocInfo(str) {
@@ -30,7 +30,7 @@ describe('ReactChildReconciler', () => {
   function createIterable(array) {
     return {
       '@@iterator': function() {
-        var i = 0;
+        let i = 0;
         return {
           next() {
             const next = {
diff --git a/packages/react-dom/src/__tests__/ReactComponent-test.js b/packages/react-dom/src/__tests__/ReactComponent-test.js
index 1f7da05821317..a0a8fde8b8661 100644
--- a/packages/react-dom/src/__tests__/ReactComponent-test.js
+++ b/packages/react-dom/src/__tests__/ReactComponent-test.js
@@ -9,10 +9,10 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
-var ReactDOMServer;
-var ReactTestUtils;
+let React;
+let ReactDOM;
+let ReactDOMServer;
+let ReactTestUtils;
 
 describe('ReactComponent', () => {
   function normalizeCodeLocInfo(str) {
@@ -27,7 +27,7 @@ describe('ReactComponent', () => {
   });
 
   it('should throw on invalid render targets', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     // jQuery objects are basically arrays; people often pass them in by mistake
     expect(function() {
       ReactDOM.render(<div />, [container]);
@@ -39,7 +39,7 @@ describe('ReactComponent', () => {
   });
 
   it('should throw when supplying a ref outside of render method', () => {
-    var instance = <div ref="badDiv" />;
+    let instance = <div ref="badDiv" />;
     expect(function() {
       instance = ReactTestUtils.renderIntoDocument(instance);
     }).toThrow();
@@ -108,8 +108,8 @@ describe('ReactComponent', () => {
   });
 
   it('should support refs on owned components', () => {
-    var innerObj = {};
-    var outerObj = {};
+    const innerObj = {};
+    const outerObj = {};
 
     class Wrapper extends React.Component {
       getObject = () => {
@@ -123,8 +123,8 @@ describe('ReactComponent', () => {
 
     class Component extends React.Component {
       render() {
-        var inner = <Wrapper object={innerObj} ref="inner" />;
-        var outer = (
+        const inner = <Wrapper object={innerObj} ref="inner" />;
+        const outer = (
           <Wrapper object={outerObj} ref="outer">
             {inner}
           </Wrapper>
@@ -166,8 +166,8 @@ describe('ReactComponent', () => {
   });
 
   it('should support new-style refs', () => {
-    var innerObj = {};
-    var outerObj = {};
+    const innerObj = {};
+    const outerObj = {};
 
     class Wrapper extends React.Component {
       getObject = () => {
@@ -179,14 +179,14 @@ describe('ReactComponent', () => {
       }
     }
 
-    var mounted = false;
+    let mounted = false;
 
     class Component extends React.Component {
       render() {
-        var inner = (
+        const inner = (
           <Wrapper object={innerObj} ref={c => (this.innerRef = c)} />
         );
-        var outer = (
+        const outer = (
           <Wrapper object={outerObj} ref={c => (this.outerRef = c)}>
             {inner}
           </Wrapper>
@@ -216,7 +216,7 @@ describe('ReactComponent', () => {
       }
     }
 
-    var mounted = false;
+    let mounted = false;
 
     class Component extends React.Component {
       getInner = () => {
@@ -248,7 +248,7 @@ describe('ReactComponent', () => {
   });
 
   it('should call refs at the correct time', () => {
-    var log = [];
+    const log = [];
 
     class Inner extends React.Component {
       render() {
@@ -303,7 +303,7 @@ describe('ReactComponent', () => {
     }
 
     // mount, update, unmount
-    var el = document.createElement('div');
+    const el = document.createElement('div');
     log.push('start mount');
     ReactDOM.render(<Outer />, el);
     log.push('start update');
@@ -344,8 +344,8 @@ describe('ReactComponent', () => {
   });
 
   it('fires the callback after a component is rendered', () => {
-    var callback = jest.fn();
-    var container = document.createElement('div');
+    const callback = jest.fn();
+    const container = document.createElement('div');
     ReactDOM.render(<div />, container, callback);
     expect(callback.mock.calls.length).toBe(1);
     ReactDOM.render(<div className="foo" />, container, callback);
@@ -357,7 +357,7 @@ describe('ReactComponent', () => {
   it('throws usefully when rendering badly-typed elements', () => {
     spyOnDev(console, 'error');
 
-    var X = undefined;
+    const X = undefined;
     expect(() => ReactTestUtils.renderIntoDocument(<X />)).toThrowError(
       'Element type is invalid: expected a string (for built-in components) ' +
         'or a class/function (for composite components) but got: undefined.' +
@@ -367,7 +367,7 @@ describe('ReactComponent', () => {
           : ''),
     );
 
-    var Y = null;
+    const Y = null;
     expect(() => ReactTestUtils.renderIntoDocument(<Y />)).toThrowError(
       'Element type is invalid: expected a string (for built-in components) ' +
         'or a class/function (for composite components) but got: null.',
@@ -382,7 +382,7 @@ describe('ReactComponent', () => {
   it('includes owner name in the error about badly-typed elements', () => {
     spyOnDev(console, 'error');
 
-    var X = undefined;
+    const X = undefined;
 
     function Indirection(props) {
       return <div>{props.children}</div>;
@@ -417,14 +417,14 @@ describe('ReactComponent', () => {
   });
 
   it('throws if a plain object is used as a child', () => {
-    var children = {
+    const children = {
       x: <span />,
       y: <span />,
       z: <span />,
     };
-    var element = <div>{[children]}</div>;
-    var container = document.createElement('div');
-    var ex;
+    const element = <div>{[children]}</div>;
+    const container = document.createElement('div');
+    let ex;
     try {
       ReactDOM.render(element, container);
     } catch (e) {
@@ -444,7 +444,7 @@ describe('ReactComponent', () => {
   it('throws if a plain object even if it is in an owner', () => {
     class Foo extends React.Component {
       render() {
-        var children = {
+        const children = {
           a: <span />,
           b: <span />,
           c: <span />,
@@ -452,8 +452,8 @@ describe('ReactComponent', () => {
         return <div>{[children]}</div>;
       }
     }
-    var container = document.createElement('div');
-    var ex;
+    const container = document.createElement('div');
+    let ex;
     try {
       ReactDOM.render(<Foo />, container);
     } catch (e) {
@@ -472,13 +472,13 @@ describe('ReactComponent', () => {
   });
 
   it('throws if a plain object is used as a child when using SSR', async () => {
-    var children = {
+    const children = {
       x: <span />,
       y: <span />,
       z: <span />,
     };
-    var element = <div>{[children]}</div>;
-    var ex;
+    const element = <div>{[children]}</div>;
+    let ex;
     try {
       ReactDOMServer.renderToString(element);
     } catch (e) {
@@ -498,7 +498,7 @@ describe('ReactComponent', () => {
   it('throws if a plain object even if it is in an owner when using SSR', async () => {
     class Foo extends React.Component {
       render() {
-        var children = {
+        const children = {
           a: <span />,
           b: <span />,
           c: <span />,
@@ -506,8 +506,8 @@ describe('ReactComponent', () => {
         return <div>{[children]}</div>;
       }
     }
-    var container = document.createElement('div');
-    var ex;
+    const container = document.createElement('div');
+    let ex;
     try {
       ReactDOMServer.renderToString(<Foo />, container);
     } catch (e) {
@@ -531,7 +531,7 @@ describe('ReactComponent', () => {
         return Foo;
       }
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<Foo />, container);
       if (__DEV__) {
         expect(console.error.calls.count()).toBe(1);
@@ -551,7 +551,7 @@ describe('ReactComponent', () => {
         }
       }
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<Foo />, container);
       if (__DEV__) {
         expect(console.error.calls.count()).toBe(1);
@@ -573,7 +573,7 @@ describe('ReactComponent', () => {
         );
       }
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<Foo />, container);
       if (__DEV__) {
         expect(console.error.calls.count()).toBe(1);
@@ -595,7 +595,7 @@ describe('ReactComponent', () => {
       function Foo() {
         return <Bar>{() => 'Hello'}</Bar>;
       }
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<Foo />, container);
       expect(container.innerHTML).toBe('Hello');
     });
@@ -620,8 +620,8 @@ describe('ReactComponent', () => {
           );
         }
       }
-      var container = document.createElement('div');
-      var component = ReactDOM.render(<Foo />, container);
+      const container = document.createElement('div');
+      const component = ReactDOM.render(<Foo />, container);
       component.setState({type: 'portobello mushrooms'});
       if (__DEV__) {
         expect(console.error.calls.count()).toBe(2);
diff --git a/packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js b/packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js
index 4da75f8629121..33730716ac1e2 100644
--- a/packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js
+++ b/packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js
@@ -9,51 +9,51 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
-var ReactTestUtils;
-var PropTypes;
+let React;
+let ReactDOM;
+let ReactTestUtils;
+let PropTypes;
 
-var clone = function(o) {
+const clone = function(o) {
   return JSON.parse(JSON.stringify(o));
 };
 
-var GET_INIT_STATE_RETURN_VAL = {
+const GET_INIT_STATE_RETURN_VAL = {
   hasWillMountCompleted: false,
   hasRenderCompleted: false,
   hasDidMountCompleted: false,
   hasWillUnmountCompleted: false,
 };
 
-var INIT_RENDER_STATE = {
+const INIT_RENDER_STATE = {
   hasWillMountCompleted: true,
   hasRenderCompleted: false,
   hasDidMountCompleted: false,
   hasWillUnmountCompleted: false,
 };
 
-var DID_MOUNT_STATE = {
+const DID_MOUNT_STATE = {
   hasWillMountCompleted: true,
   hasRenderCompleted: true,
   hasDidMountCompleted: false,
   hasWillUnmountCompleted: false,
 };
 
-var NEXT_RENDER_STATE = {
+const NEXT_RENDER_STATE = {
   hasWillMountCompleted: true,
   hasRenderCompleted: true,
   hasDidMountCompleted: true,
   hasWillUnmountCompleted: false,
 };
 
-var WILL_UNMOUNT_STATE = {
+const WILL_UNMOUNT_STATE = {
   hasWillMountCompleted: true,
   hasDidMountCompleted: true,
   hasRenderCompleted: true,
   hasWillUnmountCompleted: false,
 };
 
-var POST_WILL_UNMOUNT_STATE = {
+const POST_WILL_UNMOUNT_STATE = {
   hasWillMountCompleted: true,
   hasDidMountCompleted: true,
   hasRenderCompleted: true,
@@ -94,7 +94,7 @@ describe('ReactComponentLifeCycle', () => {
   });
 
   it('should not reuse an instance when it has been unmounted', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
     class StatefulComponent extends React.Component {
       state = {};
@@ -104,10 +104,10 @@ describe('ReactComponentLifeCycle', () => {
       }
     }
 
-    var element = <StatefulComponent />;
-    var firstInstance = ReactDOM.render(element, container);
+    const element = <StatefulComponent />;
+    const firstInstance = ReactDOM.render(element, container);
     ReactDOM.unmountComponentAtNode(container);
-    var secondInstance = ReactDOM.render(element, container);
+    const secondInstance = ReactDOM.render(element, container);
     expect(firstInstance).not.toBe(secondInstance);
   });
 
@@ -116,7 +116,7 @@ describe('ReactComponentLifeCycle', () => {
    * that second onDOMReady should not fail.
    */
   it('it should fire onDOMReady when already in onDOMReady', () => {
-    var _testJournal = [];
+    const _testJournal = [];
 
     class Child extends React.Component {
       componentDidMount() {
@@ -174,7 +174,7 @@ describe('ReactComponentLifeCycle', () => {
       }
     }
 
-    var instance = <StatefulComponent />;
+    let instance = <StatefulComponent />;
     expect(function() {
       instance = ReactTestUtils.renderIntoDocument(instance);
     }).toThrow();
@@ -191,7 +191,7 @@ describe('ReactComponentLifeCycle', () => {
       }
     }
 
-    var instance = <StatefulComponent />;
+    let instance = <StatefulComponent />;
     expect(function() {
       instance = ReactTestUtils.renderIntoDocument(instance);
     }).not.toThrow();
@@ -251,9 +251,9 @@ describe('ReactComponentLifeCycle', () => {
       }
     }
 
-    var element = <Component />;
+    const element = <Component />;
 
-    var instance = ReactTestUtils.renderIntoDocument(element);
+    const instance = ReactTestUtils.renderIntoDocument(element);
     expect(instance._isMounted()).toBeTruthy();
 
     if (__DEV__) {
@@ -284,9 +284,9 @@ describe('ReactComponentLifeCycle', () => {
       }
     }
 
-    var element = <Component />;
+    const element = <Component />;
 
-    var instance = ReactTestUtils.renderIntoDocument(element);
+    const instance = ReactTestUtils.renderIntoDocument(element);
     expect(instance._isMounted()).toBeTruthy();
 
     if (__DEV__) {
@@ -304,8 +304,8 @@ describe('ReactComponentLifeCycle', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var instance = ReactDOM.render(<Component />, container);
+    const container = document.createElement('div');
+    const instance = ReactDOM.render(<Component />, container);
 
     // No longer a public API, but we can test that it works internally by
     // reaching into the updater.
@@ -347,7 +347,7 @@ describe('ReactComponentLifeCycle', () => {
       constructor(props, context) {
         super(props, context);
         this._testJournal = {};
-        var initState = {
+        const initState = {
           hasWillMountCompleted: false,
           hasDidMountCompleted: false,
           hasRenderCompleted: false,
@@ -373,7 +373,7 @@ describe('ReactComponentLifeCycle', () => {
       }
 
       render() {
-        var isInitialRender = !this.state.hasRenderCompleted;
+        const isInitialRender = !this.state.hasRenderCompleted;
         if (isInitialRender) {
           this._testJournal.stateInInitialRender = clone(this.state);
           this._testJournal.lifeCycleInInitialRender = getLifeCycleState(this);
@@ -398,8 +398,8 @@ describe('ReactComponentLifeCycle', () => {
     // A component that is merely "constructed" (as in "constructor") but not
     // yet initialized, or rendered.
     //
-    var container = document.createElement('div');
-    var instance = ReactDOM.render(<LifeCycleComponent />, container);
+    const container = document.createElement('div');
+    const instance = ReactDOM.render(<LifeCycleComponent />, container);
 
     // getInitialState
     expect(instance._testJournal.returnedFromGetInitialState).toEqual(
@@ -490,7 +490,7 @@ describe('ReactComponentLifeCycle', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<Component text="uno" tooltipText="one" />, container);
 
     // Since `instance` is a root component, we can set its props. This also
@@ -516,7 +516,7 @@ describe('ReactComponentLifeCycle', () => {
       }
     }
 
-    var instance = (
+    let instance = (
       <SetStateInComponentDidMount
         valueToUseInitially="hello"
         valueToUseInOnDOMReady="goodbye"
@@ -527,8 +527,8 @@ describe('ReactComponentLifeCycle', () => {
   });
 
   it('should call nested lifecycle methods in the right order', () => {
-    var log;
-    var logger = function(msg) {
+    let log;
+    const logger = function(msg) {
       return function() {
         // return true for shouldComponentUpdate
         log.push(msg);
@@ -565,7 +565,7 @@ describe('ReactComponentLifeCycle', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     log = [];
     ReactDOM.render(<Outer x={17} />, container);
     expect(log).toEqual([
diff --git a/packages/react-dom/src/__tests__/ReactCompositeComponent-test.js b/packages/react-dom/src/__tests__/ReactCompositeComponent-test.js
index 1bac93ec046c1..3ae6384dbbcc4 100644
--- a/packages/react-dom/src/__tests__/ReactCompositeComponent-test.js
+++ b/packages/react-dom/src/__tests__/ReactCompositeComponent-test.js
@@ -9,16 +9,16 @@
 
 'use strict';
 
-var ChildUpdates;
-var MorphingComponent;
-var React;
-var ReactDOM;
-var ReactDOMServer;
-var ReactCurrentOwner;
-var ReactTestUtils;
-var PropTypes;
-var shallowEqual;
-var shallowCompare;
+let ChildUpdates;
+let MorphingComponent;
+let React;
+let ReactDOM;
+let ReactDOMServer;
+let ReactCurrentOwner;
+let ReactTestUtils;
+let PropTypes;
+let shallowEqual;
+let shallowCompare;
 
 describe('ReactCompositeComponent', () => {
   beforeEach(() => {
@@ -47,7 +47,7 @@ describe('ReactCompositeComponent', () => {
       };
 
       render() {
-        var toggleActivatedState = this._toggleActivatedState;
+        const toggleActivatedState = this._toggleActivatedState;
         return !this.state.activated ? (
           <a ref="x" onClick={toggleActivatedState} />
         ) : (
@@ -66,7 +66,7 @@ describe('ReactCompositeComponent', () => {
       };
 
       render() {
-        var className = this.props.anchorClassOn ? 'anchorClass' : '';
+        const className = this.props.anchorClassOn ? 'anchorClass' : '';
         return this.props.renderAnchor ? (
           <a ref="anch" className={className} />
         ) : (
@@ -85,15 +85,15 @@ describe('ReactCompositeComponent', () => {
       };
     }
 
-    var el = document.createElement('div');
+    const el = document.createElement('div');
     ReactDOM.render(<Child test="test" />, el);
 
     expect(el.textContent).toBe('test');
   });
 
   it('should support rendering to different child types over time', () => {
-    var instance = ReactTestUtils.renderIntoDocument(<MorphingComponent />);
-    var el = ReactDOM.findDOMNode(instance);
+    const instance = ReactTestUtils.renderIntoDocument(<MorphingComponent />);
+    let el = ReactDOM.findDOMNode(instance);
     expect(el.tagName).toBe('A');
 
     instance._toggleActivatedState();
@@ -123,10 +123,10 @@ describe('ReactCompositeComponent', () => {
     }
 
     spyOnDev(console, 'warn');
-    var markup = ReactDOMServer.renderToString(<Parent />);
+    const markup = ReactDOMServer.renderToString(<Parent />);
 
     // Old API based on heuristic
-    var container = document.createElement('div');
+    let container = document.createElement('div');
     container.innerHTML = markup;
     ReactDOM.render(<Parent />, container);
     if (__DEV__) {
@@ -148,8 +148,8 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should react to state changes from callbacks', () => {
-    var instance = ReactTestUtils.renderIntoDocument(<MorphingComponent />);
-    var el = ReactDOM.findDOMNode(instance);
+    const instance = ReactTestUtils.renderIntoDocument(<MorphingComponent />);
+    let el = ReactDOM.findDOMNode(instance);
     expect(el.tagName).toBe('A');
 
     ReactTestUtils.Simulate.click(el);
@@ -158,7 +158,7 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should rewire refs when rendering to different child types', () => {
-    var instance = ReactTestUtils.renderIntoDocument(<MorphingComponent />);
+    const instance = ReactTestUtils.renderIntoDocument(<MorphingComponent />);
 
     expect(ReactDOM.findDOMNode(instance.refs.x).tagName).toBe('A');
     instance._toggleActivatedState();
@@ -168,8 +168,8 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should not cache old DOM nodes when switching constructors', () => {
-    var container = document.createElement('div');
-    var instance = ReactDOM.render(
+    const container = document.createElement('div');
+    const instance = ReactDOM.render(
       <ChildUpdates renderAnchor={true} anchorClassOn={false} />,
       container,
     );
@@ -200,15 +200,15 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var instance1 = ReactTestUtils.renderIntoDocument(<Component />);
+    const instance1 = ReactTestUtils.renderIntoDocument(<Component />);
     expect(instance1.props).toEqual({prop: 'testKey'});
 
-    var instance2 = ReactTestUtils.renderIntoDocument(
+    const instance2 = ReactTestUtils.renderIntoDocument(
       <Component prop={undefined} />,
     );
     expect(instance2.props).toEqual({prop: 'testKey'});
 
-    var instance3 = ReactTestUtils.renderIntoDocument(
+    const instance3 = ReactTestUtils.renderIntoDocument(
       <Component prop={null} />,
     );
     expect(instance3.props).toEqual({prop: null});
@@ -223,8 +223,8 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var inputProps = {};
-    var instance1 = <Component {...inputProps} />;
+    const inputProps = {};
+    let instance1 = <Component {...inputProps} />;
     instance1 = ReactTestUtils.renderIntoDocument(instance1);
     expect(instance1.props.prop).toBe('testKey');
 
@@ -236,7 +236,7 @@ describe('ReactCompositeComponent', () => {
   it('should warn about `forceUpdate` on unmounted components', () => {
     spyOnDev(console, 'error');
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     document.body.appendChild(container);
 
     class Component extends React.Component {
@@ -245,7 +245,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var instance = <Component />;
+    let instance = <Component />;
     expect(instance.forceUpdate).not.toBeDefined();
 
     instance = ReactDOM.render(instance, container);
@@ -277,10 +277,10 @@ describe('ReactCompositeComponent', () => {
   it('should warn about `setState` on unmounted components', () => {
     spyOnDev(console, 'error');
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     document.body.appendChild(container);
 
-    var renders = 0;
+    let renders = 0;
 
     class Component extends React.Component {
       state = {value: 0};
@@ -291,7 +291,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var instance = <Component />;
+    let instance = <Component />;
     expect(instance.setState).not.toBeDefined();
 
     instance = ReactDOM.render(instance, container);
@@ -323,8 +323,8 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should silently allow `setState`, not call cb on unmounting components', () => {
-    var cbCalled = false;
-    var container = document.createElement('div');
+    let cbCalled = false;
+    const container = document.createElement('div');
     document.body.appendChild(container);
 
     class Component extends React.Component {
@@ -343,7 +343,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var instance = ReactDOM.render(<Component />, container);
+    const instance = ReactDOM.render(<Component />, container);
     instance.setState({value: 1});
 
     ReactDOM.unmountComponentAtNode(container);
@@ -352,7 +352,7 @@ describe('ReactCompositeComponent', () => {
 
   it('should warn when rendering a class with a render method that does not extend React.Component', () => {
     spyOnDev(console, 'error');
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     class ClassWithRenderNotExtended {
       render() {
         return <div />;
@@ -377,10 +377,10 @@ describe('ReactCompositeComponent', () => {
   it('should warn about `setState` in render', () => {
     spyOnDev(console, 'error');
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var renderedState = -1;
-    var renderPasses = 0;
+    let renderedState = -1;
+    let renderPasses = 0;
 
     class Component extends React.Component {
       state = {value: 0};
@@ -399,7 +399,7 @@ describe('ReactCompositeComponent', () => {
       expect(console.error.calls.count()).toBe(0);
     }
 
-    var instance = ReactDOM.render(<Component />, container);
+    const instance = ReactDOM.render(<Component />, container);
 
     if (__DEV__) {
       expect(console.error.calls.count()).toBe(1);
@@ -419,7 +419,7 @@ describe('ReactCompositeComponent', () => {
     expect(instance.state.value).toBe(1);
 
     // Forcing a rerender anywhere will cause the update to happen.
-    var instance2 = ReactDOM.render(<Component prop={123} />, container);
+    const instance2 = ReactDOM.render(<Component prop={123} />, container);
     expect(instance).toBe(instance2);
     expect(renderedState).toBe(1);
     expect(instance2.state.value).toBe(1);
@@ -435,9 +435,9 @@ describe('ReactCompositeComponent', () => {
   it('should warn about `setState` in getChildContext', () => {
     spyOnDev(console, 'error');
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var renderPasses = 0;
+    let renderPasses = 0;
 
     class Component extends React.Component {
       state = {value: 0};
@@ -458,7 +458,7 @@ describe('ReactCompositeComponent', () => {
     if (__DEV__) {
       expect(console.error.calls.count()).toBe(0);
     }
-    var instance = ReactDOM.render(<Component />, container);
+    const instance = ReactDOM.render(<Component />, container);
     expect(renderPasses).toBe(2);
     expect(instance.state.value).toBe(1);
     if (__DEV__) {
@@ -483,7 +483,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var instance = <BadComponent />;
+    let instance = <BadComponent />;
 
     expect(ReactCurrentOwner.current).toBe(null);
 
@@ -495,8 +495,8 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should call componentWillUnmount before unmounting', () => {
-    var container = document.createElement('div');
-    var innerUnmounted = false;
+    const container = document.createElement('div');
+    let innerUnmounted = false;
 
     class Component extends React.Component {
       render() {
@@ -539,7 +539,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var instance = ReactTestUtils.renderIntoDocument(<Component />);
+    const instance = ReactTestUtils.renderIntoDocument(<Component />);
     instance.setState({bogus: true});
 
     if (__DEV__) {
@@ -661,7 +661,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var component = ReactTestUtils.renderIntoDocument(<Parent />);
+    const component = ReactTestUtils.renderIntoDocument(<Parent />);
     expect(ReactDOM.findDOMNode(component).innerHTML).toBe('bar');
   });
 
@@ -672,7 +672,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var childRenders = 0;
+    let childRenders = 0;
 
     class Child extends React.Component {
       render() {
@@ -681,8 +681,8 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var child = <Child />;
+    const container = document.createElement('div');
+    const child = <Child />;
 
     ReactDOM.render(<Parent>{child}</Parent>, container);
     ReactDOM.render(<Parent>{child}</Parent>, container);
@@ -690,8 +690,8 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should pass context when re-rendered for static child', () => {
-    var parentInstance = null;
-    var childInstance = null;
+    let parentInstance = null;
+    let childInstance = null;
 
     class Parent extends React.Component {
       static childContextTypes = {
@@ -790,7 +790,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var wrapper = ReactTestUtils.renderIntoDocument(<Wrapper />);
+    const wrapper = ReactTestUtils.renderIntoDocument(<Wrapper />);
 
     expect(wrapper.refs.parent.state.flag).toEqual(true);
     expect(wrapper.refs.child.context).toEqual({flag: true});
@@ -803,8 +803,8 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should pass context transitively', () => {
-    var childInstance = null;
-    var grandchildInstance = null;
+    let childInstance = null;
+    let grandchildInstance = null;
 
     class Parent extends React.Component {
       static childContextTypes = {
@@ -864,8 +864,8 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should pass context when re-rendered', () => {
-    var parentInstance = null;
-    var childInstance = null;
+    let parentInstance = null;
+    let childInstance = null;
 
     class Parent extends React.Component {
       static childContextTypes = {
@@ -885,7 +885,7 @@ describe('ReactCompositeComponent', () => {
       }
 
       render() {
-        var output = <Child />;
+        let output = <Child />;
         if (!this.state.flag) {
           output = <span>Child</span>;
         }
@@ -968,7 +968,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var div = document.createElement('div');
+    const div = document.createElement('div');
     ReactDOM.render(<Parent cntxt="noise" />, div);
     expect(div.children[0].innerHTML).toBe('noise');
     div.children[0].innerHTML = 'aliens';
@@ -981,8 +981,8 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should trigger componentWillReceiveProps for context changes', () => {
-    var contextChanges = 0;
-    var propChanges = 0;
+    let contextChanges = 0;
+    let propChanges = 0;
 
     class GrandChild extends React.Component {
       static contextTypes = {
@@ -1066,9 +1066,9 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var div = document.createElement('div');
+    const div = document.createElement('div');
 
-    var parentInstance = null;
+    let parentInstance = null;
     ReactDOM.render(
       <Parent ref={inst => (parentInstance = inst)}>
         <ChildWithoutContext>
@@ -1121,7 +1121,7 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('only renders once if updated in componentWillReceiveProps', () => {
-    var renders = 0;
+    let renders = 0;
 
     class Component extends React.Component {
       state = {updated: false};
@@ -1139,8 +1139,8 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var instance = ReactDOM.render(<Component update={0} />, container);
+    const container = document.createElement('div');
+    const instance = ReactDOM.render(<Component update={0} />, container);
     expect(renders).toBe(1);
     expect(instance.state.updated).toBe(false);
     ReactDOM.render(<Component update={1} />, container);
@@ -1149,7 +1149,7 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('only renders once if updated in componentWillReceiveProps when batching', () => {
-    var renders = 0;
+    let renders = 0;
 
     class Component extends React.Component {
       state = {updated: false};
@@ -1167,8 +1167,8 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var instance = ReactDOM.render(<Component update={0} />, container);
+    const container = document.createElement('div');
+    const instance = ReactDOM.render(<Component update={0} />, container);
     expect(renders).toBe(1);
     expect(instance.state.updated).toBe(false);
     ReactDOM.unstable_batchedUpdates(() => {
@@ -1217,8 +1217,8 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var comp = ReactDOM.render(<Component flipped={false} />, container);
+    const container = document.createElement('div');
+    const comp = ReactDOM.render(<Component flipped={false} />, container);
     expect(ReactDOM.findDOMNode(comp.refs.static0).textContent).toBe('A');
     expect(ReactDOM.findDOMNode(comp.refs.static1).textContent).toBe('B');
 
@@ -1230,8 +1230,8 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should allow access to findDOMNode in componentWillUnmount', () => {
-    var a = null;
-    var b = null;
+    let a = null;
+    let b = null;
 
     class Component extends React.Component {
       componentDidMount() {
@@ -1249,7 +1249,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     expect(a).toBe(container.firstChild);
     ReactDOM.render(<Component />, container);
     ReactDOM.unmountComponentAtNode(container);
@@ -1283,7 +1283,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var div = document.createElement('div');
+    const div = document.createElement('div');
     ReactDOM.render(
       <Parent>
         <Component />
@@ -1300,7 +1300,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var moo = ReactTestUtils.renderIntoDocument(<Moo />);
+    const moo = ReactTestUtils.renderIntoDocument(<Moo />);
     // No longer a public API, but we can test that it works internally by
     // reaching into the updater.
     moo.updater.enqueueReplaceState(moo, {y: 2});
@@ -1309,7 +1309,7 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should support objects with prototypes as state', () => {
-    var NotActuallyImmutable = function(str) {
+    const NotActuallyImmutable = function(str) {
       this.str = str;
     };
     NotActuallyImmutable.prototype.amIImmutable = function() {
@@ -1325,11 +1325,11 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var moo = ReactTestUtils.renderIntoDocument(<Moo />);
+    const moo = ReactTestUtils.renderIntoDocument(<Moo />);
     expect(moo.state.str).toBe('first');
     expect(moo.state.amIImmutable()).toBe(true);
 
-    var secondState = new NotActuallyImmutable('second');
+    const secondState = new NotActuallyImmutable('second');
     moo._replaceState(secondState);
     expect(moo.state.str).toBe('second');
     expect(moo.state.amIImmutable()).toBe(true);
@@ -1341,7 +1341,7 @@ describe('ReactCompositeComponent', () => {
     expect(moo.state.amIImmutable).toBe(undefined);
 
     // When more than one state update is enqueued, we have the same behavior
-    var fifthState = new NotActuallyImmutable('fifth');
+    const fifthState = new NotActuallyImmutable('fifth');
     ReactDOM.unstable_batchedUpdates(function() {
       moo.setState({str: 'fourth'});
       moo._replaceState(fifthState);
@@ -1349,7 +1349,7 @@ describe('ReactCompositeComponent', () => {
     expect(moo.state).toBe(fifthState);
 
     // When more than one state update is enqueued, we have the same behavior
-    var sixthState = new NotActuallyImmutable('sixth');
+    const sixthState = new NotActuallyImmutable('sixth');
     ReactDOM.unstable_batchedUpdates(function() {
       moo._replaceState(sixthState);
       moo.setState({str: 'seventh'});
@@ -1359,8 +1359,8 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should not warn about unmounting during unmounting', () => {
-    var container = document.createElement('div');
-    var layer = document.createElement('div');
+    const container = document.createElement('div');
+    const layer = document.createElement('div');
 
     class Component extends React.Component {
       componentDidMount() {
@@ -1394,11 +1394,11 @@ describe('ReactCompositeComponent', () => {
   it('should warn when mutated props are passed', () => {
     spyOnDev(console, 'error');
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
     class Foo extends React.Component {
       constructor(props) {
-        var _props = {idx: props.idx + '!'};
+        const _props = {idx: props.idx + '!'};
         super(_props);
       }
 
@@ -1423,8 +1423,8 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('should only call componentWillUnmount once', () => {
-    var app;
-    var count = 0;
+    let app;
+    let count = 0;
 
     class App extends React.Component {
       render() {
@@ -1448,9 +1448,9 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var setRef = ref => {
+    const setRef = ref => {
       if (ref) {
         app = ref;
       }
@@ -1464,7 +1464,7 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('prepares new child before unmounting old', () => {
-    var log = [];
+    const log = [];
 
     class Spy extends React.Component {
       componentWillMount() {
@@ -1488,7 +1488,7 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<Wrapper name="A" />, container);
     ReactDOM.render(<Wrapper name="B" />, container);
 
@@ -1505,7 +1505,7 @@ describe('ReactCompositeComponent', () => {
   });
 
   it('respects a shallow shouldComponentUpdate implementation', () => {
-    var renderCalls = 0;
+    let renderCalls = 0;
     class PlasticWrap extends React.Component {
       constructor(props, context) {
         super(props, context);
@@ -1548,8 +1548,8 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var instance = ReactDOM.render(<PlasticWrap />, container);
+    const container = document.createElement('div');
+    const instance = ReactDOM.render(<PlasticWrap />, container);
     expect(renderCalls).toBe(1);
 
     // Do not re-render based on props
@@ -1581,8 +1581,8 @@ describe('ReactCompositeComponent', () => {
       };
     }
 
-    var renderCalls = 0;
-    var initialSettings = getInitialState();
+    let renderCalls = 0;
+    const initialSettings = getInitialState();
 
     class Component extends React.Component {
       state = initialSettings;
@@ -1597,12 +1597,12 @@ describe('ReactCompositeComponent', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var instance = ReactDOM.render(<Component />, container);
+    const container = document.createElement('div');
+    const instance = ReactDOM.render(<Component />, container);
     expect(renderCalls).toBe(1);
 
     // Do not re-render if state is equal
-    var settings = {
+    const settings = {
       foo: initialSettings.foo,
       bar: initialSettings.bar,
     };
diff --git a/packages/react-dom/src/__tests__/ReactCompositeComponentDOMMinimalism-test.js b/packages/react-dom/src/__tests__/ReactCompositeComponentDOMMinimalism-test.js
index 75ccd6cb422f9..557c9676780d2 100644
--- a/packages/react-dom/src/__tests__/ReactCompositeComponentDOMMinimalism-test.js
+++ b/packages/react-dom/src/__tests__/ReactCompositeComponentDOMMinimalism-test.js
@@ -10,15 +10,15 @@
 'use strict';
 
 // Requires
-var React;
-var ReactDOM;
-var ReactTestUtils;
+let React;
+let ReactDOM;
+let ReactTestUtils;
 
 // Test components
-var LowerLevelComposite;
-var MyCompositeComponent;
+let LowerLevelComposite;
+let MyCompositeComponent;
 
-var expectSingleChildlessDiv;
+let expectSingleChildlessDiv;
 
 /**
  * Integration test, testing the combination of JSX with our unit of
@@ -44,20 +44,20 @@ describe('ReactCompositeComponentDOMMinimalism', () => {
     };
 
     expectSingleChildlessDiv = function(instance) {
-      var el = ReactDOM.findDOMNode(instance);
+      const el = ReactDOM.findDOMNode(instance);
       expect(el.tagName).toBe('DIV');
       expect(el.children.length).toBe(0);
     };
   });
 
   it('should not render extra nodes for non-interpolated text', () => {
-    var instance = <MyCompositeComponent>A string child</MyCompositeComponent>;
+    let instance = <MyCompositeComponent>A string child</MyCompositeComponent>;
     instance = ReactTestUtils.renderIntoDocument(instance);
     expectSingleChildlessDiv(instance);
   });
 
   it('should not render extra nodes for non-interpolated text', () => {
-    var instance = (
+    let instance = (
       <MyCompositeComponent>{'Interpolated String Child'}</MyCompositeComponent>
     );
     instance = ReactTestUtils.renderIntoDocument(instance);
@@ -65,13 +65,13 @@ describe('ReactCompositeComponentDOMMinimalism', () => {
   });
 
   it('should not render extra nodes for non-interpolated text', () => {
-    var instance = (
+    let instance = (
       <MyCompositeComponent>
         <ul>This text causes no children in ul, just innerHTML</ul>
       </MyCompositeComponent>
     );
     instance = ReactTestUtils.renderIntoDocument(instance);
-    var el = ReactDOM.findDOMNode(instance);
+    const el = ReactDOM.findDOMNode(instance);
     expect(el.tagName).toBe('DIV');
     expect(el.children.length).toBe(1);
     expect(el.children[0].tagName).toBe('UL');
diff --git a/packages/react-dom/src/__tests__/ReactCompositeComponentNestedState-test.js b/packages/react-dom/src/__tests__/ReactCompositeComponentNestedState-test.js
index 06be86dc1d8e2..94a33a457a3f0 100644
--- a/packages/react-dom/src/__tests__/ReactCompositeComponentNestedState-test.js
+++ b/packages/react-dom/src/__tests__/ReactCompositeComponentNestedState-test.js
@@ -9,9 +9,9 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
-var ReactTestUtils;
+let React;
+let ReactDOM;
+let ReactTestUtils;
 
 describe('ReactCompositeComponentNestedState-state', () => {
   beforeEach(() => {
@@ -94,10 +94,10 @@ describe('ReactCompositeComponentNestedState-state', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     document.body.appendChild(container);
 
-    var logger = jest.fn();
+    const logger = jest.fn();
 
     void ReactDOM.render(<ParentComponent logger={logger} />, container);
 
diff --git a/packages/react-dom/src/__tests__/ReactCompositeComponentState-test.js b/packages/react-dom/src/__tests__/ReactCompositeComponentState-test.js
index 131488a500e91..c69e237c4e93a 100644
--- a/packages/react-dom/src/__tests__/ReactCompositeComponentState-test.js
+++ b/packages/react-dom/src/__tests__/ReactCompositeComponentState-test.js
@@ -9,10 +9,10 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
+let React;
+let ReactDOM;
 
-var TestComponent;
+let TestComponent;
 
 describe('ReactCompositeComponent-state', () => {
   beforeEach(() => {
@@ -123,11 +123,11 @@ describe('ReactCompositeComponent-state', () => {
   });
 
   it('should support setting state', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     document.body.appendChild(container);
 
-    var stateListener = jest.fn();
-    var instance = ReactDOM.render(
+    const stateListener = jest.fn();
+    const instance = ReactDOM.render(
       <TestComponent stateListener={stateListener} />,
       container,
       function peekAtInitialCallback() {
@@ -217,12 +217,12 @@ describe('ReactCompositeComponent-state', () => {
   });
 
   it('should call componentDidUpdate of children first', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var ops = [];
+    let ops = [];
 
-    var child = null;
-    var parent = null;
+    let child = null;
+    let parent = null;
 
     class Child extends React.Component {
       state = {bar: false};
@@ -237,7 +237,7 @@ describe('ReactCompositeComponent-state', () => {
       }
     }
 
-    var shouldUpdate = true;
+    let shouldUpdate = true;
 
     class Intermediate extends React.Component {
       shouldComponentUpdate() {
@@ -284,7 +284,7 @@ describe('ReactCompositeComponent-state', () => {
   });
 
   it('should batch unmounts', () => {
-    var outer;
+    let outer;
 
     class Inner extends React.Component {
       render() {
@@ -306,7 +306,7 @@ describe('ReactCompositeComponent-state', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     outer = ReactDOM.render(<Outer />, container);
     expect(() => {
       ReactDOM.unmountComponentAtNode(container);
@@ -338,7 +338,7 @@ describe('ReactCompositeComponent-state', () => {
         return <div>{this.props.value}</div>;
       }
     }
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<Parent />, container);
     ReactDOM.render(<Parent />, container);
     expect(log).toEqual([
diff --git a/packages/react-dom/src/__tests__/ReactDOM-test.js b/packages/react-dom/src/__tests__/ReactDOM-test.js
index b974c8bf41284..ffef8526cf577 100644
--- a/packages/react-dom/src/__tests__/ReactDOM-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOM-test.js
@@ -9,9 +9,9 @@
 
 'use strict';
 
-var React = require('react');
-var ReactDOM = require('react-dom');
-var ReactTestUtils = require('react-dom/test-utils');
+let React = require('react');
+let ReactDOM = require('react-dom');
+const ReactTestUtils = require('react-dom/test-utils');
 
 describe('ReactDOM', () => {
   // TODO: uncomment this test once we can run in phantom, which
@@ -44,24 +44,24 @@ describe('ReactDOM', () => {
   */
 
   it('allows a DOM element to be used with a string', () => {
-    var element = React.createElement('div', {className: 'foo'});
-    var instance = ReactTestUtils.renderIntoDocument(element);
+    const element = React.createElement('div', {className: 'foo'});
+    const instance = ReactTestUtils.renderIntoDocument(element);
     expect(ReactDOM.findDOMNode(instance).tagName).toBe('DIV');
   });
 
   it('should allow children to be passed as an argument', () => {
-    var argDiv = ReactTestUtils.renderIntoDocument(
+    const argDiv = ReactTestUtils.renderIntoDocument(
       React.createElement('div', null, 'child'),
     );
-    var argNode = ReactDOM.findDOMNode(argDiv);
+    const argNode = ReactDOM.findDOMNode(argDiv);
     expect(argNode.innerHTML).toBe('child');
   });
 
   it('should overwrite props.children with children argument', () => {
-    var conflictDiv = ReactTestUtils.renderIntoDocument(
+    const conflictDiv = ReactTestUtils.renderIntoDocument(
       React.createElement('div', {children: 'fakechild'}, 'child'),
     );
-    var conflictNode = ReactDOM.findDOMNode(conflictDiv);
+    const conflictNode = ReactDOM.findDOMNode(conflictDiv);
     expect(conflictNode.innerHTML).toBe('child');
   });
 
@@ -70,7 +70,7 @@ describe('ReactDOM', () => {
    * DOM, instead of a stale cache.
    */
   it('should purge the DOM cache when removing nodes', () => {
-    var myDiv = ReactTestUtils.renderIntoDocument(
+    let myDiv = ReactTestUtils.renderIntoDocument(
       <div>
         <div key="theDog" className="dog" />,
         <div key="theBird" className="bird" />
@@ -103,8 +103,8 @@ describe('ReactDOM', () => {
         <div key="theBird" className="bird" />,
       </div>,
     );
-    var root = ReactDOM.findDOMNode(myDiv);
-    var dog = root.childNodes[0];
+    const root = ReactDOM.findDOMNode(myDiv);
+    const dog = root.childNodes[0];
     expect(dog.className).toBe('bigdog');
   });
 
@@ -124,7 +124,7 @@ describe('ReactDOM', () => {
       }
     }
 
-    var myDiv = document.createElement('div');
+    const myDiv = document.createElement('div');
     expect(() => ReactDOM.render(<A />, myDiv, 'no')).toThrowError(
       'Invalid argument passed as callback. Expected a function. Instead ' +
         'received: no',
@@ -174,7 +174,7 @@ describe('ReactDOM', () => {
       }
     }
 
-    var myDiv = document.createElement('div');
+    const myDiv = document.createElement('div');
     ReactDOM.render(<A />, myDiv);
     expect(() => ReactDOM.render(<A />, myDiv, 'no')).toThrowError(
       'Invalid argument passed as callback. Expected a function. Instead ' +
@@ -235,8 +235,8 @@ describe('ReactDOM', () => {
       }
     }
 
-    var log = [];
-    var container = document.createElement('div');
+    const log = [];
+    const container = document.createElement('div');
     document.body.appendChild(container);
     ReactDOM.render(<A showTwo={false} />, container);
     input.focus();
@@ -245,9 +245,9 @@ describe('ReactDOM', () => {
     // something that could happen when manipulating DOM nodes (but is hard to
     // deterministically force without relying intensely on React DOM
     // implementation details)
-    var div = container.firstChild;
+    const div = container.firstChild;
     ['appendChild', 'insertBefore'].forEach(name => {
-      var mutator = div[name];
+      const mutator = div[name];
       div[name] = function() {
         if (input) {
           input.blur();
@@ -305,7 +305,7 @@ describe('ReactDOM', () => {
     const actual = [];
 
     function click(node) {
-      var fakeNativeEvent = function() {};
+      const fakeNativeEvent = function() {};
       fakeNativeEvent.target = node;
       fakeNativeEvent.path = [node, container];
       ReactTestUtils.simulateNativeEventOnNode(
@@ -341,7 +341,7 @@ describe('ReactDOM', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<Wrapper />, container);
 
     const expected = [
diff --git a/packages/react-dom/src/__tests__/ReactDOMAttribute-test.js b/packages/react-dom/src/__tests__/ReactDOMAttribute-test.js
index fe2d647b7b292..462f76be9ff08 100644
--- a/packages/react-dom/src/__tests__/ReactDOMAttribute-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMAttribute-test.js
@@ -10,8 +10,8 @@
 'use strict';
 
 describe('ReactDOM unknown attribute', () => {
-  var React;
-  var ReactDOM;
+  let React;
+  let ReactDOM;
 
   beforeEach(() => {
     jest.resetModules();
@@ -24,7 +24,7 @@ describe('ReactDOM unknown attribute', () => {
   }
 
   function testUnknownAttributeRemoval(givenValue) {
-    var el = document.createElement('div');
+    const el = document.createElement('div');
     ReactDOM.render(<div unknown="something" />, el);
     expect(el.firstChild.getAttribute('unknown')).toBe('something');
     ReactDOM.render(<div unknown={givenValue} />, el);
@@ -32,7 +32,7 @@ describe('ReactDOM unknown attribute', () => {
   }
 
   function testUnknownAttributeAssignment(givenValue, expectedDOMValue) {
-    var el = document.createElement('div');
+    const el = document.createElement('div');
     ReactDOM.render(<div unknown="something" />, el);
     expect(el.firstChild.getAttribute('unknown')).toBe('something');
     ReactDOM.render(<div unknown={givenValue} />, el);
@@ -63,7 +63,7 @@ describe('ReactDOM unknown attribute', () => {
     });
 
     it('removes unknown attributes that were rendered but are now missing', () => {
-      var el = document.createElement('div');
+      const el = document.createElement('div');
       ReactDOM.render(<div unknown="something" />, el);
       expect(el.firstChild.getAttribute('unknown')).toBe('something');
       ReactDOM.render(<div />, el);
@@ -140,7 +140,7 @@ describe('ReactDOM unknown attribute', () => {
     it('allows camelCase unknown attributes and warns', () => {
       spyOnDev(console, 'error');
 
-      var el = document.createElement('div');
+      const el = document.createElement('div');
       ReactDOM.render(<div helloWorld="something" />, el);
       expect(el.firstChild.getAttribute('helloworld')).toBe('something');
 
diff --git a/packages/react-dom/src/__tests__/ReactDOMComponent-test.js b/packages/react-dom/src/__tests__/ReactDOMComponent-test.js
index 8a8629a3d7aa0..85fe8f949b21d 100644
--- a/packages/react-dom/src/__tests__/ReactDOMComponent-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMComponent-test.js
@@ -10,10 +10,10 @@
 'use strict';
 
 describe('ReactDOMComponent', () => {
-  var React;
-  var ReactTestUtils;
-  var ReactDOM;
-  var ReactDOMServer;
+  let React;
+  let ReactTestUtils;
+  let ReactDOM;
+  let ReactDOMServer;
 
   function normalizeCodeLocInfo(str) {
     return str && str.replace(/\(at .+?:\d+\)/g, '(at **)');
@@ -29,7 +29,7 @@ describe('ReactDOMComponent', () => {
 
   describe('updateDOM', () => {
     it('should handle className', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div style={{}} />, container);
 
       ReactDOM.render(<div className={'foo'} />, container);
@@ -41,12 +41,17 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should gracefully handle various style value types', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div style={{}} />, container);
-      var stubStyle = container.firstChild.style;
+      const stubStyle = container.firstChild.style;
 
       // set initial style
-      var setup = {display: 'block', left: '1px', top: 2, fontFamily: 'Arial'};
+      const setup = {
+        display: 'block',
+        left: '1px',
+        top: 2,
+        fontFamily: 'Arial',
+      };
       ReactDOM.render(<div style={setup} />, container);
       expect(stubStyle.display).toEqual('block');
       expect(stubStyle.left).toEqual('1px');
@@ -54,7 +59,7 @@ describe('ReactDOMComponent', () => {
       expect(stubStyle.fontFamily).toEqual('Arial');
 
       // reset the style to their default state
-      var reset = {display: '', left: null, top: false, fontFamily: true};
+      const reset = {display: '', left: null, top: false, fontFamily: true};
       ReactDOM.render(<div style={reset} />, container);
       expect(stubStyle.display).toEqual('');
       expect(stubStyle.left).toEqual('');
@@ -63,10 +68,14 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should not update styles when mutating a proxy style object', () => {
-      var styleStore = {display: 'none', fontFamily: 'Arial', lineHeight: 1.2};
+      const styleStore = {
+        display: 'none',
+        fontFamily: 'Arial',
+        lineHeight: 1.2,
+      };
       // We use a proxy style object so that we can mutate it even if it is
       // frozen in DEV.
-      var styles = {
+      const styles = {
         get display() {
           return styleStore.display;
         },
@@ -86,10 +95,10 @@ describe('ReactDOMComponent', () => {
           styleStore.lineHeight = v;
         },
       };
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div style={styles} />, container);
 
-      var stubStyle = container.firstChild.style;
+      const stubStyle = container.firstChild.style;
       stubStyle.display = styles.display;
       stubStyle.fontFamily = styles.fontFamily;
 
@@ -121,7 +130,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should throw when mutating style objects', () => {
-      var style = {border: '1px solid black'};
+      const style = {border: '1px solid black'};
 
       class App extends React.Component {
         state = {style: style};
@@ -139,7 +148,7 @@ describe('ReactDOMComponent', () => {
 
     it('should warn for unknown prop', () => {
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div foo={() => {}} />, container);
       if (__DEV__) {
         expect(console.error.calls.count(0)).toBe(1);
@@ -154,7 +163,7 @@ describe('ReactDOMComponent', () => {
 
     it('should group multiple unknown prop warnings together', () => {
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div foo={() => {}} baz={() => {}} />, container);
       if (__DEV__) {
         expect(console.error.calls.count(0)).toBe(1);
@@ -169,7 +178,7 @@ describe('ReactDOMComponent', () => {
 
     it('should warn for onDblClick prop', () => {
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div onDblClick={() => {}} />, container);
       if (__DEV__) {
         expect(console.error.calls.count(0)).toBe(1);
@@ -181,7 +190,7 @@ describe('ReactDOMComponent', () => {
 
     it('should warn for unknown string event handlers', () => {
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div onUnknown="alert(&quot;hack&quot;)" />, container);
       expect(container.firstChild.hasAttribute('onUnknown')).toBe(false);
       expect(container.firstChild.onUnknown).toBe(undefined);
@@ -207,7 +216,7 @@ describe('ReactDOMComponent', () => {
 
     it('should warn for unknown function event handlers', () => {
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div onUnknown={function() {}} />, container);
       expect(container.firstChild.hasAttribute('onUnknown')).toBe(false);
       expect(container.firstChild.onUnknown).toBe(undefined);
@@ -233,7 +242,7 @@ describe('ReactDOMComponent', () => {
 
     it('should warn for badly cased React attributes', () => {
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div CHILDREN="5" />, container);
       expect(container.firstChild.getAttribute('CHILDREN')).toBe('5');
       if (__DEV__) {
@@ -257,8 +266,8 @@ describe('ReactDOMComponent', () => {
     it('should warn nicely about NaN in style', () => {
       spyOnDev(console, 'error');
 
-      var style = {fontSize: NaN};
-      var div = document.createElement('div');
+      const style = {fontSize: NaN};
+      const div = document.createElement('div');
       ReactDOM.render(<span style={style} />, div);
       ReactDOM.render(<span style={style} />, div);
 
@@ -272,11 +281,11 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should update styles if initially null', () => {
-      var styles = null;
-      var container = document.createElement('div');
+      let styles = null;
+      const container = document.createElement('div');
       ReactDOM.render(<div style={styles} />, container);
 
-      var stubStyle = container.firstChild.style;
+      const stubStyle = container.firstChild.style;
 
       styles = {display: 'block'};
 
@@ -285,12 +294,12 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should update styles if updated to null multiple times', () => {
-      var styles = null;
-      var container = document.createElement('div');
+      let styles = null;
+      const container = document.createElement('div');
       ReactDOM.render(<div style={styles} />, container);
 
       styles = {display: 'block'};
-      var stubStyle = container.firstChild.style;
+      const stubStyle = container.firstChild.style;
 
       ReactDOM.render(<div style={styles} />, container);
       expect(stubStyle.display).toEqual('block');
@@ -306,7 +315,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should allow named slot projection on both web components and regular DOM elements', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
       ReactDOM.render(
         <my-component>
@@ -316,14 +325,14 @@ describe('ReactDOMComponent', () => {
         container,
       );
 
-      var lightDOM = container.firstChild.childNodes;
+      const lightDOM = container.firstChild.childNodes;
 
       expect(lightDOM[0].getAttribute('slot')).toBe('first');
       expect(lightDOM[1].getAttribute('slot')).toBe('second');
     });
 
     it('should skip reserved props on web components', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
       ReactDOM.render(
         <my-component
@@ -359,7 +368,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should skip dangerouslySetInnerHTML on web components', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
       ReactDOM.render(
         <my-component dangerouslySetInnerHTML={{__html: 'hi'}} />,
@@ -379,7 +388,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should render null and undefined as empty but print other falsy values', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
       ReactDOM.render(
         <div dangerouslySetInnerHTML={{__html: 'textContent'}} />,
@@ -416,7 +425,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should remove attributes', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<img height="17" />, container);
 
       expect(container.firstChild.hasAttribute('height')).toBe(true);
@@ -425,7 +434,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should remove properties', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div className="monkey" />, container);
 
       expect(container.firstChild.className).toEqual('monkey');
@@ -434,20 +443,20 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should properly update custom attributes on custom elements', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<some-custom-element foo="bar" />, container);
       ReactDOM.render(<some-custom-element bar="buzz" />, container);
-      var node = container.firstChild;
+      const node = container.firstChild;
       expect(node.hasAttribute('foo')).toBe(false);
       expect(node.getAttribute('bar')).toBe('buzz');
     });
 
     it('should clear a single style prop when changing `style`', () => {
-      var styles = {display: 'none', color: 'red'};
-      var container = document.createElement('div');
+      let styles = {display: 'none', color: 'red'};
+      const container = document.createElement('div');
       ReactDOM.render(<div style={styles} />, container);
 
-      var stubStyle = container.firstChild.style;
+      const stubStyle = container.firstChild.style;
 
       styles = {color: 'green'};
       ReactDOM.render(<div style={styles} />, container);
@@ -457,9 +466,9 @@ describe('ReactDOMComponent', () => {
 
     it('should reject attribute key injection attack on markup', () => {
       spyOnDev(console, 'error');
-      for (var i = 0; i < 3; i++) {
-        var container = document.createElement('div');
-        var element = React.createElement(
+      for (let i = 0; i < 3; i++) {
+        const container = document.createElement('div');
+        const element = React.createElement(
           'x-foo-component',
           {'blah" onclick="beevil" noise="hi': 'selected'},
           null,
@@ -476,12 +485,12 @@ describe('ReactDOMComponent', () => {
 
     it('should reject attribute key injection attack on update', () => {
       spyOnDev(console, 'error');
-      for (var i = 0; i < 3; i++) {
-        var container = document.createElement('div');
-        var beforeUpdate = React.createElement('x-foo-component', {}, null);
+      for (let i = 0; i < 3; i++) {
+        const container = document.createElement('div');
+        const beforeUpdate = React.createElement('x-foo-component', {}, null);
         ReactDOM.render(beforeUpdate, container);
 
-        var afterUpdate = React.createElement(
+        const afterUpdate = React.createElement(
           'x-foo-component',
           {'blah" onclick="beevil" noise="hi': 'selected'},
           null,
@@ -497,23 +506,23 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should update arbitrary attributes for tags containing dashes', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
-      var beforeUpdate = React.createElement('x-foo-component', {}, null);
+      const beforeUpdate = React.createElement('x-foo-component', {}, null);
       ReactDOM.render(beforeUpdate, container);
 
-      var afterUpdate = <x-foo-component myattr="myval" />;
+      const afterUpdate = <x-foo-component myattr="myval" />;
       ReactDOM.render(afterUpdate, container);
 
       expect(container.childNodes[0].getAttribute('myattr')).toBe('myval');
     });
 
     it('should clear all the styles when removing `style`', () => {
-      var styles = {display: 'none', color: 'red'};
-      var container = document.createElement('div');
+      const styles = {display: 'none', color: 'red'};
+      const container = document.createElement('div');
       ReactDOM.render(<div style={styles} />, container);
 
-      var stubStyle = container.firstChild.style;
+      const stubStyle = container.firstChild.style;
 
       ReactDOM.render(<div />, container);
       expect(stubStyle.display).toEqual('');
@@ -521,18 +530,18 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should update styles when `style` changes from null to object', () => {
-      var container = document.createElement('div');
-      var styles = {color: 'red'};
+      const container = document.createElement('div');
+      const styles = {color: 'red'};
       ReactDOM.render(<div style={styles} />, container);
       ReactDOM.render(<div />, container);
       ReactDOM.render(<div style={styles} />, container);
 
-      var stubStyle = container.firstChild.style;
+      const stubStyle = container.firstChild.style;
       expect(stubStyle.color).toEqual('red');
     });
 
     it('should not reset innerHTML for when children is null', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div />, container);
       container.firstChild.innerHTML = 'bonjour';
       expect(container.firstChild.innerHTML).toEqual('bonjour');
@@ -544,7 +553,7 @@ describe('ReactDOMComponent', () => {
     it('should reset innerHTML when switching from a direct text child to an empty child', () => {
       const transitionToValues = [null, undefined, false];
       transitionToValues.forEach(transitionToValue => {
-        var container = document.createElement('div');
+        const container = document.createElement('div');
         ReactDOM.render(<div>bonjour</div>, container);
         expect(container.firstChild.innerHTML).toEqual('bonjour');
 
@@ -554,7 +563,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should empty element when removing innerHTML', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(
         <div dangerouslySetInnerHTML={{__html: ':)'}} />,
         container,
@@ -566,7 +575,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should transition from string content to innerHTML', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div>hello</div>, container);
 
       expect(container.firstChild.innerHTML).toEqual('hello');
@@ -578,7 +587,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should transition from innerHTML to string content', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(
         <div dangerouslySetInnerHTML={{__html: 'bonjour'}} />,
         container,
@@ -590,7 +599,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should transition from innerHTML to children in nested el', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(
         <div>
           <div dangerouslySetInnerHTML={{__html: 'bonjour'}} />
@@ -611,7 +620,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should transition from children to innerHTML in nested el', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(
         <div>
           <div>
@@ -632,15 +641,15 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should not incur unnecessary DOM mutations for attributes', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div id="" />, container);
 
-      var node = container.firstChild;
-      var nodeSetAttribute = node.setAttribute;
+      const node = container.firstChild;
+      const nodeSetAttribute = node.setAttribute;
       node.setAttribute = jest.fn();
       node.setAttribute.mockImplementation(nodeSetAttribute);
 
-      var nodeRemoveAttribute = node.removeAttribute;
+      const nodeRemoveAttribute = node.removeAttribute;
       node.removeAttribute = jest.fn();
       node.removeAttribute.mockImplementation(nodeRemoveAttribute);
 
@@ -670,14 +679,14 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should not incur unnecessary DOM mutations for string properties', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div value="" />, container);
 
-      var node = container.firstChild;
+      const node = container.firstChild;
 
-      var nodeValueSetter = jest.genMockFn();
+      const nodeValueSetter = jest.genMockFn();
 
-      var oldSetAttribute = node.setAttribute.bind(node);
+      const oldSetAttribute = node.setAttribute.bind(node);
       node.setAttribute = function(key, value) {
         oldSetAttribute(key, value);
         nodeValueSetter(key, value);
@@ -703,12 +712,12 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should not incur unnecessary DOM mutations for boolean properties', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div checked={true} />, container);
 
-      var node = container.firstChild;
-      var nodeValue = true;
-      var nodeValueSetter = jest.fn();
+      const node = container.firstChild;
+      let nodeValue = true;
+      const nodeValueSetter = jest.fn();
       Object.defineProperty(node, 'checked', {
         get: function() {
           return nodeValue;
@@ -732,14 +741,14 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should ignore attribute whitelist for elements with the "is" attribute', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<button is="test" cowabunga="chevynova" />, container);
       expect(container.firstChild.hasAttribute('cowabunga')).toBe(true);
     });
 
     it('should warn about non-string "is" attribute', () => {
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<button is={function() {}} />, container);
 
       if (__DEV__) {
@@ -752,10 +761,10 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should not update when switching between null/undefined', () => {
-      var container = document.createElement('div');
-      var node = ReactDOM.render(<div />, container);
+      const container = document.createElement('div');
+      const node = ReactDOM.render(<div />, container);
 
-      var setter = jest.fn();
+      const setter = jest.fn();
       node.setAttribute = setter;
 
       ReactDOM.render(<div dir={null} />, container);
@@ -770,7 +779,7 @@ describe('ReactDOMComponent', () => {
       // This test might look like it's just testing ReactMultiChild but the
       // last bug in this was actually in DOMChildrenOperations so this test
       // needs to be in some DOM-specific test file.
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
       // ABCD
       ReactDOM.render(
@@ -811,8 +820,8 @@ describe('ReactDOMComponent', () => {
     }
 
     function toHaveAttribute(actual, expected) {
-      var [attr, value] = expected;
-      var re = '(?:^|\\s)' + attr + '=[\\\'"]';
+      const [attr, value] = expected;
+      let re = '(?:^|\\s)' + attr + '=[\\\'"]';
       if (typeof value !== 'undefined') {
         re += quoteRegexp(value) + '[\\\'"]';
       }
@@ -851,12 +860,12 @@ describe('ReactDOMComponent', () => {
     }
 
     function toHaveInnerhtml(actual, expected) {
-      var re = '^' + quoteRegexp(expected) + '$';
+      const re = '^' + quoteRegexp(expected) + '$';
       return new RegExp(re).test(actual);
     }
 
     it('should handle dangerouslySetInnerHTML', () => {
-      var innerHTML = {__html: 'testContent'};
+      const innerHTML = {__html: 'testContent'};
       expect(
         toHaveInnerhtml(
           genMarkup({dangerouslySetInnerHTML: innerHTML}),
@@ -867,18 +876,18 @@ describe('ReactDOMComponent', () => {
   });
 
   describe('mountComponent', () => {
-    var mountComponent;
+    let mountComponent;
 
     beforeEach(() => {
       mountComponent = function(props) {
-        var container = document.createElement('div');
+        const container = document.createElement('div');
         ReactDOM.render(<div {...props} />, container);
       };
     });
 
     it('should work error event on <source> element', () => {
       spyOnDevAndProd(console, 'log');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(
         <video>
           <source
@@ -890,7 +899,7 @@ describe('ReactDOMComponent', () => {
         container,
       );
 
-      var errorEvent = document.createEvent('Event');
+      const errorEvent = document.createEvent('Event');
       errorEvent.initEvent('error', false, false);
       container.getElementsByTagName('source')[0].dispatchEvent(errorEvent);
 
@@ -908,7 +917,7 @@ describe('ReactDOMComponent', () => {
         }
       }
 
-      var returnedValue = ReactDOMServer.renderToString(<Container />);
+      const returnedValue = ReactDOMServer.renderToString(<Container />);
       expect(returnedValue).not.toContain('</BR>');
       if (__DEV__) {
         expect(console.error.calls.count()).toBe(1);
@@ -1041,11 +1050,11 @@ describe('ReactDOMComponent', () => {
     it('should emit a warning once for a named custom component using shady DOM', () => {
       spyOnDev(console, 'error');
 
-      var defaultCreateElement = document.createElement.bind(document);
+      const defaultCreateElement = document.createElement.bind(document);
 
       try {
         document.createElement = element => {
-          var container = defaultCreateElement(element);
+          const container = defaultCreateElement(element);
           container.shadyRoot = {};
           return container;
         };
@@ -1054,7 +1063,7 @@ describe('ReactDOMComponent', () => {
             return <polymer-component />;
           }
         }
-        var node = document.createElement('div');
+        const node = document.createElement('div');
         ReactDOM.render(<ShadyComponent />, node);
         if (__DEV__) {
           expect(console.error.calls.count()).toBe(1);
@@ -1075,11 +1084,11 @@ describe('ReactDOMComponent', () => {
     it('should emit a warning once for an unnamed custom component using shady DOM', () => {
       spyOnDev(console, 'error');
 
-      var defaultCreateElement = document.createElement.bind(document);
+      const defaultCreateElement = document.createElement.bind(document);
 
       try {
         document.createElement = element => {
-          var container = defaultCreateElement(element);
+          const container = defaultCreateElement(element);
           container.shadyRoot = {};
           return container;
         };
@@ -1105,9 +1114,9 @@ describe('ReactDOMComponent', () => {
     it('should treat menuitem as a void element but still create the closing tag', () => {
       // menuitem is not implemented in jsdom, so this triggers the unknown warning error
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
-      var returnedValue = ReactDOMServer.renderToString(
+      const returnedValue = ReactDOMServer.renderToString(
         <menu>
           <menuitem />
         </menu>,
@@ -1238,7 +1247,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('should support custom elements which extend native elements', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       spyOnDevAndProd(document, 'createElement').and.callThrough();
       ReactDOM.render(<div is="custom-div" />, container);
       expect(document.createElement).toHaveBeenCalledWith('div', {
@@ -1248,7 +1257,7 @@ describe('ReactDOMComponent', () => {
 
     it('should work load and error events on <image> element in SVG', () => {
       spyOnDevAndProd(console, 'log');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(
         <svg>
           <image
@@ -1260,8 +1269,8 @@ describe('ReactDOMComponent', () => {
         container,
       );
 
-      var loadEvent = document.createEvent('Event');
-      var errorEvent = document.createEvent('Event');
+      const loadEvent = document.createEvent('Event');
+      const errorEvent = document.createEvent('Event');
 
       loadEvent.initEvent('load', false, false);
       errorEvent.initEvent('error', false, false);
@@ -1278,7 +1287,7 @@ describe('ReactDOMComponent', () => {
   });
 
   describe('updateComponent', () => {
-    var container;
+    let container;
 
     beforeEach(() => {
       container = document.createElement('div');
@@ -1406,7 +1415,7 @@ describe('ReactDOMComponent', () => {
         }
       }
 
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(
         <div>
           <Inner />
@@ -1419,26 +1428,26 @@ describe('ReactDOMComponent', () => {
 
   describe('tag sanitization', () => {
     it('should throw when an invalid tag name is used server-side', () => {
-      var hackzor = React.createElement('script tag');
+      const hackzor = React.createElement('script tag');
       expect(() => ReactDOMServer.renderToString(hackzor)).toThrowError(
         'Invalid tag: script tag',
       );
     });
 
     it('should throw when an attack vector is used server-side', () => {
-      var hackzor = React.createElement('div><img /><div');
+      const hackzor = React.createElement('div><img /><div');
       expect(() => ReactDOMServer.renderToString(hackzor)).toThrowError(
         'Invalid tag: div><img /><div',
       );
     });
 
     it('should throw when an invalid tag name is used', () => {
-      var hackzor = React.createElement('script tag');
+      const hackzor = React.createElement('script tag');
       expect(() => ReactTestUtils.renderIntoDocument(hackzor)).toThrow();
     });
 
     it('should throw when an attack vector is used', () => {
-      var hackzor = React.createElement('div><img /><div');
+      const hackzor = React.createElement('div><img /><div');
       expect(() => ReactTestUtils.renderIntoDocument(hackzor)).toThrow();
     });
   });
@@ -1472,7 +1481,7 @@ describe('ReactDOMComponent', () => {
 
     it('warns on invalid nesting at root', () => {
       spyOnDev(console, 'error');
-      var p = document.createElement('p');
+      const p = document.createElement('p');
       ReactDOM.render(
         <span>
           <p />
@@ -1890,7 +1899,7 @@ describe('ReactDOMComponent', () => {
 
     it('gives source code refs for unknown prop warning for update render', () => {
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
       ReactTestUtils.renderIntoDocument(<div className="paladin" />, container);
       if (__DEV__) {
@@ -1923,14 +1932,14 @@ describe('ReactDOMComponent', () => {
         expect(console.error.calls.count()).toBe(2);
 
         expect(console.error.calls.argsFor(0)[0]).toContain('className');
-        var matches = console.error.calls
+        let matches = console.error.calls
           .argsFor(0)[0]
           .match(/.*\(.*:(\d+)\).*/);
-        var previousLine = matches[1];
+        const previousLine = matches[1];
 
         expect(console.error.calls.argsFor(1)[0]).toContain('onClick');
         matches = console.error.calls.argsFor(1)[0].match(/.*\(.*:(\d+)\).*/);
-        var currentLine = matches[1];
+        const currentLine = matches[1];
 
         //verify line number has a proper relative difference,
         //since hard coding the line number would make test too brittle
@@ -1955,15 +1964,15 @@ describe('ReactDOMComponent', () => {
         expect(console.error.calls.count()).toBe(2);
 
         expect(console.error.calls.argsFor(0)[0]).toContain('className');
-        var matches = console.error.calls
+        let matches = console.error.calls
           .argsFor(0)[0]
           .match(/.*\(.*:(\d+)\).*/);
-        var previousLine = (matches || [])[1];
+        const previousLine = (matches || [])[1];
 
         expect(console.error.calls.argsFor(1)[0]).toContain('onClick');
         matches =
           console.error.calls.argsFor(1)[0].match(/.*\(.*:(\d+)\).*/) || {};
-        var currentLine = (matches || [])[1];
+        const currentLine = (matches || [])[1];
 
         //verify line number has a proper relative difference,
         //since hard coding the line number would make test too brittle
@@ -1973,7 +1982,7 @@ describe('ReactDOMComponent', () => {
 
     it('gives source code refs for unknown prop warning for exact elements in composition', () => {
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
       class Parent extends React.Component {
         render() {
@@ -2018,14 +2027,14 @@ describe('ReactDOMComponent', () => {
         expect(console.error.calls.count()).toBe(2);
 
         expect(console.error.calls.argsFor(0)[0]).toContain('className');
-        var matches = console.error.calls
+        let matches = console.error.calls
           .argsFor(0)[0]
           .match(/.*\(.*:(\d+)\).*/);
-        var previousLine = (matches || [])[1];
+        const previousLine = (matches || [])[1];
 
         expect(console.error.calls.argsFor(1)[0]).toContain('onClick');
         matches = console.error.calls.argsFor(1)[0].match(/.*\(.*:(\d+)\).*/);
-        var currentLine = (matches || [])[1];
+        const currentLine = (matches || [])[1];
 
         //verify line number has a proper relative difference,
         //since hard coding the line number would make test too brittle
@@ -2035,7 +2044,7 @@ describe('ReactDOMComponent', () => {
 
     it('gives source code refs for unknown prop warning for exact elements in composition (ssr)', () => {
       spyOnDev(console, 'error');
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
       class Parent extends React.Component {
         render() {
@@ -2080,14 +2089,14 @@ describe('ReactDOMComponent', () => {
         expect(console.error.calls.count()).toBe(2);
 
         expect(console.error.calls.argsFor(0)[0]).toContain('className');
-        var matches = console.error.calls
+        let matches = console.error.calls
           .argsFor(0)[0]
           .match(/.*\(.*:(\d+)\).*/);
-        var previousLine = (matches || [])[1];
+        const previousLine = (matches || [])[1];
 
         expect(console.error.calls.argsFor(1)[0]).toContain('onClick');
         matches = console.error.calls.argsFor(1)[0].match(/.*\(.*:(\d+)\).*/);
-        var currentLine = (matches || [])[1];
+        const currentLine = (matches || [])[1];
 
         //verify line number has a proper relative difference,
         //since hard coding the line number would make test too brittle
@@ -2170,7 +2179,7 @@ describe('ReactDOMComponent', () => {
     it('sets aliased attributes on HTML attributes', function() {
       spyOnDev(console, 'error');
 
-      var el = ReactTestUtils.renderIntoDocument(<div class="test" />);
+      const el = ReactTestUtils.renderIntoDocument(<div class="test" />);
 
       expect(el.className).toBe('test');
 
@@ -2184,7 +2193,7 @@ describe('ReactDOMComponent', () => {
     it('sets incorrectly cased aliased attributes on HTML attributes with a warning', function() {
       spyOnDev(console, 'error');
 
-      var el = ReactTestUtils.renderIntoDocument(<div cLASS="test" />);
+      const el = ReactTestUtils.renderIntoDocument(<div cLASS="test" />);
 
       expect(el.className).toBe('test');
 
@@ -2198,12 +2207,12 @@ describe('ReactDOMComponent', () => {
     it('sets aliased attributes on SVG elements with a warning', function() {
       spyOnDev(console, 'error');
 
-      var el = ReactTestUtils.renderIntoDocument(
+      const el = ReactTestUtils.renderIntoDocument(
         <svg>
           <text arabic-form="initial" />
         </svg>,
       );
-      var text = el.querySelector('text');
+      const text = el.querySelector('text');
 
       expect(text.hasAttribute('arabic-form')).toBe(true);
 
@@ -2215,7 +2224,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('sets aliased attributes on custom elements', function() {
-      var el = ReactTestUtils.renderIntoDocument(
+      const el = ReactTestUtils.renderIntoDocument(
         <div is="custom-element" class="test" />,
       );
 
@@ -2223,7 +2232,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('aliased attributes on custom elements with bad casing', function() {
-      var el = ReactTestUtils.renderIntoDocument(
+      const el = ReactTestUtils.renderIntoDocument(
         <div is="custom-element" claSS="test" />,
       );
 
@@ -2231,7 +2240,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('updates aliased attributes on custom elements', function() {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div is="custom-element" class="foo" />, container);
       ReactDOM.render(<div is="custom-element" class="bar" />, container);
 
@@ -2241,7 +2250,7 @@ describe('ReactDOMComponent', () => {
 
   describe('Custom attributes', function() {
     it('allows assignment of custom attributes with string values', function() {
-      var el = ReactTestUtils.renderIntoDocument(<div whatever="30" />);
+      const el = ReactTestUtils.renderIntoDocument(<div whatever="30" />);
 
       expect(el.getAttribute('whatever')).toBe('30');
     });
@@ -2260,7 +2269,7 @@ describe('ReactDOMComponent', () => {
     it('does not assign a boolean custom attributes as a string', function() {
       spyOnDev(console, 'error');
 
-      var el = ReactTestUtils.renderIntoDocument(<div whatever={true} />);
+      const el = ReactTestUtils.renderIntoDocument(<div whatever={true} />);
 
       expect(el.hasAttribute('whatever')).toBe(false);
 
@@ -2277,7 +2286,7 @@ describe('ReactDOMComponent', () => {
       spyOnDev(console, 'error');
 
       // eslint-disable-next-line react/jsx-boolean-value
-      var el = ReactTestUtils.renderIntoDocument(<div whatever />);
+      const el = ReactTestUtils.renderIntoDocument(<div whatever />);
 
       expect(el.hasAttribute('whatever')).toBe(false);
 
@@ -2291,7 +2300,7 @@ describe('ReactDOMComponent', () => {
     });
 
     it('assigns a numeric custom attributes as a string', function() {
-      var el = ReactTestUtils.renderIntoDocument(<div whatever={3} />);
+      const el = ReactTestUtils.renderIntoDocument(<div whatever={3} />);
 
       expect(el.getAttribute('whatever')).toBe('3');
     });
@@ -2299,7 +2308,7 @@ describe('ReactDOMComponent', () => {
     it('will not assign a function custom attributes', function() {
       spyOnDev(console, 'error');
 
-      var el = ReactTestUtils.renderIntoDocument(<div whatever={() => {}} />);
+      const el = ReactTestUtils.renderIntoDocument(<div whatever={() => {}} />);
 
       expect(el.hasAttribute('whatever')).toBe(false);
 
@@ -2311,14 +2320,14 @@ describe('ReactDOMComponent', () => {
     });
 
     it('will assign an object custom attributes', function() {
-      var el = ReactTestUtils.renderIntoDocument(<div whatever={{}} />);
+      const el = ReactTestUtils.renderIntoDocument(<div whatever={{}} />);
       expect(el.getAttribute('whatever')).toBe('[object Object]');
     });
 
     it('allows cased data attributes', function() {
       spyOnDev(console, 'error');
 
-      var el = ReactTestUtils.renderIntoDocument(<div data-fooBar="true" />);
+      const el = ReactTestUtils.renderIntoDocument(<div data-fooBar="true" />);
       expect(el.getAttribute('data-foobar')).toBe('true');
 
       if (__DEV__) {
@@ -2337,7 +2346,7 @@ describe('ReactDOMComponent', () => {
     it('allows cased custom attributes', function() {
       spyOnDev(console, 'error');
 
-      var el = ReactTestUtils.renderIntoDocument(<div fooBar="true" />);
+      const el = ReactTestUtils.renderIntoDocument(<div fooBar="true" />);
       expect(el.getAttribute('foobar')).toBe('true');
 
       if (__DEV__) {
@@ -2356,7 +2365,7 @@ describe('ReactDOMComponent', () => {
     it('warns on NaN attributes', function() {
       spyOnDev(console, 'error');
 
-      var el = ReactTestUtils.renderIntoDocument(<div whatever={NaN} />);
+      const el = ReactTestUtils.renderIntoDocument(<div whatever={NaN} />);
 
       expect(el.getAttribute('whatever')).toBe('NaN');
 
@@ -2371,10 +2380,10 @@ describe('ReactDOMComponent', () => {
     it('removes a property when it becomes invalid', function() {
       spyOnDev(console, 'error');
 
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div whatever={0} />, container);
       ReactDOM.render(<div whatever={() => {}} />, container);
-      var el = container.firstChild;
+      const el = container.firstChild;
 
       expect(el.hasAttribute('whatever')).toBe(false);
 
@@ -2388,7 +2397,7 @@ describe('ReactDOMComponent', () => {
     it('warns on bad casing of known HTML attributes', function() {
       spyOnDev(console, 'error');
 
-      var el = ReactTestUtils.renderIntoDocument(<div SiZe="30" />);
+      const el = ReactTestUtils.renderIntoDocument(<div SiZe="30" />);
 
       expect(el.getAttribute('size')).toBe('30');
 
@@ -2402,17 +2411,17 @@ describe('ReactDOMComponent', () => {
 
   describe('Object stringification', function() {
     it('allows objects on known properties', function() {
-      var el = ReactTestUtils.renderIntoDocument(<div acceptCharset={{}} />);
+      const el = ReactTestUtils.renderIntoDocument(<div acceptCharset={{}} />);
       expect(el.getAttribute('accept-charset')).toBe('[object Object]');
     });
 
     it('should pass objects as attributes if they define toString', () => {
-      var obj = {
+      const obj = {
         toString() {
           return 'hello';
         },
       };
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
       ReactDOM.render(<img src={obj} />, container);
       expect(container.firstChild.src).toBe('hello');
@@ -2425,8 +2434,8 @@ describe('ReactDOMComponent', () => {
     });
 
     it('passes objects on known SVG attributes if they do not define toString', () => {
-      var obj = {};
-      var container = document.createElement('div');
+      const obj = {};
+      const container = document.createElement('div');
 
       ReactDOM.render(<svg arabicForm={obj} />, container);
       expect(container.firstChild.getAttribute('arabic-form')).toBe(
@@ -2435,8 +2444,8 @@ describe('ReactDOMComponent', () => {
     });
 
     it('passes objects on custom attributes if they do not define toString', () => {
-      var obj = {};
-      var container = document.createElement('div');
+      const obj = {};
+      const container = document.createElement('div');
 
       ReactDOM.render(<div unknown={obj} />, container);
       expect(container.firstChild.getAttribute('unknown')).toBe(
@@ -2445,16 +2454,16 @@ describe('ReactDOMComponent', () => {
     });
 
     it('allows objects that inherit a custom toString method', function() {
-      var parent = {toString: () => 'hello.jpg'};
-      var child = Object.create(parent);
-      var el = ReactTestUtils.renderIntoDocument(<img src={child} />);
+      const parent = {toString: () => 'hello.jpg'};
+      const child = Object.create(parent);
+      const el = ReactTestUtils.renderIntoDocument(<img src={child} />);
 
       expect(el.src).toBe('hello.jpg');
     });
 
     it('assigns ajaxify (an important internal FB attribute)', function() {
-      var options = {toString: () => 'ajaxy'};
-      var el = ReactTestUtils.renderIntoDocument(<div ajaxify={options} />);
+      const options = {toString: () => 'ajaxy'};
+      const el = ReactTestUtils.renderIntoDocument(<div ajaxify={options} />);
 
       expect(el.getAttribute('ajaxify')).toBe('ajaxy');
     });
@@ -2464,7 +2473,7 @@ describe('ReactDOMComponent', () => {
     it('does not assign string boolean attributes for custom attributes', function() {
       spyOnDev(console, 'error');
 
-      var el = ReactTestUtils.renderIntoDocument(<div whatever={true} />);
+      const el = ReactTestUtils.renderIntoDocument(<div whatever={true} />);
 
       expect(el.hasAttribute('whatever')).toBe(false);
 
@@ -2478,20 +2487,20 @@ describe('ReactDOMComponent', () => {
     });
 
     it('stringifies the boolean true for allowed attributes', function() {
-      var el = ReactTestUtils.renderIntoDocument(<div spellCheck={true} />);
+      const el = ReactTestUtils.renderIntoDocument(<div spellCheck={true} />);
 
       expect(el.getAttribute('spellCheck')).toBe('true');
     });
 
     it('stringifies the boolean false for allowed attributes', function() {
-      var el = ReactTestUtils.renderIntoDocument(<div spellCheck={false} />);
+      const el = ReactTestUtils.renderIntoDocument(<div spellCheck={false} />);
 
       expect(el.getAttribute('spellCheck')).toBe('false');
     });
 
     it('stringifies implicit booleans for allowed attributes', function() {
       // eslint-disable-next-line react/jsx-boolean-value
-      var el = ReactTestUtils.renderIntoDocument(<div spellCheck />);
+      const el = ReactTestUtils.renderIntoDocument(<div spellCheck />);
 
       expect(el.getAttribute('spellCheck')).toBe('true');
     });
@@ -2500,7 +2509,7 @@ describe('ReactDOMComponent', () => {
   describe('Hyphenated SVG elements', function() {
     it('the font-face element is not a custom element', function() {
       spyOnDev(console, 'error');
-      var el = ReactTestUtils.renderIntoDocument(
+      const el = ReactTestUtils.renderIntoDocument(
         <svg>
           <font-face x-height={false} />
         </svg>,
@@ -2520,7 +2529,7 @@ describe('ReactDOMComponent', () => {
 
     it('the font-face element does not allow unknown boolean values', function() {
       spyOnDev(console, 'error');
-      var el = ReactTestUtils.renderIntoDocument(
+      const el = ReactTestUtils.renderIntoDocument(
         <svg>
           <font-face whatever={false} />
         </svg>,
diff --git a/packages/react-dom/src/__tests__/ReactDOMEventListener-test.js b/packages/react-dom/src/__tests__/ReactDOMEventListener-test.js
index 5ffe51333861c..8f11847aacbf7 100644
--- a/packages/react-dom/src/__tests__/ReactDOMEventListener-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMEventListener-test.js
@@ -10,8 +10,8 @@
 'use strict';
 
 describe('ReactDOMEventListener', () => {
-  var React;
-  var ReactDOM;
+  let React;
+  let ReactDOM;
 
   beforeEach(() => {
     jest.resetModules();
@@ -20,11 +20,11 @@ describe('ReactDOMEventListener', () => {
   });
 
   it('should dispatch events from outside React tree', () => {
-    var mock = jest.fn();
+    const mock = jest.fn();
 
-    var container = document.createElement('div');
-    var node = ReactDOM.render(<div onMouseEnter={mock} />, container);
-    var otherNode = document.createElement('h1');
+    const container = document.createElement('div');
+    const node = ReactDOM.render(<div onMouseEnter={mock} />, container);
+    const otherNode = document.createElement('h1');
     document.body.appendChild(container);
     document.body.appendChild(otherNode);
 
@@ -40,23 +40,23 @@ describe('ReactDOMEventListener', () => {
 
   describe('Propagation', () => {
     it('should propagate events one level down', () => {
-      var mouseOut = jest.fn();
-      var onMouseOut = event => mouseOut(event.currentTarget);
+      const mouseOut = jest.fn();
+      const onMouseOut = event => mouseOut(event.currentTarget);
 
-      var childContainer = document.createElement('div');
-      var parentContainer = document.createElement('div');
-      var childNode = ReactDOM.render(
+      const childContainer = document.createElement('div');
+      const parentContainer = document.createElement('div');
+      const childNode = ReactDOM.render(
         <div onMouseOut={onMouseOut}>Child</div>,
         childContainer,
       );
-      var parentNode = ReactDOM.render(
+      const parentNode = ReactDOM.render(
         <div onMouseOut={onMouseOut}>div</div>,
         parentContainer,
       );
       parentNode.appendChild(childContainer);
       document.body.appendChild(parentContainer);
 
-      var nativeEvent = document.createEvent('Event');
+      const nativeEvent = document.createEvent('Event');
       nativeEvent.initEvent('mouseout', true, true);
       childNode.dispatchEvent(nativeEvent);
 
@@ -69,21 +69,21 @@ describe('ReactDOMEventListener', () => {
     });
 
     it('should propagate events two levels down', () => {
-      var mouseOut = jest.fn();
-      var onMouseOut = event => mouseOut(event.currentTarget);
+      const mouseOut = jest.fn();
+      const onMouseOut = event => mouseOut(event.currentTarget);
 
-      var childContainer = document.createElement('div');
-      var parentContainer = document.createElement('div');
-      var grandParentContainer = document.createElement('div');
-      var childNode = ReactDOM.render(
+      const childContainer = document.createElement('div');
+      const parentContainer = document.createElement('div');
+      const grandParentContainer = document.createElement('div');
+      const childNode = ReactDOM.render(
         <div onMouseOut={onMouseOut}>Child</div>,
         childContainer,
       );
-      var parentNode = ReactDOM.render(
+      const parentNode = ReactDOM.render(
         <div onMouseOut={onMouseOut}>Parent</div>,
         parentContainer,
       );
-      var grandParentNode = ReactDOM.render(
+      const grandParentNode = ReactDOM.render(
         <div onMouseOut={onMouseOut}>Parent</div>,
         grandParentContainer,
       );
@@ -92,7 +92,7 @@ describe('ReactDOMEventListener', () => {
 
       document.body.appendChild(grandParentContainer);
 
-      var nativeEvent = document.createEvent('Event');
+      const nativeEvent = document.createEvent('Event');
       nativeEvent.initEvent('mouseout', true, true);
       childNode.dispatchEvent(nativeEvent);
 
@@ -107,7 +107,7 @@ describe('ReactDOMEventListener', () => {
 
     // Regression test for https://github.com/facebook/react/issues/1105
     it('should not get confused by disappearing elements', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       document.body.appendChild(container);
       class MyComponent extends React.Component {
         state = {clicked: false};
@@ -139,32 +139,32 @@ describe('ReactDOMEventListener', () => {
     });
 
     it('should batch between handlers from different roots', () => {
-      var mock = jest.fn();
+      const mock = jest.fn();
 
-      var childContainer = document.createElement('div');
-      var handleChildMouseOut = () => {
+      const childContainer = document.createElement('div');
+      const handleChildMouseOut = () => {
         ReactDOM.render(<div>1</div>, childContainer);
         mock(childNode.textContent);
       };
 
-      var parentContainer = document.createElement('div');
-      var handleParentMouseOut = () => {
+      const parentContainer = document.createElement('div');
+      const handleParentMouseOut = () => {
         ReactDOM.render(<div>2</div>, childContainer);
         mock(childNode.textContent);
       };
 
-      var childNode = ReactDOM.render(
+      const childNode = ReactDOM.render(
         <div onMouseOut={handleChildMouseOut}>Child</div>,
         childContainer,
       );
-      var parentNode = ReactDOM.render(
+      const parentNode = ReactDOM.render(
         <div onMouseOut={handleParentMouseOut}>Parent</div>,
         parentContainer,
       );
       parentNode.appendChild(childContainer);
       document.body.appendChild(parentContainer);
 
-      var nativeEvent = document.createEvent('Event');
+      const nativeEvent = document.createEvent('Event');
       nativeEvent.initEvent('mouseout', true, true);
       childNode.dispatchEvent(nativeEvent);
 
@@ -183,8 +183,8 @@ describe('ReactDOMEventListener', () => {
   });
 
   it('should not fire duplicate events for a React DOM tree', () => {
-    var mouseOut = jest.fn();
-    var onMouseOut = event => mouseOut(event.target);
+    const mouseOut = jest.fn();
+    const onMouseOut = event => mouseOut(event.target);
 
     class Wrapper extends React.Component {
       getInner = () => {
@@ -192,7 +192,7 @@ describe('ReactDOMEventListener', () => {
       };
 
       render() {
-        var inner = <div ref="inner">Inner</div>;
+        const inner = <div ref="inner">Inner</div>;
         return (
           <div>
             <div onMouseOut={onMouseOut} id="outer">
@@ -203,12 +203,12 @@ describe('ReactDOMEventListener', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var instance = ReactDOM.render(<Wrapper />, container);
+    const container = document.createElement('div');
+    const instance = ReactDOM.render(<Wrapper />, container);
 
     document.body.appendChild(container);
 
-    var nativeEvent = document.createEvent('Event');
+    const nativeEvent = document.createEvent('Event');
     nativeEvent.initEvent('mouseout', true, true);
     instance.getInner().dispatchEvent(nativeEvent);
 
diff --git a/packages/react-dom/src/__tests__/ReactDOMFiber-test.js b/packages/react-dom/src/__tests__/ReactDOMFiber-test.js
index 66a0d34057d9a..3db78615a179d 100644
--- a/packages/react-dom/src/__tests__/ReactDOMFiber-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMFiber-test.js
@@ -9,17 +9,17 @@
 
 'use strict';
 
-var React = require('react');
-var ReactDOM = require('react-dom');
-var ReactTestUtils = require('react-dom/test-utils');
-var PropTypes = require('prop-types');
+const React = require('react');
+const ReactDOM = require('react-dom');
+const ReactTestUtils = require('react-dom/test-utils');
+const PropTypes = require('prop-types');
 
 describe('ReactDOMFiber', () => {
   function normalizeCodeLocInfo(str) {
     return str && str.replace(/\(at .+?:\d+\)/g, '(at **)');
   }
 
-  var container;
+  let container;
 
   beforeEach(() => {
     container = document.createElement('div');
@@ -175,16 +175,16 @@ describe('ReactDOMFiber', () => {
     expect(firstNode.tagName).toBe('DIV');
   });
 
-  var svgEls, htmlEls, mathEls;
-  var expectSVG = {ref: el => svgEls.push(el)};
-  var expectHTML = {ref: el => htmlEls.push(el)};
-  var expectMath = {ref: el => mathEls.push(el)};
+  let svgEls, htmlEls, mathEls;
+  const expectSVG = {ref: el => svgEls.push(el)};
+  const expectHTML = {ref: el => htmlEls.push(el)};
+  const expectMath = {ref: el => mathEls.push(el)};
 
-  var usePortal = function(tree) {
+  const usePortal = function(tree) {
     return ReactDOM.createPortal(tree, document.createElement('div'));
   };
 
-  var assertNamespacesMatch = function(tree) {
+  const assertNamespacesMatch = function(tree) {
     container = document.createElement('div');
     svgEls = [];
     htmlEls = [];
@@ -206,7 +206,7 @@ describe('ReactDOMFiber', () => {
   };
 
   it('should render one portal', () => {
-    var portalContainer = document.createElement('div');
+    const portalContainer = document.createElement('div');
 
     ReactDOM.render(
       <div>{ReactDOM.createPortal(<div>portal</div>, portalContainer)}</div>,
@@ -222,7 +222,7 @@ describe('ReactDOMFiber', () => {
 
   // TODO: remove in React 17
   it('should support unstable_createPortal alias', () => {
-    var portalContainer = document.createElement('div');
+    const portalContainer = document.createElement('div');
 
     ReactDOM.render(
       <div>
@@ -239,10 +239,10 @@ describe('ReactDOMFiber', () => {
   });
 
   it('should render many portals', () => {
-    var portalContainer1 = document.createElement('div');
-    var portalContainer2 = document.createElement('div');
+    const portalContainer1 = document.createElement('div');
+    const portalContainer2 = document.createElement('div');
 
-    var ops = [];
+    const ops = [];
     class Child extends React.Component {
       componentDidMount() {
         ops.push(`${this.props.name} componentDidMount`);
@@ -339,9 +339,9 @@ describe('ReactDOMFiber', () => {
   });
 
   it('should render nested portals', () => {
-    var portalContainer1 = document.createElement('div');
-    var portalContainer2 = document.createElement('div');
-    var portalContainer3 = document.createElement('div');
+    const portalContainer1 = document.createElement('div');
+    const portalContainer2 = document.createElement('div');
+    const portalContainer3 = document.createElement('div');
 
     ReactDOM.render(
       [
@@ -382,7 +382,7 @@ describe('ReactDOMFiber', () => {
   });
 
   it('should reconcile portal children', () => {
-    var portalContainer = document.createElement('div');
+    const portalContainer = document.createElement('div');
 
     ReactDOM.render(
       <div>{ReactDOM.createPortal(<div>portal:1</div>, portalContainer)}</div>,
@@ -676,7 +676,7 @@ describe('ReactDOMFiber', () => {
   });
 
   it('should pass portal context when rendering subtree elsewhere', () => {
-    var portalContainer = document.createElement('div');
+    const portalContainer = document.createElement('div');
 
     class Component extends React.Component {
       static contextTypes = {
@@ -710,7 +710,7 @@ describe('ReactDOMFiber', () => {
   });
 
   it('should update portal context if it changes due to setState', () => {
-    var portalContainer = document.createElement('div');
+    const portalContainer = document.createElement('div');
 
     class Component extends React.Component {
       static contextTypes = {
@@ -745,7 +745,7 @@ describe('ReactDOMFiber', () => {
       }
     }
 
-    var instance = ReactDOM.render(<Parent />, container);
+    const instance = ReactDOM.render(<Parent />, container);
     expect(portalContainer.innerHTML).toBe('<div>initial-initial</div>');
     expect(container.innerHTML).toBe('');
     instance.setState({bar: 'changed'});
@@ -754,7 +754,7 @@ describe('ReactDOMFiber', () => {
   });
 
   it('should update portal context if it changes due to re-render', () => {
-    var portalContainer = document.createElement('div');
+    const portalContainer = document.createElement('div');
 
     class Component extends React.Component {
       static contextTypes = {
@@ -802,22 +802,22 @@ describe('ReactDOMFiber', () => {
       }
     }
 
-    var myNodeA = ReactDOM.render(<MyNode />, container);
-    var a = ReactDOM.findDOMNode(myNodeA);
+    const myNodeA = ReactDOM.render(<MyNode />, container);
+    const a = ReactDOM.findDOMNode(myNodeA);
     expect(a.tagName).toBe('DIV');
 
-    var myNodeB = ReactDOM.render(<MyNode flag={true} />, container);
+    const myNodeB = ReactDOM.render(<MyNode flag={true} />, container);
     expect(myNodeA === myNodeB).toBe(true);
 
-    var b = ReactDOM.findDOMNode(myNodeB);
+    const b = ReactDOM.findDOMNode(myNodeB);
     expect(b.tagName).toBe('SPAN');
   });
 
   it('should bubble events from the portal to the parent', () => {
-    var portalContainer = document.createElement('div');
+    const portalContainer = document.createElement('div');
 
-    var ops = [];
-    var portal = null;
+    const ops = [];
+    let portal = null;
 
     ReactDOM.render(
       <div onClick={() => ops.push('parent clicked')}>
@@ -835,7 +835,7 @@ describe('ReactDOMFiber', () => {
 
     expect(portal.tagName).toBe('DIV');
 
-    var fakeNativeEvent = {};
+    const fakeNativeEvent = {};
     ReactTestUtils.simulateNativeEventOnNode(
       'topClick',
       portal,
@@ -846,12 +846,12 @@ describe('ReactDOMFiber', () => {
   });
 
   it('should not onMouseLeave when staying in the portal', () => {
-    var portalContainer = document.createElement('div');
+    const portalContainer = document.createElement('div');
 
-    var ops = [];
-    var firstTarget = null;
-    var secondTarget = null;
-    var thirdTarget = null;
+    let ops = [];
+    let firstTarget = null;
+    let secondTarget = null;
+    let thirdTarget = null;
 
     function simulateMouseMove(from, to) {
       if (from) {
@@ -990,7 +990,7 @@ describe('ReactDOMFiber', () => {
     expect(node.tagName).toEqual('DIV');
 
     function click(target) {
-      var fakeNativeEvent = {};
+      const fakeNativeEvent = {};
       ReactTestUtils.simulateNativeEventOnNode(
         'topClick',
         target,
@@ -1136,18 +1136,18 @@ describe('ReactDOMFiber', () => {
     // 1. Create a new document through the use of iframe
     // 2. Set up the spy to make asserts when a text component
     //    is rendered inside the iframe container
-    var textContent = 'Hello world';
-    var iframe = document.createElement('iframe');
+    const textContent = 'Hello world';
+    const iframe = document.createElement('iframe');
     document.body.appendChild(iframe);
-    var iframeDocument = iframe.contentDocument;
+    const iframeDocument = iframe.contentDocument;
     iframeDocument.write(
       '<!DOCTYPE html><html><head></head><body><div></div></body></html>',
     );
     iframeDocument.close();
-    var iframeContainer = iframeDocument.body.firstChild;
+    const iframeContainer = iframeDocument.body.firstChild;
 
-    var actualDocument;
-    var textNode;
+    let actualDocument;
+    let textNode;
 
     spyOnDevAndProd(iframeContainer, 'appendChild').and.callFake(node => {
       actualDocument = node.ownerDocument;
@@ -1163,7 +1163,7 @@ describe('ReactDOMFiber', () => {
   });
 
   it('should mount into a document fragment', () => {
-    var fragment = document.createDocumentFragment();
+    const fragment = document.createDocumentFragment();
     ReactDOM.render(<div>foo</div>, fragment);
     expect(container.innerHTML).toBe('');
     container.appendChild(fragment);
diff --git a/packages/react-dom/src/__tests__/ReactDOMFiberAsync-test.internal.js b/packages/react-dom/src/__tests__/ReactDOMFiberAsync-test.internal.js
index 85f13de1befce..6ea40d66d8ed4 100644
--- a/packages/react-dom/src/__tests__/ReactDOMFiberAsync-test.internal.js
+++ b/packages/react-dom/src/__tests__/ReactDOMFiberAsync-test.internal.js
@@ -9,15 +9,15 @@
 
 'use strict';
 
-var React = require('react');
-var ReactFeatureFlags = require('shared/ReactFeatureFlags');
+const React = require('react');
+let ReactFeatureFlags = require('shared/ReactFeatureFlags');
 
-var ReactDOM;
+let ReactDOM;
 
-var AsyncComponent = React.unstable_AsyncComponent;
+const AsyncComponent = React.unstable_AsyncComponent;
 
 describe('ReactDOMFiberAsync', () => {
-  var container;
+  let container;
 
   beforeEach(() => {
     container = document.createElement('div');
@@ -25,7 +25,7 @@ describe('ReactDOMFiberAsync', () => {
   });
 
   it('renders synchronously by default', () => {
-    var ops = [];
+    const ops = [];
     ReactDOM.render(<div>Hi</div>, container, () => {
       ops.push(container.textContent);
     });
diff --git a/packages/react-dom/src/__tests__/ReactDOMIframe-test.js b/packages/react-dom/src/__tests__/ReactDOMIframe-test.js
index a3c3c019d2ac7..074a1d96f2a82 100644
--- a/packages/react-dom/src/__tests__/ReactDOMIframe-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMIframe-test.js
@@ -10,9 +10,9 @@
 'use strict';
 
 describe('ReactDOMIframe', () => {
-  var React;
-  var ReactDOM;
-  var ReactTestUtils;
+  let React;
+  let ReactDOM;
+  let ReactTestUtils;
 
   beforeEach(() => {
     React = require('react');
@@ -21,11 +21,11 @@ describe('ReactDOMIframe', () => {
   });
 
   it('should trigger load events', () => {
-    var onLoadSpy = jasmine.createSpy();
-    var iframe = React.createElement('iframe', {onLoad: onLoadSpy});
+    const onLoadSpy = jasmine.createSpy();
+    let iframe = React.createElement('iframe', {onLoad: onLoadSpy});
     iframe = ReactTestUtils.renderIntoDocument(iframe);
 
-    var loadEvent = document.createEvent('Event');
+    const loadEvent = document.createEvent('Event');
     loadEvent.initEvent('load', false, false);
 
     ReactDOM.findDOMNode(iframe).dispatchEvent(loadEvent);
diff --git a/packages/react-dom/src/__tests__/ReactDOMInput-test.js b/packages/react-dom/src/__tests__/ReactDOMInput-test.js
index c89802c202917..695bf89cbe795 100644
--- a/packages/react-dom/src/__tests__/ReactDOMInput-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMInput-test.js
@@ -9,14 +9,14 @@
 
 'use strict';
 
-var emptyFunction = require('fbjs/lib/emptyFunction');
+const emptyFunction = require('fbjs/lib/emptyFunction');
 
 describe('ReactDOMInput', () => {
-  var React;
-  var ReactDOM;
-  var ReactDOMServer;
-  var ReactTestUtils;
-  var setUntrackedValue;
+  let React;
+  let ReactDOM;
+  let ReactDOMServer;
+  let ReactTestUtils;
+  let setUntrackedValue;
 
   function normalizeCodeLocInfo(str) {
     return str && str.replace(/\(at .+?:\d+\)/g, '(at **)');
@@ -42,12 +42,12 @@ describe('ReactDOMInput', () => {
 
   it('should properly control a value even if no event listener exists', () => {
     spyOnDev(console, 'error');
-    var container = document.createElement('div');
-    var stub = ReactDOM.render(<input type="text" value="lion" />, container);
+    const container = document.createElement('div');
+    const stub = ReactDOM.render(<input type="text" value="lion" />, container);
 
     document.body.appendChild(container);
 
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
     if (__DEV__) {
       expect(console.error.calls.count()).toBe(1);
     }
@@ -99,8 +99,8 @@ describe('ReactDOMInput', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var instance = ReactDOM.render(<ControlledInputs />, container);
+    const container = document.createElement('div');
+    const instance = ReactDOM.render(<ControlledInputs />, container);
 
     // We need it to be in the body to test native event dispatching.
     document.body.appendChild(container);
@@ -152,8 +152,8 @@ describe('ReactDOMInput', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var instance = ReactDOM.render(<ControlledInputs />, container);
+    const container = document.createElement('div');
+    const instance = ReactDOM.render(<ControlledInputs />, container);
 
     // We need it to be in the body to test native event dispatching.
     document.body.appendChild(container);
@@ -171,9 +171,9 @@ describe('ReactDOMInput', () => {
 
   describe('switching text inputs between numeric and string numbers', () => {
     it('does change the number 2 to "2.0" with no change handler', () => {
-      var stub = <input type="text" value={2} onChange={jest.fn()} />;
+      let stub = <input type="text" value={2} onChange={jest.fn()} />;
       stub = ReactTestUtils.renderIntoDocument(stub);
-      var node = ReactDOM.findDOMNode(stub);
+      const node = ReactDOM.findDOMNode(stub);
 
       node.value = '2.0';
 
@@ -184,9 +184,9 @@ describe('ReactDOMInput', () => {
     });
 
     it('does change the string "2" to "2.0" with no change handler', () => {
-      var stub = <input type="text" value={'2'} onChange={jest.fn()} />;
+      let stub = <input type="text" value={'2'} onChange={jest.fn()} />;
       stub = ReactTestUtils.renderIntoDocument(stub);
-      var node = ReactDOM.findDOMNode(stub);
+      const node = ReactDOM.findDOMNode(stub);
 
       node.value = '2.0';
 
@@ -211,8 +211,8 @@ describe('ReactDOMInput', () => {
         }
       }
 
-      var stub = ReactTestUtils.renderIntoDocument(<Stub />);
-      var node = ReactDOM.findDOMNode(stub);
+      const stub = ReactTestUtils.renderIntoDocument(<Stub />);
+      const node = ReactDOM.findDOMNode(stub);
 
       node.value = '2.0';
 
@@ -234,8 +234,8 @@ describe('ReactDOMInput', () => {
       }
     }
 
-    var stub = ReactTestUtils.renderIntoDocument(<Stub />);
-    var node = ReactDOM.findDOMNode(stub);
+    const stub = ReactTestUtils.renderIntoDocument(<Stub />);
+    const node = ReactDOM.findDOMNode(stub);
     stub.setState({value: '0.98'});
 
     expect(node.value).toEqual('0.98');
@@ -276,8 +276,8 @@ describe('ReactDOMInput', () => {
       }
     }
 
-    var stub = ReactTestUtils.renderIntoDocument(<Stub />);
-    var node = ReactDOM.findDOMNode(stub);
+    const stub = ReactTestUtils.renderIntoDocument(<Stub />);
+    const node = ReactDOM.findDOMNode(stub);
     stub.setState({value: '3'});
 
     expect(node.value).toEqual('3');
@@ -291,9 +291,9 @@ describe('ReactDOMInput', () => {
   });
 
   it('should display `defaultValue` of number 0', () => {
-    var stub = <input type="text" defaultValue={0} />;
+    let stub = <input type="text" defaultValue={0} />;
     stub = ReactTestUtils.renderIntoDocument(stub);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.getAttribute('value')).toBe('0');
     expect(node.value).toBe('0');
@@ -306,8 +306,8 @@ describe('ReactDOMInput', () => {
       }
     }
 
-    var component = ReactTestUtils.renderIntoDocument(<Test />);
-    var node = ReactDOM.findDOMNode(component);
+    const component = ReactTestUtils.renderIntoDocument(<Test />);
+    const node = ReactDOM.findDOMNode(component);
 
     Object.defineProperty(node, 'defaultValue', {
       get() {
@@ -324,25 +324,25 @@ describe('ReactDOMInput', () => {
   });
 
   it('should display "true" for `defaultValue` of `true`', () => {
-    var stub = <input type="text" defaultValue={true} />;
+    let stub = <input type="text" defaultValue={true} />;
     stub = ReactTestUtils.renderIntoDocument(stub);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.value).toBe('true');
   });
 
   it('should display "false" for `defaultValue` of `false`', () => {
-    var stub = <input type="text" defaultValue={false} />;
+    let stub = <input type="text" defaultValue={false} />;
     stub = ReactTestUtils.renderIntoDocument(stub);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.value).toBe('false');
   });
 
   it('should update `defaultValue` for uncontrolled input', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var node = ReactDOM.render(
+    const node = ReactDOM.render(
       <input type="text" defaultValue="0" />,
       container,
     );
@@ -356,9 +356,9 @@ describe('ReactDOMInput', () => {
   });
 
   it('should update `defaultValue` for uncontrolled date/time input', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var node = ReactDOM.render(
+    const node = ReactDOM.render(
       <input type="date" defaultValue="1980-01-01" />,
       container,
     );
@@ -375,8 +375,8 @@ describe('ReactDOMInput', () => {
 
   it('should take `defaultValue` when changing to uncontrolled input', () => {
     spyOnDev(console, 'error');
-    var container = document.createElement('div');
-    var node = ReactDOM.render(
+    const container = document.createElement('div');
+    const node = ReactDOM.render(
       <input type="text" value="0" readOnly="true" />,
       container,
     );
@@ -394,79 +394,79 @@ describe('ReactDOMInput', () => {
   });
 
   it('should render defaultValue for SSR', () => {
-    var markup = ReactDOMServer.renderToString(
+    const markup = ReactDOMServer.renderToString(
       <input type="text" defaultValue="1" />,
     );
-    var div = document.createElement('div');
+    const div = document.createElement('div');
     div.innerHTML = markup;
     expect(div.firstChild.getAttribute('value')).toBe('1');
     expect(div.firstChild.getAttribute('defaultValue')).toBe(null);
   });
 
   it('should render value for SSR', () => {
-    var element = <input type="text" value="1" onChange={() => {}} />;
-    var markup = ReactDOMServer.renderToString(element);
-    var div = document.createElement('div');
+    const element = <input type="text" value="1" onChange={() => {}} />;
+    const markup = ReactDOMServer.renderToString(element);
+    const div = document.createElement('div');
     div.innerHTML = markup;
     expect(div.firstChild.getAttribute('value')).toBe('1');
     expect(div.firstChild.getAttribute('defaultValue')).toBe(null);
   });
 
   it('should render name attribute if it is supplied', () => {
-    var container = document.createElement('div');
-    var node = ReactDOM.render(<input type="text" name="name" />, container);
+    const container = document.createElement('div');
+    const node = ReactDOM.render(<input type="text" name="name" />, container);
     expect(node.name).toBe('name');
     expect(container.firstChild.getAttribute('name')).toBe('name');
   });
 
   it('should render name attribute if it is supplied for SSR', () => {
-    var element = <input type="text" name="name" />;
-    var markup = ReactDOMServer.renderToString(element);
-    var div = document.createElement('div');
+    const element = <input type="text" name="name" />;
+    const markup = ReactDOMServer.renderToString(element);
+    const div = document.createElement('div');
     div.innerHTML = markup;
     expect(div.firstChild.getAttribute('name')).toBe('name');
   });
 
   it('should not render name attribute if it is not supplied', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<input type="text" />, container);
     expect(container.firstChild.getAttribute('name')).toBe(null);
   });
 
   it('should not render name attribute if it is not supplied for SSR', () => {
-    var element = <input type="text" />;
-    var markup = ReactDOMServer.renderToString(element);
-    var div = document.createElement('div');
+    const element = <input type="text" />;
+    const markup = ReactDOMServer.renderToString(element);
+    const div = document.createElement('div');
     div.innerHTML = markup;
     expect(div.firstChild.getAttribute('name')).toBe(null);
   });
 
   it('should display "foobar" for `defaultValue` of `objToString`', () => {
-    var objToString = {
+    const objToString = {
       toString: function() {
         return 'foobar';
       },
     };
 
-    var stub = <input type="text" defaultValue={objToString} />;
+    let stub = <input type="text" defaultValue={objToString} />;
     stub = ReactTestUtils.renderIntoDocument(stub);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.value).toBe('foobar');
   });
 
   it('should display `value` of number 0', () => {
-    var stub = <input type="text" value={0} />;
+    let stub = <input type="text" value={0} />;
     stub = ReactTestUtils.renderIntoDocument(stub);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.value).toBe('0');
   });
 
   it('should allow setting `value` to `true`', () => {
-    var container = document.createElement('div');
-    var stub = <input type="text" value="yolo" onChange={emptyFunction} />;
-    var node = ReactDOM.render(stub, container);
+    const container = document.createElement('div');
+    let stub = <input type="text" value="yolo" onChange={emptyFunction} />;
+    const node = ReactDOM.render(stub, container);
 
     expect(node.value).toBe('yolo');
 
@@ -478,9 +478,9 @@ describe('ReactDOMInput', () => {
   });
 
   it('should allow setting `value` to `false`', () => {
-    var container = document.createElement('div');
-    var stub = <input type="text" value="yolo" onChange={emptyFunction} />;
-    var node = ReactDOM.render(stub, container);
+    const container = document.createElement('div');
+    let stub = <input type="text" value="yolo" onChange={emptyFunction} />;
+    const node = ReactDOM.render(stub, container);
 
     expect(node.value).toBe('yolo');
 
@@ -492,13 +492,13 @@ describe('ReactDOMInput', () => {
   });
 
   it('should allow setting `value` to `objToString`', () => {
-    var container = document.createElement('div');
-    var stub = <input type="text" value="foo" onChange={emptyFunction} />;
-    var node = ReactDOM.render(stub, container);
+    const container = document.createElement('div');
+    let stub = <input type="text" value="foo" onChange={emptyFunction} />;
+    const node = ReactDOM.render(stub, container);
 
     expect(node.value).toBe('foo');
 
-    var objToString = {
+    const objToString = {
       toString: function() {
         return 'foobar';
       },
@@ -511,12 +511,12 @@ describe('ReactDOMInput', () => {
   });
 
   it('should not incur unnecessary DOM mutations', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<input value="a" onChange={() => {}} />, container);
 
-    var node = container.firstChild;
-    var nodeValue = 'a';
-    var nodeValueSetter = jest.genMockFn();
+    const node = container.firstChild;
+    let nodeValue = 'a';
+    const nodeValueSetter = jest.genMockFn();
     Object.defineProperty(node, 'value', {
       get: function() {
         return nodeValue;
@@ -534,12 +534,12 @@ describe('ReactDOMInput', () => {
   });
 
   it('should not incur unnecessary DOM mutations for numeric type conversion', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<input value="0" onChange={() => {}} />, container);
 
-    var node = container.firstChild;
-    var nodeValue = '0';
-    var nodeValueSetter = jest.genMockFn();
+    const node = container.firstChild;
+    let nodeValue = '0';
+    const nodeValueSetter = jest.genMockFn();
     Object.defineProperty(node, 'value', {
       get: function() {
         return nodeValue;
@@ -554,12 +554,12 @@ describe('ReactDOMInput', () => {
   });
 
   it('should not incur unnecessary DOM mutations for the boolean type conversion', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<input value="true" onChange={() => {}} />, container);
 
-    var node = container.firstChild;
-    var nodeValue = 'true';
-    var nodeValueSetter = jest.genMockFn();
+    const node = container.firstChild;
+    let nodeValue = 'true';
+    const nodeValueSetter = jest.genMockFn();
     Object.defineProperty(node, 'value', {
       get: function() {
         return nodeValue;
@@ -574,9 +574,9 @@ describe('ReactDOMInput', () => {
   });
 
   it('should properly control a value of number `0`', () => {
-    var stub = <input type="text" value={0} onChange={emptyFunction} />;
+    let stub = <input type="text" value={0} onChange={emptyFunction} />;
     stub = ReactTestUtils.renderIntoDocument(stub);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     node.value = 'giraffe';
     ReactTestUtils.Simulate.change(node);
@@ -584,9 +584,9 @@ describe('ReactDOMInput', () => {
   });
 
   it('should properly control 0.0 for a text input', () => {
-    var stub = <input type="text" value={0} onChange={emptyFunction} />;
+    let stub = <input type="text" value={0} onChange={emptyFunction} />;
     stub = ReactTestUtils.renderIntoDocument(stub);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     node.value = '0.0';
     ReactTestUtils.Simulate.change(node, {target: {value: '0.0'}});
@@ -594,9 +594,9 @@ describe('ReactDOMInput', () => {
   });
 
   it('should properly control 0.0 for a number input', () => {
-    var stub = <input type="number" value={0} onChange={emptyFunction} />;
+    let stub = <input type="number" value={0} onChange={emptyFunction} />;
     stub = ReactTestUtils.renderIntoDocument(stub);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     node.value = '0.0';
     ReactTestUtils.Simulate.change(node, {target: {value: '0.0'}});
@@ -604,24 +604,24 @@ describe('ReactDOMInput', () => {
   });
 
   it('should properly transition from an empty value to 0', function() {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
     ReactDOM.render(<input type="text" value="" />, container);
     ReactDOM.render(<input type="text" value={0} />, container);
 
-    var node = container.firstChild;
+    const node = container.firstChild;
 
     expect(node.value).toBe('0');
     expect(node.defaultValue).toBe('0');
   });
 
   it('should properly transition from 0 to an empty value', function() {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
     ReactDOM.render(<input type="text" value={0} />, container);
     ReactDOM.render(<input type="text" value="" />, container);
 
-    var node = container.firstChild;
+    const node = container.firstChild;
 
     expect(node.value).toBe('');
     expect(node.defaultValue).toBe('');
@@ -664,18 +664,18 @@ describe('ReactDOMInput', () => {
   });
 
   it('should have the correct target value', () => {
-    var handled = false;
-    var handler = function(event) {
+    let handled = false;
+    const handler = function(event) {
       expect(event.target.nodeName).toBe('INPUT');
       handled = true;
     };
-    var stub = <input type="text" value={0} onChange={handler} />;
-    var container = document.createElement('div');
-    var node = ReactDOM.render(stub, container);
+    const stub = <input type="text" value={0} onChange={handler} />;
+    const container = document.createElement('div');
+    const node = ReactDOM.render(stub, container);
 
     setUntrackedValue.call(node, 'giraffe');
 
-    var fakeNativeEvent = function() {};
+    const fakeNativeEvent = function() {};
     fakeNativeEvent.target = node;
     fakeNativeEvent.path = [node, container];
     ReactTestUtils.simulateNativeEventOnNode('topInput', node, fakeNativeEvent);
@@ -684,9 +684,9 @@ describe('ReactDOMInput', () => {
   });
 
   it('should not set a value for submit buttons unnecessarily', () => {
-    var stub = <input type="submit" />;
+    let stub = <input type="submit" />;
     stub = ReactTestUtils.renderIntoDocument(stub);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     // The value shouldn't be '', or else the button will have no text; it
     // should have the default "Submit" or "Submit Query" label. Most browsers
@@ -726,10 +726,10 @@ describe('ReactDOMInput', () => {
       }
     }
 
-    var stub = ReactTestUtils.renderIntoDocument(<RadioGroup />);
-    var aNode = stub.refs.a;
-    var bNode = stub.refs.b;
-    var cNode = stub.refs.c;
+    const stub = ReactTestUtils.renderIntoDocument(<RadioGroup />);
+    const aNode = stub.refs.a;
+    const bNode = stub.refs.b;
+    const cNode = stub.refs.c;
 
     expect(aNode.checked).toBe(true);
     expect(bNode.checked).toBe(false);
@@ -782,23 +782,23 @@ describe('ReactDOMInput', () => {
       }
     }
 
-    var stub = ReactTestUtils.renderIntoDocument(<App />);
-    var buttonNode = ReactDOM.findDOMNode(stub).childNodes[0];
-    var firstRadioNode = ReactDOM.findDOMNode(stub).childNodes[1];
+    const stub = ReactTestUtils.renderIntoDocument(<App />);
+    const buttonNode = ReactDOM.findDOMNode(stub).childNodes[0];
+    const firstRadioNode = ReactDOM.findDOMNode(stub).childNodes[1];
     expect(firstRadioNode.checked).toBe(false);
     ReactTestUtils.Simulate.click(buttonNode);
     expect(firstRadioNode.checked).toBe(true);
   });
 
   it('should control radio buttons if the tree updates during render', () => {
-    var sharedParent = document.createElement('div');
-    var container1 = document.createElement('div');
-    var container2 = document.createElement('div');
+    const sharedParent = document.createElement('div');
+    const container1 = document.createElement('div');
+    const container2 = document.createElement('div');
 
     sharedParent.appendChild(container1);
 
-    var aNode;
-    var bNode;
+    let aNode;
+    let bNode;
     class ComponentA extends React.Component {
       componentDidMount() {
         ReactDOM.render(<ComponentB />, container2);
@@ -889,11 +889,11 @@ describe('ReactDOMInput', () => {
   });
 
   it('should have a this value of undefined if bind is not used', () => {
-    var unboundInputOnChange = function() {
+    const unboundInputOnChange = function() {
       expect(this).toBe(undefined);
     };
 
-    var instance = <input type="text" onChange={unboundInputOnChange} />;
+    let instance = <input type="text" onChange={unboundInputOnChange} />;
     instance = ReactTestUtils.renderIntoDocument(instance);
 
     ReactTestUtils.Simulate.change(instance);
@@ -917,7 +917,7 @@ describe('ReactDOMInput', () => {
   });
 
   it('should update defaultValue to empty string', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<input type="text" defaultValue={'foo'} />, container);
     ReactDOM.render(<input type="text" defaultValue={''} />, container);
     expect(container.firstChild.defaultValue).toBe('');
@@ -1000,10 +1000,10 @@ describe('ReactDOMInput', () => {
 
   it('should warn if controlled input switches to uncontrolled (value is undefined)', () => {
     spyOnDev(console, 'error');
-    var stub = (
+    const stub = (
       <input type="text" value="controlled" onChange={emptyFunction} />
     );
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="text" />, container);
     if (__DEV__) {
@@ -1020,10 +1020,10 @@ describe('ReactDOMInput', () => {
 
   it('should warn if controlled input switches to uncontrolled (value is null)', () => {
     spyOnDev(console, 'error');
-    var stub = (
+    const stub = (
       <input type="text" value="controlled" onChange={emptyFunction} />
     );
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="text" value={null} />, container);
     if (__DEV__) {
@@ -1040,10 +1040,10 @@ describe('ReactDOMInput', () => {
 
   it('should warn if controlled input switches to uncontrolled with defaultValue', () => {
     spyOnDev(console, 'error');
-    var stub = (
+    const stub = (
       <input type="text" value="controlled" onChange={emptyFunction} />
     );
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(
       <input type="text" defaultValue="uncontrolled" />,
@@ -1063,8 +1063,8 @@ describe('ReactDOMInput', () => {
 
   it('should warn if uncontrolled input (value is undefined) switches to controlled', () => {
     spyOnDev(console, 'error');
-    var stub = <input type="text" />;
-    var container = document.createElement('div');
+    const stub = <input type="text" />;
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="text" value="controlled" />, container);
     if (__DEV__) {
@@ -1081,8 +1081,8 @@ describe('ReactDOMInput', () => {
 
   it('should warn if uncontrolled input (value is null) switches to controlled', () => {
     spyOnDev(console, 'error');
-    var stub = <input type="text" value={null} />;
-    var container = document.createElement('div');
+    const stub = <input type="text" value={null} />;
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="text" value="controlled" />, container);
     if (__DEV__) {
@@ -1099,10 +1099,10 @@ describe('ReactDOMInput', () => {
 
   it('should warn if controlled checkbox switches to uncontrolled (checked is undefined)', () => {
     spyOnDev(console, 'error');
-    var stub = (
+    const stub = (
       <input type="checkbox" checked={true} onChange={emptyFunction} />
     );
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="checkbox" />, container);
     if (__DEV__) {
@@ -1119,10 +1119,10 @@ describe('ReactDOMInput', () => {
 
   it('should warn if controlled checkbox switches to uncontrolled (checked is null)', () => {
     spyOnDev(console, 'error');
-    var stub = (
+    const stub = (
       <input type="checkbox" checked={true} onChange={emptyFunction} />
     );
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="checkbox" checked={null} />, container);
     if (__DEV__) {
@@ -1139,10 +1139,10 @@ describe('ReactDOMInput', () => {
 
   it('should warn if controlled checkbox switches to uncontrolled with defaultChecked', () => {
     spyOnDev(console, 'error');
-    var stub = (
+    const stub = (
       <input type="checkbox" checked={true} onChange={emptyFunction} />
     );
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="checkbox" defaultChecked={true} />, container);
     if (__DEV__) {
@@ -1159,8 +1159,8 @@ describe('ReactDOMInput', () => {
 
   it('should warn if uncontrolled checkbox (checked is undefined) switches to controlled', () => {
     spyOnDev(console, 'error');
-    var stub = <input type="checkbox" />;
-    var container = document.createElement('div');
+    const stub = <input type="checkbox" />;
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="checkbox" checked={true} />, container);
     if (__DEV__) {
@@ -1177,8 +1177,8 @@ describe('ReactDOMInput', () => {
 
   it('should warn if uncontrolled checkbox (checked is null) switches to controlled', () => {
     spyOnDev(console, 'error');
-    var stub = <input type="checkbox" checked={null} />;
-    var container = document.createElement('div');
+    const stub = <input type="checkbox" checked={null} />;
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="checkbox" checked={true} />, container);
     if (__DEV__) {
@@ -1195,8 +1195,8 @@ describe('ReactDOMInput', () => {
 
   it('should warn if controlled radio switches to uncontrolled (checked is undefined)', () => {
     spyOnDev(console, 'error');
-    var stub = <input type="radio" checked={true} onChange={emptyFunction} />;
-    var container = document.createElement('div');
+    const stub = <input type="radio" checked={true} onChange={emptyFunction} />;
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="radio" />, container);
     if (__DEV__) {
@@ -1213,8 +1213,8 @@ describe('ReactDOMInput', () => {
 
   it('should warn if controlled radio switches to uncontrolled (checked is null)', () => {
     spyOnDev(console, 'error');
-    var stub = <input type="radio" checked={true} onChange={emptyFunction} />;
-    var container = document.createElement('div');
+    const stub = <input type="radio" checked={true} onChange={emptyFunction} />;
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="radio" checked={null} />, container);
     if (__DEV__) {
@@ -1231,8 +1231,8 @@ describe('ReactDOMInput', () => {
 
   it('should warn if controlled radio switches to uncontrolled with defaultChecked', () => {
     spyOnDev(console, 'error');
-    var stub = <input type="radio" checked={true} onChange={emptyFunction} />;
-    var container = document.createElement('div');
+    const stub = <input type="radio" checked={true} onChange={emptyFunction} />;
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="radio" defaultChecked={true} />, container);
     if (__DEV__) {
@@ -1249,8 +1249,8 @@ describe('ReactDOMInput', () => {
 
   it('should warn if uncontrolled radio (checked is undefined) switches to controlled', () => {
     spyOnDev(console, 'error');
-    var stub = <input type="radio" />;
-    var container = document.createElement('div');
+    const stub = <input type="radio" />;
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="radio" checked={true} />, container);
     if (__DEV__) {
@@ -1267,8 +1267,8 @@ describe('ReactDOMInput', () => {
 
   it('should warn if uncontrolled radio (checked is null) switches to controlled', () => {
     spyOnDev(console, 'error');
-    var stub = <input type="radio" checked={null} />;
-    var container = document.createElement('div');
+    const stub = <input type="radio" checked={null} />;
+    const container = document.createElement('div');
     ReactDOM.render(stub, container);
     ReactDOM.render(<input type="radio" checked={true} />, container);
     if (__DEV__) {
@@ -1284,7 +1284,7 @@ describe('ReactDOMInput', () => {
   });
 
   it('should not warn if radio value changes but never becomes controlled', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<input type="radio" value="value" />, container);
     ReactDOM.render(<input type="radio" />, container);
     ReactDOM.render(
@@ -1299,7 +1299,7 @@ describe('ReactDOMInput', () => {
   });
 
   it('should not warn if radio value changes but never becomes uncontrolled', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <input type="radio" checked={false} onChange={() => null} />,
       container,
@@ -1318,7 +1318,7 @@ describe('ReactDOMInput', () => {
 
   it('should warn if radio checked false changes to become uncontrolled', () => {
     spyOnDev(console, 'error');
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <input
         type="radio"
@@ -1341,11 +1341,11 @@ describe('ReactDOMInput', () => {
   });
 
   it('sets type, step, min, max before value always', () => {
-    var log = [];
-    var originalCreateElement = document.createElement;
+    const log = [];
+    const originalCreateElement = document.createElement;
     spyOnDevAndProd(document, 'createElement').and.callFake(function(type) {
-      var el = originalCreateElement.apply(this, arguments);
-      var value = '';
+      const el = originalCreateElement.apply(this, arguments);
+      let value = '';
 
       if (type === 'input') {
         Object.defineProperty(el, 'value', {
@@ -1387,7 +1387,7 @@ describe('ReactDOMInput', () => {
   });
 
   it('sets value properly with type coming later in props', () => {
-    var input = ReactTestUtils.renderIntoDocument(
+    const input = ReactTestUtils.renderIntoDocument(
       <input value="hi" type="radio" />,
     );
     expect(input.value).toBe('hi');
@@ -1398,13 +1398,13 @@ describe('ReactDOMInput', () => {
       state = {type: 'number', value: 1000};
 
       render() {
-        var {value, type} = this.state;
+        const {value, type} = this.state;
         return <input onChange={() => {}} type={type} value={value} />;
       }
     }
 
-    var input = ReactTestUtils.renderIntoDocument(<Input />);
-    var node = ReactDOM.findDOMNode(input);
+    const input = ReactTestUtils.renderIntoDocument(<Input />);
+    const node = ReactDOM.findDOMNode(input);
 
     // If the value is set before the type, a validation warning will raise and
     // the value will not be assigned.
@@ -1417,11 +1417,11 @@ describe('ReactDOMInput', () => {
       return JSON.stringify(x, null, 2);
     }
 
-    var log = [];
-    var originalCreateElement = document.createElement;
+    const log = [];
+    const originalCreateElement = document.createElement;
     spyOnDevAndProd(document, 'createElement').and.callFake(function(type) {
-      var el = originalCreateElement.apply(this, arguments);
-      var value = '';
+      const el = originalCreateElement.apply(this, arguments);
+      let value = '';
       if (type === 'input') {
         Object.defineProperty(el, 'value', {
           get: function() {
@@ -1461,8 +1461,8 @@ describe('ReactDOMInput', () => {
           this.setState({value: event.target.value});
         };
         render() {
-          var type = this.props.type;
-          var value = this.state.value;
+          const type = this.props.type;
+          const value = this.state.value;
 
           return <input type={type} value={value} onChange={this.onChange} />;
         }
@@ -1470,9 +1470,9 @@ describe('ReactDOMInput', () => {
     }
 
     it('always sets the attribute when values change on text inputs', function() {
-      var Input = getTestInput();
-      var stub = ReactTestUtils.renderIntoDocument(<Input type="text" />);
-      var node = ReactDOM.findDOMNode(stub);
+      const Input = getTestInput();
+      const stub = ReactTestUtils.renderIntoDocument(<Input type="text" />);
+      const node = ReactDOM.findDOMNode(stub);
 
       ReactTestUtils.Simulate.change(node, {target: {value: '2'}});
 
@@ -1480,11 +1480,11 @@ describe('ReactDOMInput', () => {
     });
 
     it('does not set the value attribute on number inputs if focused', () => {
-      var Input = getTestInput();
-      var stub = ReactTestUtils.renderIntoDocument(
+      const Input = getTestInput();
+      const stub = ReactTestUtils.renderIntoDocument(
         <Input type="number" value="1" />,
       );
-      var node = ReactDOM.findDOMNode(stub);
+      const node = ReactDOM.findDOMNode(stub);
 
       node.focus();
 
@@ -1494,11 +1494,11 @@ describe('ReactDOMInput', () => {
     });
 
     it('sets the value attribute on number inputs on blur', () => {
-      var Input = getTestInput();
-      var stub = ReactTestUtils.renderIntoDocument(
+      const Input = getTestInput();
+      const stub = ReactTestUtils.renderIntoDocument(
         <Input type="number" value="1" />,
       );
-      var node = ReactDOM.findDOMNode(stub);
+      const node = ReactDOM.findDOMNode(stub);
 
       ReactTestUtils.Simulate.change(node, {target: {value: '2'}});
       ReactTestUtils.SimulateNative.blur(node);
@@ -1507,10 +1507,10 @@ describe('ReactDOMInput', () => {
     });
 
     it('an uncontrolled number input will not update the value attribute on blur', () => {
-      var stub = ReactTestUtils.renderIntoDocument(
+      const stub = ReactTestUtils.renderIntoDocument(
         <input type="number" defaultValue="1" />,
       );
-      var node = ReactDOM.findDOMNode(stub);
+      const node = ReactDOM.findDOMNode(stub);
 
       node.value = 4;
 
@@ -1520,10 +1520,10 @@ describe('ReactDOMInput', () => {
     });
 
     it('an uncontrolled text input will not update the value attribute on blur', () => {
-      var stub = ReactTestUtils.renderIntoDocument(
+      const stub = ReactTestUtils.renderIntoDocument(
         <input type="text" defaultValue="1" />,
       );
-      var node = ReactDOM.findDOMNode(stub);
+      const node = ReactDOM.findDOMNode(stub);
 
       node.value = 4;
 
@@ -1534,7 +1534,7 @@ describe('ReactDOMInput', () => {
   });
 
   describe('setting a controlled input to undefined', () => {
-    var input;
+    let input;
 
     function renderInputWithStringThenWithUndefined() {
       class Input extends React.Component {
@@ -1549,7 +1549,7 @@ describe('ReactDOMInput', () => {
         }
       }
 
-      var stub = ReactTestUtils.renderIntoDocument(<Input />);
+      const stub = ReactTestUtils.renderIntoDocument(<Input />);
       input = ReactDOM.findDOMNode(stub);
       ReactTestUtils.Simulate.change(input, {target: {value: 'latest'}});
       ReactTestUtils.Simulate.change(input, {target: {value: undefined}});
@@ -1583,7 +1583,7 @@ describe('ReactDOMInput', () => {
   });
 
   describe('setting a controlled input to null', () => {
-    var input;
+    let input;
 
     function renderInputWithStringThenWithNull() {
       class Input extends React.Component {
@@ -1598,7 +1598,7 @@ describe('ReactDOMInput', () => {
         }
       }
 
-      var stub = ReactTestUtils.renderIntoDocument(<Input />);
+      const stub = ReactTestUtils.renderIntoDocument(<Input />);
       input = ReactDOM.findDOMNode(stub);
       ReactTestUtils.Simulate.change(input, {target: {value: 'latest'}});
       ReactTestUtils.Simulate.change(input, {target: {value: null}});
diff --git a/packages/react-dom/src/__tests__/ReactDOMInvalidARIAHook-test.js b/packages/react-dom/src/__tests__/ReactDOMInvalidARIAHook-test.js
index 518e177de5b35..2235282ec3725 100644
--- a/packages/react-dom/src/__tests__/ReactDOMInvalidARIAHook-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMInvalidARIAHook-test.js
@@ -10,9 +10,9 @@
 'use strict';
 
 describe('ReactDOMInvalidARIAHook', () => {
-  var React;
-  var ReactTestUtils;
-  var mountComponent;
+  let React;
+  let ReactTestUtils;
+  let mountComponent;
 
   beforeEach(() => {
     jest.resetModules();
diff --git a/packages/react-dom/src/__tests__/ReactDOMOption-test.js b/packages/react-dom/src/__tests__/ReactDOMOption-test.js
index 8a28a32297b00..72e4361113ac0 100644
--- a/packages/react-dom/src/__tests__/ReactDOMOption-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMOption-test.js
@@ -14,9 +14,9 @@ describe('ReactDOMOption', () => {
     return str && str.replace(/\(at .+?:\d+\)/g, '(at **)');
   }
 
-  var React;
-  var ReactDOM;
-  var ReactTestUtils;
+  let React;
+  let ReactDOM;
+  let ReactTestUtils;
 
   beforeEach(() => {
     React = require('react');
@@ -25,25 +25,25 @@ describe('ReactDOMOption', () => {
   });
 
   it('should flatten children to a string', () => {
-    var stub = (
+    let stub = (
       <option>
         {1} {'foo'}
       </option>
     );
     stub = ReactTestUtils.renderIntoDocument(stub);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.innerHTML).toBe('1 foo');
   });
 
   it('should ignore and warn invalid children types', () => {
     spyOnDev(console, 'error');
-    var el = (
+    const el = (
       <option>
         {1} <div /> {2}
       </option>
     );
-    var node = ReactTestUtils.renderIntoDocument(el);
+    const node = ReactTestUtils.renderIntoDocument(el);
     expect(node.innerHTML).toBe('1  2');
     ReactTestUtils.renderIntoDocument(el);
     if (__DEV__) {
@@ -58,7 +58,7 @@ describe('ReactDOMOption', () => {
   });
 
   it('should ignore null/undefined/false children without warning', () => {
-    var stub = (
+    let stub = (
       <option>
         {1} {false}
         {true}
@@ -69,7 +69,7 @@ describe('ReactDOMOption', () => {
     spyOnDev(console, 'error');
     stub = ReactTestUtils.renderIntoDocument(stub);
 
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     if (__DEV__) {
       expect(console.error.calls.count()).toBe(0);
@@ -78,16 +78,16 @@ describe('ReactDOMOption', () => {
   });
 
   it('should be able to use dangerouslySetInnerHTML on option', () => {
-    var stub = <option dangerouslySetInnerHTML={{__html: 'foobar'}} />;
+    let stub = <option dangerouslySetInnerHTML={{__html: 'foobar'}} />;
     stub = ReactTestUtils.renderIntoDocument(stub);
 
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
     expect(node.innerHTML).toBe('foobar');
   });
 
   it('should set attribute for empty value', () => {
-    var container = document.createElement('div');
-    var option = ReactDOM.render(<option value="" />, container);
+    const container = document.createElement('div');
+    const option = ReactDOM.render(<option value="" />, container);
     expect(option.hasAttribute('value')).toBe(true);
     expect(option.getAttribute('value')).toBe('');
 
@@ -97,18 +97,18 @@ describe('ReactDOMOption', () => {
   });
 
   it('should allow ignoring `value` on option', () => {
-    var a = 'a';
-    var stub = (
+    const a = 'a';
+    let stub = (
       <select value="giraffe" onChange={() => {}}>
         <option>monkey</option>
         <option>gir{a}ffe</option>
         <option>gorill{a}</option>
       </select>
     );
-    var options = stub.props.children;
-    var container = document.createElement('div');
+    const options = stub.props.children;
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.selectedIndex).toBe(1);
 
diff --git a/packages/react-dom/src/__tests__/ReactDOMRoot-test.internal.js b/packages/react-dom/src/__tests__/ReactDOMRoot-test.internal.js
index a16ad96d44e78..f0387d7df443a 100644
--- a/packages/react-dom/src/__tests__/ReactDOMRoot-test.internal.js
+++ b/packages/react-dom/src/__tests__/ReactDOMRoot-test.internal.js
@@ -9,10 +9,10 @@
 
 'use strict';
 
-var React = require('react');
-var ReactDOM = require('react-dom');
-var ReactDOMServer = require('react-dom/server');
-var AsyncComponent = React.unstable_AsyncComponent;
+let React = require('react');
+let ReactDOM = require('react-dom');
+let ReactDOMServer = require('react-dom/server');
+let AsyncComponent = React.unstable_AsyncComponent;
 
 describe('ReactDOMRoot', () => {
   let container;
@@ -29,7 +29,7 @@ describe('ReactDOMRoot', () => {
     scheduledCallback = null;
     flush = function(units = Infinity) {
       if (scheduledCallback !== null) {
-        var didStop = false;
+        let didStop = false;
         while (scheduledCallback !== null && !didStop) {
           const cb = scheduledCallback;
           scheduledCallback = null;
diff --git a/packages/react-dom/src/__tests__/ReactDOMSVG-test.js b/packages/react-dom/src/__tests__/ReactDOMSVG-test.js
index 60aa5aeecb38b..10e6b88831447 100644
--- a/packages/react-dom/src/__tests__/ReactDOMSVG-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMSVG-test.js
@@ -9,9 +9,9 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
-var ReactDOMServer;
+let React;
+let ReactDOM;
+let ReactDOMServer;
 
 describe('ReactDOMSVG', () => {
   beforeEach(() => {
@@ -21,7 +21,7 @@ describe('ReactDOMSVG', () => {
   });
 
   it('creates initial namespaced markup', () => {
-    var markup = ReactDOMServer.renderToString(
+    const markup = ReactDOMServer.renderToString(
       <svg>
         <image xlinkHref="http://i.imgur.com/w7GCRPb.png" />
       </svg>,
@@ -30,8 +30,8 @@ describe('ReactDOMSVG', () => {
   });
 
   it('creates elements with SVG namespace inside SVG tag during mount', () => {
-    var node = document.createElement('div');
-    var div,
+    const node = document.createElement('div');
+    let div,
       div2,
       div3,
       foreignObject,
@@ -111,7 +111,7 @@ describe('ReactDOMSVG', () => {
   });
 
   it('creates elements with SVG namespace inside SVG tag during update', () => {
-    var inst,
+    let inst,
       div,
       div2,
       foreignObject,
@@ -158,7 +158,7 @@ describe('ReactDOMSVG', () => {
       }
     }
 
-    var node = document.createElement('div');
+    const node = document.createElement('div');
     ReactDOM.render(
       <svg ref={el => (svg = el)}>
         <App />
@@ -194,29 +194,32 @@ describe('ReactDOMSVG', () => {
   });
 
   it('can render SVG into a non-React SVG tree', () => {
-    var outerSVGRoot = document.createElementNS(
+    const outerSVGRoot = document.createElementNS(
       'http://www.w3.org/2000/svg',
       'svg',
     );
-    var container = document.createElementNS('http://www.w3.org/2000/svg', 'g');
+    const container = document.createElementNS(
+      'http://www.w3.org/2000/svg',
+      'g',
+    );
     outerSVGRoot.appendChild(container);
-    var image;
+    let image;
     ReactDOM.render(<image ref={el => (image = el)} />, container);
     expect(image.namespaceURI).toBe('http://www.w3.org/2000/svg');
     expect(image.tagName).toBe('image');
   });
 
   it('can render HTML into a foreignObject in non-React SVG tree', () => {
-    var outerSVGRoot = document.createElementNS(
+    const outerSVGRoot = document.createElementNS(
       'http://www.w3.org/2000/svg',
       'svg',
     );
-    var container = document.createElementNS(
+    const container = document.createElementNS(
       'http://www.w3.org/2000/svg',
       'foreignObject',
     );
     outerSVGRoot.appendChild(container);
-    var div;
+    let div;
     ReactDOM.render(<div ref={el => (div = el)} />, container);
     expect(div.namespaceURI).toBe('http://www.w3.org/1999/xhtml');
     expect(div.tagName).toBe('DIV');
diff --git a/packages/react-dom/src/__tests__/ReactDOMSelect-test.js b/packages/react-dom/src/__tests__/ReactDOMSelect-test.js
index df206e7355eb0..96470b75d55e4 100644
--- a/packages/react-dom/src/__tests__/ReactDOMSelect-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMSelect-test.js
@@ -10,12 +10,12 @@
 'use strict';
 
 describe('ReactDOMSelect', () => {
-  var React;
-  var ReactDOM;
-  var ReactDOMServer;
-  var ReactTestUtils;
+  let React;
+  let ReactDOM;
+  let ReactDOMServer;
+  let ReactTestUtils;
 
-  var noop = function() {};
+  const noop = function() {};
 
   beforeEach(() => {
     jest.resetModules();
@@ -26,17 +26,17 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should allow setting `defaultValue`', () => {
-    var stub = (
+    let stub = (
       <select defaultValue="giraffe">
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var options = stub.props.children;
-    var container = document.createElement('div');
+    const options = stub.props.children;
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.value).toBe('giraffe');
 
@@ -49,7 +49,7 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should not throw with `defaultValue` and without children', () => {
-    var stub = <select defaultValue="dummy" />;
+    const stub = <select defaultValue="dummy" />;
 
     expect(() => {
       ReactTestUtils.renderIntoDocument(stub);
@@ -57,16 +57,16 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should not control when using `defaultValue`', () => {
-    var el = (
+    const el = (
       <select defaultValue="giraffe">
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var container = document.createElement('div');
-    var stub = ReactDOM.render(el, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const container = document.createElement('div');
+    const stub = ReactDOM.render(el, container);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.value).toBe('giraffe');
 
@@ -77,17 +77,17 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should allow setting `defaultValue` with multiple', () => {
-    var stub = (
+    let stub = (
       <select multiple={true} defaultValue={['giraffe', 'gorilla']}>
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var options = stub.props.children;
-    var container = document.createElement('div');
+    const options = stub.props.children;
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.options[0].selected).toBe(false); // monkey
     expect(node.options[1].selected).toBe(true); // giraffe
@@ -107,17 +107,17 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should allow setting `value`', () => {
-    var stub = (
+    let stub = (
       <select value="giraffe" onChange={noop}>
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var options = stub.props.children;
-    var container = document.createElement('div');
+    const options = stub.props.children;
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.value).toBe('giraffe');
 
@@ -132,7 +132,7 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should default to the first non-disabled option', () => {
-    var stub = (
+    let stub = (
       <select defaultValue="">
         <option disabled={true}>Disabled</option>
         <option disabled={true}>Still Disabled</option>
@@ -140,25 +140,25 @@ describe('ReactDOMSelect', () => {
         <option disabled={true}>Also Disabled</option>
       </select>
     );
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
     expect(node.options[0].selected).toBe(false);
     expect(node.options[2].selected).toBe(true);
   });
 
   it('should allow setting `value` to __proto__', () => {
-    var stub = (
+    let stub = (
       <select value="__proto__" onChange={noop}>
         <option value="monkey">A monkey!</option>
         <option value="__proto__">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var options = stub.props.children;
-    var container = document.createElement('div');
+    const options = stub.props.children;
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.value).toBe('__proto__');
 
@@ -173,7 +173,7 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should not throw with `value` and without children', () => {
-    var stub = <select value="dummy" onChange={noop} />;
+    const stub = <select value="dummy" onChange={noop} />;
 
     expect(() => {
       ReactTestUtils.renderIntoDocument(stub);
@@ -181,17 +181,17 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should allow setting `value` with multiple', () => {
-    var stub = (
+    let stub = (
       <select multiple={true} value={['giraffe', 'gorilla']} onChange={noop}>
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var options = stub.props.children;
-    var container = document.createElement('div');
+    const options = stub.props.children;
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.options[0].selected).toBe(false); // monkey
     expect(node.options[1].selected).toBe(true); // giraffe
@@ -211,17 +211,17 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should allow setting `value` to __proto__ with multiple', () => {
-    var stub = (
+    let stub = (
       <select multiple={true} value={['__proto__', 'gorilla']} onChange={noop}>
         <option value="monkey">A monkey!</option>
         <option value="__proto__">A __proto__!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var options = stub.props.children;
-    var container = document.createElement('div');
+    const options = stub.props.children;
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.options[0].selected).toBe(false); // monkey
     expect(node.options[1].selected).toBe(true); // __proto__
@@ -241,7 +241,7 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should not select other options automatically', () => {
-    var stub = (
+    let stub = (
       <select multiple={true} value={['12']} onChange={noop}>
         <option value="1">one</option>
         <option value="2">two</option>
@@ -249,7 +249,7 @@ describe('ReactDOMSelect', () => {
       </select>
     );
     stub = ReactTestUtils.renderIntoDocument(stub);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.options[0].selected).toBe(false); // one
     expect(node.options[1].selected).toBe(false); // two
@@ -257,8 +257,8 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should reset child options selected when they are changed and `value` is set', () => {
-    var stub = <select multiple={true} value={['a', 'b']} onChange={noop} />;
-    var container = document.createElement('div');
+    let stub = <select multiple={true} value={['a', 'b']} onChange={noop} />;
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
 
     ReactDOM.render(
@@ -270,7 +270,7 @@ describe('ReactDOMSelect', () => {
       container,
     );
 
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.options[0].selected).toBe(true); // a
     expect(node.options[1].selected).toBe(true); // b
@@ -278,23 +278,23 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should allow setting `value` with `objectToString`', () => {
-    var objectToString = {
+    const objectToString = {
       animal: 'giraffe',
       toString: function() {
         return this.animal;
       },
     };
 
-    var el = (
+    const el = (
       <select multiple={true} value={[objectToString]} onChange={noop}>
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var container = document.createElement('div');
-    var stub = ReactDOM.render(el, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const container = document.createElement('div');
+    const stub = ReactDOM.render(el, container);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.options[0].selected).toBe(false); // monkey
     expect(node.options[1].selected).toBe(true); // giraffe
@@ -303,7 +303,7 @@ describe('ReactDOMSelect', () => {
     // Changing the `value` prop should change the selected options.
     objectToString.animal = 'monkey';
 
-    var el2 = (
+    const el2 = (
       <select multiple={true} value={[objectToString]}>
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
@@ -318,17 +318,17 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should allow switching to multiple', () => {
-    var stub = (
+    let stub = (
       <select defaultValue="giraffe">
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var options = stub.props.children;
-    var container = document.createElement('div');
+    const options = stub.props.children;
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.options[0].selected).toBe(false); // monkey
     expect(node.options[1].selected).toBe(true); // giraffe
@@ -348,17 +348,17 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should allow switching from multiple', () => {
-    var stub = (
+    let stub = (
       <select multiple={true} defaultValue={['giraffe', 'gorilla']}>
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var options = stub.props.children;
-    var container = document.createElement('div');
+    const options = stub.props.children;
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.options[0].selected).toBe(false); // monkey
     expect(node.options[1].selected).toBe(true); // giraffe
@@ -377,17 +377,17 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should remember value when switching to uncontrolled', () => {
-    var stub = (
+    let stub = (
       <select value={'giraffe'} onChange={noop}>
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var options = stub.props.children;
-    var container = document.createElement('div');
+    const options = stub.props.children;
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(node.options[0].selected).toBe(false); // monkey
     expect(node.options[1].selected).toBe(true); // giraffe
@@ -401,17 +401,17 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should remember updated value when switching to uncontrolled', () => {
-    var stub = (
+    let stub = (
       <select value={'giraffe'} onChange={noop}>
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var options = stub.props.children;
-    var container = document.createElement('div');
+    const options = stub.props.children;
+    const container = document.createElement('div');
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     ReactDOM.render(
       <select value="gorilla" onChange={noop}>
@@ -432,51 +432,51 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should support server-side rendering', () => {
-    var stub = (
+    const stub = (
       <select value="giraffe" onChange={noop}>
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var markup = ReactDOMServer.renderToString(stub);
+    const markup = ReactDOMServer.renderToString(stub);
     expect(markup).toContain('<option selected value="giraffe"');
     expect(markup).not.toContain('<option selected value="monkey"');
     expect(markup).not.toContain('<option selected value="gorilla"');
   });
 
   it('should support server-side rendering with defaultValue', () => {
-    var stub = (
+    const stub = (
       <select defaultValue="giraffe">
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var markup = ReactDOMServer.renderToString(stub);
+    const markup = ReactDOMServer.renderToString(stub);
     expect(markup).toContain('<option selected value="giraffe"');
     expect(markup).not.toContain('<option selected value="monkey"');
     expect(markup).not.toContain('<option selected value="gorilla"');
   });
 
   it('should support server-side rendering with multiple', () => {
-    var stub = (
+    const stub = (
       <select multiple={true} value={['giraffe', 'gorilla']} onChange={noop}>
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
         <option value="gorilla">A gorilla!</option>
       </select>
     );
-    var markup = ReactDOMServer.renderToString(stub);
+    const markup = ReactDOMServer.renderToString(stub);
     expect(markup).toContain('<option selected value="giraffe"');
     expect(markup).toContain('<option selected value="gorilla"');
     expect(markup).not.toContain('<option selected value="monkey"');
   });
 
   it('should not control defaultValue if readding options', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var select = ReactDOM.render(
+    const select = ReactDOM.render(
       <select multiple={true} defaultValue={['giraffe']}>
         <option key="monkey" value="monkey">
           A monkey!
@@ -490,7 +490,7 @@ describe('ReactDOMSelect', () => {
       </select>,
       container,
     );
-    var node = ReactDOM.findDOMNode(select);
+    const node = ReactDOM.findDOMNode(select);
 
     expect(node.options[0].selected).toBe(false); // monkey
     expect(node.options[1].selected).toBe(true); // giraffe
@@ -586,7 +586,7 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should refresh state on change', () => {
-    var stub = (
+    let stub = (
       <select value="giraffe" onChange={noop}>
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
@@ -594,7 +594,7 @@ describe('ReactDOMSelect', () => {
       </select>
     );
     stub = ReactTestUtils.renderIntoDocument(stub);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     ReactTestUtils.Simulate.change(node);
 
@@ -637,8 +637,8 @@ describe('ReactDOMSelect', () => {
       ReactDOM.unmountComponentAtNode(container);
     }
 
-    var container = document.createElement('div');
-    var stub = (
+    const container = document.createElement('div');
+    let stub = (
       <select value="giraffe" onChange={changeView}>
         <option value="monkey">A monkey!</option>
         <option value="giraffe">A giraffe!</option>
@@ -646,13 +646,13 @@ describe('ReactDOMSelect', () => {
       </select>
     );
     stub = ReactDOM.render(stub, container);
-    var node = ReactDOM.findDOMNode(stub);
+    const node = ReactDOM.findDOMNode(stub);
 
     expect(() => ReactTestUtils.Simulate.change(node)).not.toThrow();
   });
 
   it('should select grandchild options nested inside an optgroup', () => {
-    var stub = (
+    const stub = (
       <select value="b" onChange={noop}>
         <optgroup label="group">
           <option value="a">a</option>
@@ -661,8 +661,8 @@ describe('ReactDOMSelect', () => {
         </optgroup>
       </select>
     );
-    var container = document.createElement('div');
-    var node = ReactDOM.render(stub, container);
+    const container = document.createElement('div');
+    const node = ReactDOM.render(stub, container);
 
     expect(node.options[0].selected).toBe(false); // a
     expect(node.options[1].selected).toBe(true); // b
@@ -670,7 +670,7 @@ describe('ReactDOMSelect', () => {
   });
 
   it('should allow controlling `value` in a nested render', () => {
-    var selectNode;
+    let selectNode;
 
     class Parent extends React.Component {
       state = {
@@ -708,7 +708,7 @@ describe('ReactDOMSelect', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
     document.body.appendChild(container);
 
diff --git a/packages/react-dom/src/__tests__/ReactDOMSelection-test.internal.js b/packages/react-dom/src/__tests__/ReactDOMSelection-test.internal.js
index 74ff592b0ab75..b167084ad87ba 100644
--- a/packages/react-dom/src/__tests__/ReactDOMSelection-test.internal.js
+++ b/packages/react-dom/src/__tests__/ReactDOMSelection-test.internal.js
@@ -9,12 +9,12 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
-var ReactDOMSelection;
-var invariant;
+let React;
+let ReactDOM;
+let ReactDOMSelection;
+let invariant;
 
-var getModernOffsetsFromPoints;
+let getModernOffsetsFromPoints;
 
 describe('ReactDOMSelection', () => {
   beforeEach(() => {
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationBasic-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationBasic-test.js
index 5ba73558cd9a0..4a64b29f0d2a2 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationBasic-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationBasic-test.js
@@ -113,7 +113,7 @@ describe('ReactDOMServerIntegration', () => {
     itRenders('an iterable', async render => {
       const threeDivIterable = {
         '@@iterator': function() {
-          var i = 0;
+          let i = 0;
           return {
             next: function() {
               if (i++ < 3) {
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationElements-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationElements-test.js
index bdf2557534e01..9ec22278354d4 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationElements-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationElements-test.js
@@ -550,7 +550,7 @@ describe('ReactDOMServerIntegration', () => {
             </Component>
           </Component>,
         );
-        for (var i = 0; i < 3; i++) {
+        for (let i = 0; i < 3; i++) {
           expect(e.tagName).toBe('DIV');
           expect(e.childNodes.length).toBe(1);
           e = e.firstChild;
@@ -575,12 +575,12 @@ describe('ReactDOMServerIntegration', () => {
         );
         expect(e.tagName).toBe('DIV');
         expect(e.childNodes.length).toBe(2);
-        for (var i = 0; i < 2; i++) {
-          var child = e.childNodes[i];
+        for (let i = 0; i < 2; i++) {
+          const child = e.childNodes[i];
           expect(child.tagName).toBe('DIV');
           expect(child.childNodes.length).toBe(2);
-          for (var j = 0; j < 2; j++) {
-            var grandchild = child.childNodes[j];
+          for (let j = 0; j < 2; j++) {
+            const grandchild = child.childNodes[j];
             expect(grandchild.tagName).toBe('DIV');
             expect(grandchild.childNodes.length).toBe(0);
           }
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationForms-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationForms-test.js
index 0034e42233229..fc4594261eba2 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationForms-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationForms-test.js
@@ -226,7 +226,7 @@ describe('ReactDOMServerIntegration', () => {
     });
 
     describe('selects', function() {
-      var options;
+      let options;
       beforeEach(function() {
         options = [
           <option key={1} value="foo" id="foo">
@@ -255,7 +255,7 @@ describe('ReactDOMServerIntegration', () => {
 
         ['foo', 'bar', 'baz'].forEach(value => {
           const expectedValue = selected.indexOf(value) !== -1;
-          var option = element.querySelector(`#${value}`);
+          const option = element.querySelector(`#${value}`);
           expect(option.selected).toBe(expectedValue);
         });
       };
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationReconnecting-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationReconnecting-test.js
index 60d33cde35787..c10450c436f1d 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationReconnecting-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationReconnecting-test.js
@@ -42,7 +42,7 @@ describe('ReactDOMServerIntegration', () => {
   });
 
   describe('reconnecting to server markup', function() {
-    var EmptyComponent;
+    let EmptyComponent;
     beforeEach(() => {
       EmptyComponent = class extends React.Component {
         render() {
diff --git a/packages/react-dom/src/__tests__/ReactDOMTextComponent-test.js b/packages/react-dom/src/__tests__/ReactDOMTextComponent-test.js
index b163ff115ba7a..f3f9dc5485b35 100644
--- a/packages/react-dom/src/__tests__/ReactDOMTextComponent-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMTextComponent-test.js
@@ -9,9 +9,9 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
-var ReactDOMServer;
+let React;
+let ReactDOM;
+let ReactDOMServer;
 
 // In standard React, TextComponent keeps track of different Text templates
 // using comments. However, in React Fiber, those comments are not outputted due
@@ -30,8 +30,8 @@ describe('ReactDOMTextComponent', () => {
   });
 
   it('updates a mounted text component in place', () => {
-    var el = document.createElement('div');
-    var inst = ReactDOM.render(
+    const el = document.createElement('div');
+    let inst = ReactDOM.render(
       <div>
         <span />
         {'foo'}
@@ -64,8 +64,8 @@ describe('ReactDOMTextComponent', () => {
   });
 
   it('can be toggled in and out of the markup', () => {
-    var el = document.createElement('div');
-    var inst = ReactDOM.render(
+    const el = document.createElement('div');
+    let inst = ReactDOM.render(
       <div>
         {'foo'}
         <div />
@@ -74,7 +74,7 @@ describe('ReactDOMTextComponent', () => {
       el,
     );
 
-    var container = ReactDOM.findDOMNode(inst);
+    let container = ReactDOM.findDOMNode(inst);
     let childNodes = filterOutComments(container.childNodes);
     let childDiv = childNodes[1];
 
@@ -113,8 +113,8 @@ describe('ReactDOMTextComponent', () => {
    */
 
   xit('can reconcile text merged by Node.normalize() alongside other elements', () => {
-    var el = document.createElement('div');
-    var inst = ReactDOM.render(
+    const el = document.createElement('div');
+    let inst = ReactDOM.render(
       <div>
         {'foo'}
         {'bar'}
@@ -125,7 +125,7 @@ describe('ReactDOMTextComponent', () => {
       el,
     );
 
-    var container = ReactDOM.findDOMNode(inst);
+    let container = ReactDOM.findDOMNode(inst);
     container.normalize();
 
     inst = ReactDOM.render(
@@ -143,8 +143,8 @@ describe('ReactDOMTextComponent', () => {
   });
 
   xit('can reconcile text merged by Node.normalize()', () => {
-    var el = document.createElement('div');
-    var inst = ReactDOM.render(
+    const el = document.createElement('div');
+    let inst = ReactDOM.render(
       <div>
         {'foo'}
         {'bar'}
@@ -153,7 +153,7 @@ describe('ReactDOMTextComponent', () => {
       el,
     );
 
-    var container = ReactDOM.findDOMNode(inst);
+    let container = ReactDOM.findDOMNode(inst);
     container.normalize();
 
     inst = ReactDOM.render(
@@ -169,8 +169,8 @@ describe('ReactDOMTextComponent', () => {
   });
 
   it('can reconcile text from pre-rendered markup', () => {
-    var el = document.createElement('div');
-    var reactEl = (
+    const el = document.createElement('div');
+    let reactEl = (
       <div>
         {'foo'}
         {'bar'}
@@ -198,8 +198,8 @@ describe('ReactDOMTextComponent', () => {
   });
 
   xit('can reconcile text arbitrarily split into multiple nodes', () => {
-    var el = document.createElement('div');
-    var inst = ReactDOM.render(
+    const el = document.createElement('div');
+    let inst = ReactDOM.render(
       <div>
         <span />
         {'foobarbaz'}
@@ -207,7 +207,7 @@ describe('ReactDOMTextComponent', () => {
       el,
     );
 
-    var container = ReactDOM.findDOMNode(inst);
+    let container = ReactDOM.findDOMNode(inst);
     let childNodes = filterOutComments(ReactDOM.findDOMNode(inst).childNodes);
     let textNode = childNodes[1];
     textNode.textContent = 'foo';
@@ -232,8 +232,8 @@ describe('ReactDOMTextComponent', () => {
   });
 
   xit('can reconcile text arbitrarily split into multiple nodes on some substitutions only', () => {
-    var el = document.createElement('div');
-    var inst = ReactDOM.render(
+    const el = document.createElement('div');
+    let inst = ReactDOM.render(
       <div>
         <span />
         {'bar'}
@@ -246,7 +246,7 @@ describe('ReactDOMTextComponent', () => {
       el,
     );
 
-    var container = ReactDOM.findDOMNode(inst);
+    let container = ReactDOM.findDOMNode(inst);
     let childNodes = filterOutComments(ReactDOM.findDOMNode(inst).childNodes);
     let textNode = childNodes[3];
     textNode.textContent = 'foo';
@@ -282,7 +282,7 @@ describe('ReactDOMTextComponent', () => {
   });
 
   xit('can unmount normalized text nodes', () => {
-    var el = document.createElement('div');
+    const el = document.createElement('div');
     ReactDOM.render(
       <div>
         {''}
diff --git a/packages/react-dom/src/__tests__/ReactDOMTextarea-test.js b/packages/react-dom/src/__tests__/ReactDOMTextarea-test.js
index 6196c556bee4a..569ff91eafcee 100644
--- a/packages/react-dom/src/__tests__/ReactDOMTextarea-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMTextarea-test.js
@@ -9,15 +9,15 @@
 
 'use strict';
 
-var emptyFunction = require('fbjs/lib/emptyFunction');
+const emptyFunction = require('fbjs/lib/emptyFunction');
 
 describe('ReactDOMTextarea', () => {
-  var React;
-  var ReactDOM;
-  var ReactDOMServer;
-  var ReactTestUtils;
+  let React;
+  let ReactDOM;
+  let ReactDOMServer;
+  let ReactTestUtils;
 
-  var renderTextarea;
+  let renderTextarea;
 
   beforeEach(() => {
     React = require('react');
@@ -29,7 +29,7 @@ describe('ReactDOMTextarea', () => {
       if (!container) {
         container = document.createElement('div');
       }
-      var node = ReactDOM.render(component, container);
+      const node = ReactDOM.render(component, container);
 
       // Fixing jsdom's quirky behavior -- in reality, the parser should strip
       // off the leading newline but we need to do it by hand here.
@@ -39,8 +39,8 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should allow setting `defaultValue`', () => {
-    var container = document.createElement('div');
-    var node = renderTextarea(<textarea defaultValue="giraffe" />, container);
+    const container = document.createElement('div');
+    const node = renderTextarea(<textarea defaultValue="giraffe" />, container);
 
     expect(node.value).toBe('giraffe');
 
@@ -55,34 +55,34 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should display `defaultValue` of number 0', () => {
-    var stub = <textarea defaultValue={0} />;
-    var node = renderTextarea(stub);
+    const stub = <textarea defaultValue={0} />;
+    const node = renderTextarea(stub);
 
     expect(node.value).toBe('0');
   });
 
   it('should display "false" for `defaultValue` of `false`', () => {
-    var stub = <textarea defaultValue={false} />;
-    var node = renderTextarea(stub);
+    const stub = <textarea defaultValue={false} />;
+    const node = renderTextarea(stub);
 
     expect(node.value).toBe('false');
   });
 
   it('should display "foobar" for `defaultValue` of `objToString`', () => {
-    var objToString = {
+    const objToString = {
       toString: function() {
         return 'foobar';
       },
     };
 
-    var stub = <textarea defaultValue={objToString} />;
-    var node = renderTextarea(stub);
+    const stub = <textarea defaultValue={objToString} />;
+    const node = renderTextarea(stub);
 
     expect(node.value).toBe('foobar');
   });
 
   it('should set defaultValue', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<textarea defaultValue="foo" />, container);
     ReactDOM.render(<textarea defaultValue="bar" />, container);
     ReactDOM.render(<textarea defaultValue="noise" />, container);
@@ -90,30 +90,30 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should not render value as an attribute', () => {
-    var stub = <textarea value="giraffe" onChange={emptyFunction} />;
-    var node = renderTextarea(stub);
+    const stub = <textarea value="giraffe" onChange={emptyFunction} />;
+    const node = renderTextarea(stub);
 
     expect(node.getAttribute('value')).toBe(null);
   });
 
   it('should display `value` of number 0', () => {
-    var stub = <textarea value={0} />;
-    var node = renderTextarea(stub);
+    const stub = <textarea value={0} />;
+    const node = renderTextarea(stub);
 
     expect(node.value).toBe('0');
   });
 
   it('should update defaultValue to empty string', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<textarea defaultValue={'foo'} />, container);
     ReactDOM.render(<textarea defaultValue={''} />, container);
     expect(container.firstChild.defaultValue).toBe('');
   });
 
   it('should allow setting `value` to `giraffe`', () => {
-    var container = document.createElement('div');
-    var stub = <textarea value="giraffe" onChange={emptyFunction} />;
-    var node = renderTextarea(stub, container);
+    const container = document.createElement('div');
+    let stub = <textarea value="giraffe" onChange={emptyFunction} />;
+    const node = renderTextarea(stub, container);
 
     expect(node.value).toBe('giraffe');
 
@@ -125,26 +125,26 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should render defaultValue for SSR', () => {
-    var markup = ReactDOMServer.renderToString(<textarea defaultValue="1" />);
-    var div = document.createElement('div');
+    const markup = ReactDOMServer.renderToString(<textarea defaultValue="1" />);
+    const div = document.createElement('div');
     div.innerHTML = markup;
     expect(div.firstChild.innerHTML).toBe('1');
     expect(div.firstChild.getAttribute('defaultValue')).toBe(null);
   });
 
   it('should render value for SSR', () => {
-    var element = <textarea value="1" onChange={function() {}} />;
-    var markup = ReactDOMServer.renderToString(element);
-    var div = document.createElement('div');
+    const element = <textarea value="1" onChange={function() {}} />;
+    const markup = ReactDOMServer.renderToString(element);
+    const div = document.createElement('div');
     div.innerHTML = markup;
     expect(div.firstChild.innerHTML).toBe('1');
     expect(div.firstChild.getAttribute('defaultValue')).toBe(null);
   });
 
   it('should allow setting `value` to `true`', () => {
-    var container = document.createElement('div');
-    var stub = <textarea value="giraffe" onChange={emptyFunction} />;
-    var node = renderTextarea(stub, container);
+    const container = document.createElement('div');
+    let stub = <textarea value="giraffe" onChange={emptyFunction} />;
+    const node = renderTextarea(stub, container);
 
     expect(node.value).toBe('giraffe');
 
@@ -156,9 +156,9 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should allow setting `value` to `false`', () => {
-    var container = document.createElement('div');
-    var stub = <textarea value="giraffe" onChange={emptyFunction} />;
-    var node = renderTextarea(stub, container);
+    const container = document.createElement('div');
+    let stub = <textarea value="giraffe" onChange={emptyFunction} />;
+    const node = renderTextarea(stub, container);
 
     expect(node.value).toBe('giraffe');
 
@@ -170,13 +170,13 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should allow setting `value` to `objToString`', () => {
-    var container = document.createElement('div');
-    var stub = <textarea value="giraffe" onChange={emptyFunction} />;
-    var node = renderTextarea(stub, container);
+    const container = document.createElement('div');
+    let stub = <textarea value="giraffe" onChange={emptyFunction} />;
+    const node = renderTextarea(stub, container);
 
     expect(node.value).toBe('giraffe');
 
-    var objToString = {
+    const objToString = {
       toString: function() {
         return 'foo';
       },
@@ -189,9 +189,9 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should take updates to `defaultValue` for uncontrolled textarea', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var node = ReactDOM.render(<textarea defaultValue="0" />, container);
+    const node = ReactDOM.render(<textarea defaultValue="0" />, container);
 
     expect(node.value).toBe('0');
 
@@ -201,9 +201,9 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should take updates to children in lieu of `defaultValue` for uncontrolled textarea', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var node = ReactDOM.render(<textarea defaultValue="0" />, container);
+    const node = ReactDOM.render(<textarea defaultValue="0" />, container);
 
     expect(node.value).toBe('0');
 
@@ -215,12 +215,12 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should not incur unnecessary DOM mutations', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<textarea value="a" onChange={emptyFunction} />, container);
 
-    var node = container.firstChild;
-    var nodeValue = 'a';
-    var nodeValueSetter = jest.genMockFn();
+    const node = container.firstChild;
+    let nodeValue = 'a';
+    const nodeValueSetter = jest.genMockFn();
     Object.defineProperty(node, 'value', {
       get: function() {
         return nodeValue;
@@ -238,8 +238,8 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should properly control a value of number `0`', () => {
-    var stub = <textarea value={0} onChange={emptyFunction} />;
-    var node = renderTextarea(stub);
+    const stub = <textarea value={0} onChange={emptyFunction} />;
+    const node = renderTextarea(stub);
 
     node.value = 'giraffe';
     ReactTestUtils.Simulate.change(node);
@@ -249,9 +249,9 @@ describe('ReactDOMTextarea', () => {
   it('should treat children like `defaultValue`', () => {
     spyOnDev(console, 'error');
 
-    var container = document.createElement('div');
-    var stub = <textarea>giraffe</textarea>;
-    var node = renderTextarea(stub, container);
+    const container = document.createElement('div');
+    let stub = <textarea>giraffe</textarea>;
+    const node = renderTextarea(stub, container);
 
     if (__DEV__) {
       expect(console.error.calls.count()).toBe(1);
@@ -264,9 +264,9 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should keep value when switching to uncontrolled element if not changed', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var node = renderTextarea(
+    const node = renderTextarea(
       <textarea value="kitten" onChange={emptyFunction} />,
       container,
     );
@@ -279,9 +279,9 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should keep value when switching to uncontrolled element if changed', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var node = renderTextarea(
+    const node = renderTextarea(
       <textarea value="kitten" onChange={emptyFunction} />,
       container,
     );
@@ -302,7 +302,7 @@ describe('ReactDOMTextarea', () => {
 
   it('should allow numbers as children', () => {
     spyOnDev(console, 'error');
-    var node = renderTextarea(<textarea>{17}</textarea>);
+    const node = renderTextarea(<textarea>{17}</textarea>);
     if (__DEV__) {
       expect(console.error.calls.count()).toBe(1);
     }
@@ -311,7 +311,7 @@ describe('ReactDOMTextarea', () => {
 
   it('should allow booleans as children', () => {
     spyOnDev(console, 'error');
-    var node = renderTextarea(<textarea>{false}</textarea>);
+    const node = renderTextarea(<textarea>{false}</textarea>);
     if (__DEV__) {
       expect(console.error.calls.count()).toBe(1);
     }
@@ -320,12 +320,12 @@ describe('ReactDOMTextarea', () => {
 
   it('should allow objects as children', () => {
     spyOnDev(console, 'error');
-    var obj = {
+    const obj = {
       toString: function() {
         return 'sharkswithlasers';
       },
     };
-    var node = renderTextarea(<textarea>{obj}</textarea>);
+    const node = renderTextarea(<textarea>{obj}</textarea>);
     if (__DEV__) {
       expect(console.error.calls.count()).toBe(1);
     }
@@ -348,7 +348,7 @@ describe('ReactDOMTextarea', () => {
       expect(console.error.calls.count()).toBe(1);
     }
 
-    var node;
+    let node;
     expect(function() {
       node = renderTextarea(
         <textarea>
@@ -365,7 +365,7 @@ describe('ReactDOMTextarea', () => {
   });
 
   it('should unmount', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     renderTextarea(<textarea />, container);
     ReactDOM.unmountComponentAtNode(container);
   });
diff --git a/packages/react-dom/src/__tests__/ReactEmptyComponent-test.js b/packages/react-dom/src/__tests__/ReactEmptyComponent-test.js
index e8a61a6833f5f..be849a8e7f699 100644
--- a/packages/react-dom/src/__tests__/ReactEmptyComponent-test.js
+++ b/packages/react-dom/src/__tests__/ReactEmptyComponent-test.js
@@ -9,12 +9,12 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
-var ReactTestUtils;
-var TogglingComponent;
+let React;
+let ReactDOM;
+let ReactTestUtils;
+let TogglingComponent;
 
-var log;
+let log;
 
 describe('ReactEmptyComponent', () => {
   beforeEach(() => {
@@ -39,7 +39,7 @@ describe('ReactEmptyComponent', () => {
       }
 
       render() {
-        var Component = this.state.component;
+        const Component = this.state.component;
         return Component ? <Component /> : null;
       }
     };
@@ -58,11 +58,11 @@ describe('ReactEmptyComponent', () => {
       }
     }
 
-    var container1 = document.createElement('div');
+    const container1 = document.createElement('div');
     ReactDOM.render(<Component1 />, container1);
     expect(container1.children.length).toBe(0);
 
-    var container2 = document.createElement('div');
+    const container2 = document.createElement('div');
     ReactDOM.render(<Component2 />, container2);
     expect(container2.children.length).toBe(0);
   });
@@ -81,10 +81,10 @@ describe('ReactEmptyComponent', () => {
   });
 
   it('should be able to switch between rendering null and a normal tag', () => {
-    var instance1 = (
+    const instance1 = (
       <TogglingComponent firstComponent={null} secondComponent={'div'} />
     );
-    var instance2 = (
+    const instance2 = (
       <TogglingComponent firstComponent={'div'} secondComponent={null} />
     );
 
@@ -99,7 +99,7 @@ describe('ReactEmptyComponent', () => {
   });
 
   it('should be able to switch in a list of children', () => {
-    var instance1 = (
+    const instance1 = (
       <TogglingComponent firstComponent={null} secondComponent={'div'} />
     );
 
@@ -121,10 +121,10 @@ describe('ReactEmptyComponent', () => {
   });
 
   it('should distinguish between a script placeholder and an actual script tag', () => {
-    var instance1 = (
+    const instance1 = (
       <TogglingComponent firstComponent={null} secondComponent={'script'} />
     );
-    var instance2 = (
+    const instance2 = (
       <TogglingComponent firstComponent={'script'} secondComponent={null} />
     );
 
@@ -158,10 +158,10 @@ describe('ReactEmptyComponent', () => {
         }
       }
 
-      var instance1 = (
+      const instance1 = (
         <TogglingComponent firstComponent={'div'} secondComponent={Child} />
       );
-      var instance2 = (
+      const instance2 = (
         <TogglingComponent firstComponent={Child} secondComponent={'div'} />
       );
 
@@ -181,7 +181,7 @@ describe('ReactEmptyComponent', () => {
   );
 
   it('works when switching components', () => {
-    var assertions = 0;
+    let assertions = 0;
 
     class Inner extends React.Component {
       render() {
@@ -209,8 +209,8 @@ describe('ReactEmptyComponent', () => {
       }
     }
 
-    var el = document.createElement('div');
-    var component;
+    const el = document.createElement('div');
+    let component;
 
     // Render the <Inner /> component...
     component = ReactDOM.render(<Wrapper showInner={true} />, el);
@@ -228,7 +228,7 @@ describe('ReactEmptyComponent', () => {
   });
 
   it('can render null at the top level', () => {
-    var div = document.createElement('div');
+    const div = document.createElement('div');
     ReactDOM.render(null, div);
     expect(div.innerHTML).toBe('');
   });
@@ -278,15 +278,15 @@ describe('ReactEmptyComponent', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
     ReactDOM.render(<Empty />, container);
-    var noscript1 = container.firstChild;
+    const noscript1 = container.firstChild;
     expect(noscript1).toBe(null);
 
     // This update shouldn't create a DOM node
     ReactDOM.render(<Empty />, container);
-    var noscript2 = container.firstChild;
+    const noscript2 = container.firstChild;
     expect(noscript2).toBe(null);
   });
 });
diff --git a/packages/react-dom/src/__tests__/ReactErrorBoundaries-test.js b/packages/react-dom/src/__tests__/ReactErrorBoundaries-test.js
index a3ef26a8374ea..04cfc5525e6a1 100644
--- a/packages/react-dom/src/__tests__/ReactErrorBoundaries-test.js
+++ b/packages/react-dom/src/__tests__/ReactErrorBoundaries-test.js
@@ -9,29 +9,29 @@
 
 'use strict';
 
-var PropTypes;
-var React;
-var ReactDOM;
+let PropTypes;
+let React;
+let ReactDOM;
 
 describe('ReactErrorBoundaries', () => {
-  var log;
-
-  var BrokenConstructor;
-  var BrokenComponentWillMount;
-  var BrokenComponentDidMount;
-  var BrokenComponentWillReceiveProps;
-  var BrokenComponentWillUpdate;
-  var BrokenComponentDidUpdate;
-  var BrokenComponentWillUnmount;
-  var BrokenRenderErrorBoundary;
-  var BrokenComponentWillMountErrorBoundary;
-  var BrokenComponentDidMountErrorBoundary;
-  var BrokenRender;
-  var ErrorBoundary;
-  var ErrorMessage;
-  var NoopErrorBoundary;
-  var RetryErrorBoundary;
-  var Normal;
+  let log;
+
+  let BrokenConstructor;
+  let BrokenComponentWillMount;
+  let BrokenComponentDidMount;
+  let BrokenComponentWillReceiveProps;
+  let BrokenComponentWillUpdate;
+  let BrokenComponentDidUpdate;
+  let BrokenComponentWillUnmount;
+  let BrokenRenderErrorBoundary;
+  let BrokenComponentWillMountErrorBoundary;
+  let BrokenComponentDidMountErrorBoundary;
+  let BrokenRender;
+  let ErrorBoundary;
+  let ErrorMessage;
+  let NoopErrorBoundary;
+  let RetryErrorBoundary;
+  let Normal;
 
   beforeEach(() => {
     PropTypes = require('prop-types');
@@ -528,7 +528,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('does not swallow exceptions on mounting without boundaries', () => {
-    var container = document.createElement('div');
+    let container = document.createElement('div');
     expect(() => {
       ReactDOM.render(<BrokenRender />, container);
     }).toThrow('Hello');
@@ -545,7 +545,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('does not swallow exceptions on updating without boundaries', () => {
-    var container = document.createElement('div');
+    let container = document.createElement('div');
     ReactDOM.render(<BrokenComponentWillUpdate />, container);
     expect(() => {
       ReactDOM.render(<BrokenComponentWillUpdate />, container);
@@ -565,7 +565,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('does not swallow exceptions on unmounting without boundaries', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<BrokenComponentWillUnmount />, container);
     expect(() => {
       ReactDOM.unmountComponentAtNode(container);
@@ -573,9 +573,9 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('prevents errors from leaking into other roots', () => {
-    var container1 = document.createElement('div');
-    var container2 = document.createElement('div');
-    var container3 = document.createElement('div');
+    const container1 = document.createElement('div');
+    const container2 = document.createElement('div');
+    const container3 = document.createElement('div');
 
     ReactDOM.render(<span>Before 1</span>, container1);
     expect(() => {
@@ -610,7 +610,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('renders an error state if child throws in render', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenRender />
@@ -640,7 +640,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('renders an error state if child throws in constructor', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenConstructor />
@@ -668,7 +668,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('renders an error state if child throws in componentWillMount', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenComponentWillMount />
@@ -708,7 +708,7 @@ describe('ReactErrorBoundaries', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenComponentWillMountWithContext />
@@ -736,7 +736,7 @@ describe('ReactErrorBoundaries', () => {
       foo: PropTypes.number,
     };
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenComponentWillMountWithContext />
@@ -751,7 +751,7 @@ describe('ReactErrorBoundaries', () => {
       return <ErrorMessage message={error.message} />;
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary renderError={renderError}>
         <BrokenRender />
@@ -785,7 +785,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('propagates errors on retry on mounting', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <RetryErrorBoundary>
@@ -829,7 +829,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('propagates errors inside boundary during componentWillMount', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenComponentWillMountErrorBoundary />
@@ -857,7 +857,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('propagates errors inside boundary while rendering error state', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenRenderErrorBoundary>
@@ -900,7 +900,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('does not call componentWillUnmount when aborting initial mount', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <Normal />
@@ -945,7 +945,7 @@ describe('ReactErrorBoundaries', () => {
       log.push('Error message ref is set to ' + x);
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary errorMessageRef={errorMessageRef}>
         <div ref={childRef} />
@@ -982,7 +982,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('successfully mounts if no error occurs', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <div>Mounted successfully.</div>
@@ -1003,7 +1003,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('catches if child throws in constructor during update', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <Normal />
@@ -1051,7 +1051,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('catches if child throws in componentWillMount during update', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <Normal />
@@ -1100,7 +1100,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('catches if child throws in componentWillReceiveProps during update', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <Normal />
@@ -1144,7 +1144,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('catches if child throws in componentWillUpdate during update', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <Normal />
@@ -1189,7 +1189,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('catches if child throws in render during update', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <Normal />
@@ -1248,7 +1248,7 @@ describe('ReactErrorBoundaries', () => {
       log.push('Error message ref is set to ' + x);
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary errorMessageRef={errorMessageRef}>
         <div ref={child1Ref} />
@@ -1302,7 +1302,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('recovers from componentWillUnmount errors on update', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenComponentWillUnmount />
@@ -1354,7 +1354,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('recovers from nested componentWillUnmount errors on update', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <Normal>
@@ -1418,7 +1418,7 @@ describe('ReactErrorBoundaries', () => {
       return <div>Caught an outer error: {error.message}.</div>;
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary
         logName="OuterErrorBoundary"
@@ -1480,7 +1480,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('can recover from error state', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenRender />
@@ -1528,7 +1528,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('can update multiple times in error state', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenRender />
@@ -1552,7 +1552,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it("doesn't get into inconsistent state during removals", () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <Normal />
@@ -1571,7 +1571,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it("doesn't get into inconsistent state during additions", () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<ErrorBoundary />, container);
     ReactDOM.render(
       <ErrorBoundary>
@@ -1590,17 +1590,17 @@ describe('ReactErrorBoundaries', () => {
 
   it("doesn't get into inconsistent state during reorders", () => {
     function getAMixOfNormalAndBrokenRenderElements() {
-      var elements = [];
-      for (var i = 0; i < 100; i++) {
+      const elements = [];
+      for (let i = 0; i < 100; i++) {
         elements.push(<Normal key={i} />);
       }
       elements.push(<MaybeBrokenRender key={100} />);
 
-      var currentIndex = elements.length;
+      let currentIndex = elements.length;
       while (0 !== currentIndex) {
-        var randomIndex = Math.floor(Math.random() * currentIndex);
+        const randomIndex = Math.floor(Math.random() * currentIndex);
         currentIndex -= 1;
-        var temporaryValue = elements[currentIndex];
+        const temporaryValue = elements[currentIndex];
         elements[currentIndex] = elements[randomIndex];
         elements[randomIndex] = temporaryValue;
       }
@@ -1616,8 +1616,8 @@ describe('ReactErrorBoundaries', () => {
       }
     }
 
-    var fail = false;
-    var container = document.createElement('div');
+    let fail = false;
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>{getAMixOfNormalAndBrokenRenderElements()}</ErrorBoundary>,
       container,
@@ -1637,7 +1637,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('catches errors originating downstream', () => {
-    var fail = false;
+    let fail = false;
     class Stateful extends React.Component {
       state = {shouldThrow: false};
 
@@ -1650,8 +1650,8 @@ describe('ReactErrorBoundaries', () => {
       }
     }
 
-    var statefulInst;
-    var container = document.createElement('div');
+    let statefulInst;
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <Stateful ref={inst => (statefulInst = inst)} />
@@ -1679,7 +1679,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('catches errors in componentDidMount', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenComponentWillUnmount>
@@ -1734,7 +1734,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('catches errors in componentDidUpdate', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenComponentDidUpdate />
@@ -1773,7 +1773,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('propagates errors inside boundary during componentDidMount', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary>
         <BrokenComponentDidMountErrorBoundary
@@ -1817,7 +1817,7 @@ describe('ReactErrorBoundaries', () => {
       return <div>Caught an updating error: {error.message}.</div>;
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <ErrorBoundary logName="OuterErrorBoundary">
         <ErrorBoundary
@@ -1934,7 +1934,7 @@ describe('ReactErrorBoundaries', () => {
   });
 
   it('renders empty output if error boundary does not handle the error', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <div>
         Sibling
@@ -1985,7 +1985,7 @@ describe('ReactErrorBoundaries', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     try {
       // Here, we test the behavior where there is no error boundary and we
       // delegate to the host root.
diff --git a/packages/react-dom/src/__tests__/ReactEventIndependence-test.js b/packages/react-dom/src/__tests__/ReactEventIndependence-test.js
index 72f1c1773bcca..5133c3d8181b1 100644
--- a/packages/react-dom/src/__tests__/ReactEventIndependence-test.js
+++ b/packages/react-dom/src/__tests__/ReactEventIndependence-test.js
@@ -9,9 +9,9 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
-var ReactTestUtils;
+let React;
+let ReactDOM;
+let ReactTestUtils;
 
 describe('ReactEventIndependence', () => {
   beforeEach(() => {
@@ -23,8 +23,8 @@ describe('ReactEventIndependence', () => {
   });
 
   it('does not crash with other react inside', () => {
-    var clicks = 0;
-    var div = ReactTestUtils.renderIntoDocument(
+    let clicks = 0;
+    const div = ReactTestUtils.renderIntoDocument(
       <div
         onClick={() => clicks++}
         dangerouslySetInnerHTML={{
@@ -37,10 +37,10 @@ describe('ReactEventIndependence', () => {
   });
 
   it('does not crash with other react outside', () => {
-    var clicks = 0;
-    var outer = document.createElement('div');
+    let clicks = 0;
+    const outer = document.createElement('div');
     outer.setAttribute('data-reactid', '.z');
-    var inner = ReactDOM.render(
+    const inner = ReactDOM.render(
       <button onClick={() => clicks++}>click me</button>,
       outer,
     );
@@ -49,9 +49,9 @@ describe('ReactEventIndependence', () => {
   });
 
   it('does not when event fired on unmounted tree', () => {
-    var clicks = 0;
-    var container = document.createElement('div');
-    var button = ReactDOM.render(
+    let clicks = 0;
+    const container = document.createElement('div');
+    const button = ReactDOM.render(
       <button onClick={() => clicks++}>click me</button>,
       container,
     );
diff --git a/packages/react-dom/src/__tests__/ReactIdentity-test.js b/packages/react-dom/src/__tests__/ReactIdentity-test.js
index f5e118f192233..7849d7d3b7523 100644
--- a/packages/react-dom/src/__tests__/ReactIdentity-test.js
+++ b/packages/react-dom/src/__tests__/ReactIdentity-test.js
@@ -9,9 +9,9 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
-var ReactTestUtils;
+let React;
+let ReactDOM;
+let ReactTestUtils;
 
 describe('ReactIdentity', () => {
   beforeEach(() => {
@@ -22,19 +22,19 @@ describe('ReactIdentity', () => {
   });
 
   it('should allow key property to express identity', () => {
-    var node;
-    var Component = props => (
+    let node;
+    const Component = props => (
       <div ref={c => (node = c)}>
         <div key={props.swap ? 'banana' : 'apple'} />
         <div key={props.swap ? 'apple' : 'banana'} />
       </div>
     );
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<Component />, container);
-    var origChildren = Array.from(node.childNodes);
+    const origChildren = Array.from(node.childNodes);
     ReactDOM.render(<Component swap={true} />, container);
-    var newChildren = Array.from(node.childNodes);
+    const newChildren = Array.from(node.childNodes);
     expect(origChildren[0]).toBe(newChildren[1]);
     expect(origChildren[1]).toBe(newChildren[0]);
   });
@@ -46,9 +46,9 @@ describe('ReactIdentity', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var node1;
-    var node2;
+    const container = document.createElement('div');
+    let node1;
+    let node2;
     ReactDOM.render(
       <Wrapper key="wrap1">
         <span ref={c => (node1 = c)} />
@@ -76,20 +76,20 @@ describe('ReactIdentity', () => {
       }
     }
 
-    var instance = ReactDOM.render(<Wrapper />, container);
-    var span = instance.refs.span;
+    const instance = ReactDOM.render(<Wrapper />, container);
+    const span = instance.refs.span;
     expect(ReactDOM.findDOMNode(span)).not.toBe(null);
   }
 
   it('should allow any character as a key, in a detached parent', () => {
-    var detachedContainer = document.createElement('div');
+    const detachedContainer = document.createElement('div');
     renderAComponentWithKeyIntoContainer("<'WEIRD/&\\key'>", detachedContainer);
   });
 
   it('should allow any character as a key, in an attached parent', () => {
     // This test exists to protect against implementation details that
     // incorrectly query escaped IDs using DOM tools like getElementById.
-    var attachedContainer = document.createElement('div');
+    const attachedContainer = document.createElement('div');
     document.body.appendChild(attachedContainer);
 
     renderAComponentWithKeyIntoContainer("<'WEIRD/&\\key'>", attachedContainer);
@@ -98,10 +98,10 @@ describe('ReactIdentity', () => {
   });
 
   it('should not allow scripts in keys to execute', () => {
-    var h4x0rKey =
+    const h4x0rKey =
       '"><script>window[\'YOUVEBEENH4X0RED\']=true;</script><div id="';
 
-    var attachedContainer = document.createElement('div');
+    const attachedContainer = document.createElement('div');
     document.body.appendChild(attachedContainer);
 
     renderAComponentWithKeyIntoContainer(h4x0rKey, attachedContainer);
@@ -113,9 +113,9 @@ describe('ReactIdentity', () => {
   });
 
   it('should let restructured components retain their uniqueness', () => {
-    var instance0 = <span />;
-    var instance1 = <span />;
-    var instance2 = <span />;
+    const instance0 = <span />;
+    const instance1 = <span />;
+    const instance2 = <span />;
 
     class TestComponent extends React.Component {
       render() {
@@ -146,9 +146,9 @@ describe('ReactIdentity', () => {
   });
 
   it('should let nested restructures retain their uniqueness', () => {
-    var instance0 = <span />;
-    var instance1 = <span />;
-    var instance2 = <span />;
+    const instance0 = <span />;
+    const instance1 = <span />;
+    const instance2 = <span />;
 
     class TestComponent extends React.Component {
       render() {
@@ -232,26 +232,26 @@ describe('ReactIdentity', () => {
       }
     }
 
-    var instance0 = <span key="A" />;
-    var instance1 = <span key="B" />;
+    const instance0 = <span key="A" />;
+    const instance1 = <span key="B" />;
 
-    var wrapped = <TestContainer first={instance0} second={instance1} />;
+    let wrapped = <TestContainer first={instance0} second={instance1} />;
 
     wrapped = ReactDOM.render(wrapped, document.createElement('div'));
-    var div = ReactDOM.findDOMNode(wrapped);
+    const div = ReactDOM.findDOMNode(wrapped);
 
-    var beforeA = div.childNodes[0];
-    var beforeB = div.childNodes[1];
+    const beforeA = div.childNodes[0];
+    const beforeB = div.childNodes[1];
     wrapped.swap();
-    var afterA = div.childNodes[1];
-    var afterB = div.childNodes[0];
+    const afterA = div.childNodes[1];
+    const afterB = div.childNodes[0];
 
     expect(beforeA).toBe(afterA);
     expect(beforeB).toBe(afterB);
   });
 
   it('should not allow implicit and explicit keys to collide', () => {
-    var component = (
+    const component = (
       <div>
         <span />
         <span key="0" />
diff --git a/packages/react-dom/src/__tests__/ReactMockedComponent-test.js b/packages/react-dom/src/__tests__/ReactMockedComponent-test.js
index 608a664edf034..ff5ee23210c7d 100644
--- a/packages/react-dom/src/__tests__/ReactMockedComponent-test.js
+++ b/packages/react-dom/src/__tests__/ReactMockedComponent-test.js
@@ -9,11 +9,11 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
+let React;
+let ReactDOM;
 
-var MockedComponent;
-var ReactDOMServer;
+let MockedComponent;
+let ReactDOMServer;
 
 describe('ReactMockedComponent', () => {
   beforeEach(() => {
@@ -31,7 +31,7 @@ describe('ReactMockedComponent', () => {
   });
 
   it('should allow a mocked component to be rendered in dev', () => {
-    var container = document.createElement('container');
+    const container = document.createElement('container');
     if (__DEV__) {
       ReactDOM.render(<MockedComponent />, container);
     } else {
@@ -42,7 +42,7 @@ describe('ReactMockedComponent', () => {
   });
 
   it('should allow a mocked component to be updated in dev', () => {
-    var container = document.createElement('container');
+    const container = document.createElement('container');
     if (__DEV__) {
       ReactDOM.render(<MockedComponent />, container);
     } else {
diff --git a/packages/react-dom/src/__tests__/ReactMount-test.js b/packages/react-dom/src/__tests__/ReactMount-test.js
index aeef5fd76d467..5baf0a736cc59 100644
--- a/packages/react-dom/src/__tests__/ReactMount-test.js
+++ b/packages/react-dom/src/__tests__/ReactMount-test.js
@@ -11,10 +11,10 @@
 
 const {COMMENT_NODE} = require('../shared/HTMLNodeType');
 
-var React;
-var ReactDOM;
-var ReactDOMServer;
-var ReactTestUtils;
+let React;
+let ReactDOM;
+let ReactDOMServer;
+let ReactTestUtils;
 
 describe('ReactMount', () => {
   beforeEach(() => {
@@ -28,7 +28,7 @@ describe('ReactMount', () => {
 
   describe('unmountComponentAtNode', () => {
     it('throws when given a non-node', () => {
-      var nodeArray = document.getElementsByTagName('div');
+      const nodeArray = document.getElementsByTagName('div');
       expect(function() {
         ReactDOM.unmountComponentAtNode(nodeArray);
       }).toThrowError(
@@ -37,14 +37,14 @@ describe('ReactMount', () => {
     });
 
     it('returns false on non-React containers', () => {
-      var d = document.createElement('div');
+      const d = document.createElement('div');
       d.innerHTML = '<b>hellooo</b>';
       expect(ReactDOM.unmountComponentAtNode(d)).toBe(false);
       expect(d.textContent).toBe('hellooo');
     });
 
     it('returns true on React containers', () => {
-      var d = document.createElement('div');
+      const d = document.createElement('div');
       ReactDOM.render(<b>hellooo</b>, d);
       expect(d.textContent).toBe('hellooo');
       expect(ReactDOM.unmountComponentAtNode(d)).toBe(true);
@@ -72,7 +72,7 @@ describe('ReactMount', () => {
   });
 
   it('should render different components in same root', () => {
-    var container = document.createElement('container');
+    const container = document.createElement('container');
     document.body.appendChild(container);
 
     ReactDOM.render(<div />, container);
@@ -83,10 +83,10 @@ describe('ReactMount', () => {
   });
 
   it('should unmount and remount if the key changes', () => {
-    var container = document.createElement('container');
+    const container = document.createElement('container');
 
-    var mockMount = jest.fn();
-    var mockUnmount = jest.fn();
+    const mockMount = jest.fn();
+    const mockUnmount = jest.fn();
 
     class Component extends React.Component {
       componentDidMount = mockMount;
@@ -118,15 +118,15 @@ describe('ReactMount', () => {
   });
 
   it('should reuse markup if rendering to the same target twice', () => {
-    var container = document.createElement('container');
-    var instance1 = ReactDOM.render(<div />, container);
-    var instance2 = ReactDOM.render(<div />, container);
+    const container = document.createElement('container');
+    const instance1 = ReactDOM.render(<div />, container);
+    const instance2 = ReactDOM.render(<div />, container);
 
     expect(instance1 === instance2).toBe(true);
   });
 
   it('should warn if mounting into left padded rendered markup', () => {
-    var container = document.createElement('container');
+    const container = document.createElement('container');
     container.innerHTML = ReactDOMServer.renderToString(<div />) + ' ';
 
     spyOnDev(console, 'error');
@@ -140,7 +140,7 @@ describe('ReactMount', () => {
   });
 
   it('should warn if mounting into right padded rendered markup', () => {
-    var container = document.createElement('container');
+    const container = document.createElement('container');
     container.innerHTML = ' ' + ReactDOMServer.renderToString(<div />);
 
     spyOnDev(console, 'error');
@@ -154,7 +154,7 @@ describe('ReactMount', () => {
   });
 
   it('should not warn if mounting into non-empty node', () => {
-    var container = document.createElement('container');
+    const container = document.createElement('container');
     container.innerHTML = '<div></div>';
 
     spyOnDev(console, 'error');
@@ -165,7 +165,7 @@ describe('ReactMount', () => {
   });
 
   it('should warn when mounting into document.body', () => {
-    var iFrame = document.createElement('iframe');
+    const iFrame = document.createElement('iframe');
     document.body.appendChild(iFrame);
     spyOnDev(console, 'error');
 
@@ -180,8 +180,8 @@ describe('ReactMount', () => {
   });
 
   it('should account for escaping on a checksum mismatch', () => {
-    var div = document.createElement('div');
-    var markup = ReactDOMServer.renderToString(
+    const div = document.createElement('div');
+    const markup = ReactDOMServer.renderToString(
       <div>This markup contains an nbsp entity: &nbsp; server text</div>,
     );
     div.innerHTML = markup;
@@ -201,7 +201,7 @@ describe('ReactMount', () => {
   });
 
   it('should warn if render removes React-rendered children', () => {
-    var container = document.createElement('container');
+    const container = document.createElement('container');
 
     class Component extends React.Component {
       render() {
@@ -217,7 +217,7 @@ describe('ReactMount', () => {
 
     // Test that blasting away children throws a warning
     spyOnDev(console, 'error');
-    var rootNode = container.firstChild;
+    const rootNode = container.firstChild;
     ReactDOM.render(<span />, rootNode);
     if (__DEV__) {
       expect(console.error.calls.count()).toBe(1);
@@ -232,8 +232,8 @@ describe('ReactMount', () => {
 
   it('should warn if the unmounted node was rendered by another copy of React', () => {
     jest.resetModules();
-    var ReactDOMOther = require('react-dom');
-    var container = document.createElement('div');
+    const ReactDOMOther = require('react-dom');
+    const container = document.createElement('div');
 
     class Component extends React.Component {
       render() {
@@ -267,8 +267,8 @@ describe('ReactMount', () => {
   });
 
   it('passes the correct callback context', () => {
-    var container = document.createElement('div');
-    var calls = 0;
+    const container = document.createElement('div');
+    let calls = 0;
 
     ReactDOM.render(<div />, container, function() {
       expect(this.nodeName).toBe('DIV');
@@ -307,8 +307,8 @@ describe('ReactMount', () => {
   });
 
   it('initial mount is sync inside batchedUpdates, but task work is deferred until the end of the batch', () => {
-    var container1 = document.createElement('div');
-    var container2 = document.createElement('div');
+    const container1 = document.createElement('div');
+    const container2 = document.createElement('div');
 
     class Foo extends React.Component {
       state = {active: false};
diff --git a/packages/react-dom/src/__tests__/ReactMountDestruction-test.js b/packages/react-dom/src/__tests__/ReactMountDestruction-test.js
index 4a43846a3c591..6b204d7e7191b 100644
--- a/packages/react-dom/src/__tests__/ReactMountDestruction-test.js
+++ b/packages/react-dom/src/__tests__/ReactMountDestruction-test.js
@@ -9,21 +9,21 @@
 
 'use strict';
 
-var React = require('react');
-var ReactDOM = require('react-dom');
+const React = require('react');
+const ReactDOM = require('react-dom');
 
 describe('ReactMount', () => {
   it('should destroy a react root upon request', () => {
-    var mainContainerDiv = document.createElement('div');
+    const mainContainerDiv = document.createElement('div');
     document.body.appendChild(mainContainerDiv);
 
-    var instanceOne = <div className="firstReactDiv" />;
-    var firstRootDiv = document.createElement('div');
+    const instanceOne = <div className="firstReactDiv" />;
+    const firstRootDiv = document.createElement('div');
     mainContainerDiv.appendChild(firstRootDiv);
     ReactDOM.render(instanceOne, firstRootDiv);
 
-    var instanceTwo = <div className="secondReactDiv" />;
-    var secondRootDiv = document.createElement('div');
+    const instanceTwo = <div className="secondReactDiv" />;
+    const secondRootDiv = document.createElement('div');
     mainContainerDiv.appendChild(secondRootDiv);
     ReactDOM.render(instanceTwo, secondRootDiv);
 
@@ -39,9 +39,9 @@ describe('ReactMount', () => {
   });
 
   it('should warn when unmounting a non-container root node', () => {
-    var mainContainerDiv = document.createElement('div');
+    const mainContainerDiv = document.createElement('div');
 
-    var component = (
+    const component = (
       <div>
         <div />
       </div>
@@ -49,7 +49,7 @@ describe('ReactMount', () => {
     ReactDOM.render(component, mainContainerDiv);
 
     // Test that unmounting at a root node gives a helpful warning
-    var rootDiv = mainContainerDiv.firstChild;
+    const rootDiv = mainContainerDiv.firstChild;
     spyOnDev(console, 'error');
     ReactDOM.unmountComponentAtNode(rootDiv);
     if (__DEV__) {
@@ -64,9 +64,9 @@ describe('ReactMount', () => {
   });
 
   it('should warn when unmounting a non-container, non-root node', () => {
-    var mainContainerDiv = document.createElement('div');
+    const mainContainerDiv = document.createElement('div');
 
-    var component = (
+    const component = (
       <div>
         <div>
           <div />
@@ -76,7 +76,7 @@ describe('ReactMount', () => {
     ReactDOM.render(component, mainContainerDiv);
 
     // Test that unmounting at a non-root node gives a different warning
-    var nonRootDiv = mainContainerDiv.firstChild.firstChild;
+    const nonRootDiv = mainContainerDiv.firstChild.firstChild;
     spyOnDev(console, 'error');
     ReactDOM.unmountComponentAtNode(nonRootDiv);
     if (__DEV__) {
diff --git a/packages/react-dom/src/__tests__/ReactMultiChild-test.js b/packages/react-dom/src/__tests__/ReactMultiChild-test.js
index e86599f4ccdb2..f34eda9ffccb4 100644
--- a/packages/react-dom/src/__tests__/ReactMultiChild-test.js
+++ b/packages/react-dom/src/__tests__/ReactMultiChild-test.js
@@ -14,8 +14,8 @@ describe('ReactMultiChild', () => {
     return str && str.replace(/\(at .+?:\d+\)/g, '(at **)');
   }
 
-  var React;
-  var ReactDOM;
+  let React;
+  let ReactDOM;
 
   beforeEach(() => {
     jest.resetModules();
@@ -25,11 +25,11 @@ describe('ReactMultiChild', () => {
 
   describe('reconciliation', () => {
     it('should update children when possible', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
-      var mockMount = jest.fn();
-      var mockUpdate = jest.fn();
-      var mockUnmount = jest.fn();
+      const mockMount = jest.fn();
+      const mockUpdate = jest.fn();
+      const mockUnmount = jest.fn();
 
       class MockComponent extends React.Component {
         componentDidMount = mockMount;
@@ -68,10 +68,10 @@ describe('ReactMultiChild', () => {
     });
 
     it('should replace children with different constructors', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
-      var mockMount = jest.fn();
-      var mockUnmount = jest.fn();
+      const mockMount = jest.fn();
+      const mockUnmount = jest.fn();
 
       class MockComponent extends React.Component {
         componentDidMount = mockMount;
@@ -106,10 +106,10 @@ describe('ReactMultiChild', () => {
     });
 
     it('should NOT replace children with different owners', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
-      var mockMount = jest.fn();
-      var mockUnmount = jest.fn();
+      const mockMount = jest.fn();
+      const mockUnmount = jest.fn();
 
       class MockComponent extends React.Component {
         componentDidMount = mockMount;
@@ -145,10 +145,10 @@ describe('ReactMultiChild', () => {
     });
 
     it('should replace children with different keys', () => {
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
-      var mockMount = jest.fn();
-      var mockUnmount = jest.fn();
+      const mockMount = jest.fn();
+      const mockUnmount = jest.fn();
 
       class MockComponent extends React.Component {
         componentDidMount = mockMount;
@@ -185,7 +185,7 @@ describe('ReactMultiChild', () => {
     it('should warn for duplicated array keys with component stack info', () => {
       spyOnDev(console, 'error');
 
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
       class WrapperComponent extends React.Component {
         render() {
@@ -231,7 +231,7 @@ describe('ReactMultiChild', () => {
     it('should warn for duplicated iterable keys with component stack info', () => {
       spyOnDev(console, 'error');
 
-      var container = document.createElement('div');
+      const container = document.createElement('div');
 
       class WrapperComponent extends React.Component {
         render() {
@@ -252,7 +252,7 @@ describe('ReactMultiChild', () => {
       function createIterable(array) {
         return {
           '@@iterator': function() {
-            var i = 0;
+            let i = 0;
             return {
               next() {
                 const next = {
@@ -303,7 +303,7 @@ describe('ReactMultiChild', () => {
         return <div>{new Map([['foo', 0], ['bar', 1]])}</div>;
       }
     }
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<Parent />, container);
     if (__DEV__) {
       expect(console.error.calls.count()).toBe(1);
@@ -342,7 +342,7 @@ describe('ReactMultiChild', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
     // Two random strings -- some additions, some removals, some moves
     ReactDOM.render(<Letters letters="XKwHomsNjIkBcQWFbiZU" />, container);
@@ -352,7 +352,7 @@ describe('ReactMultiChild', () => {
   });
 
   it('prepares new children before unmounting old', () => {
-    var log = [];
+    const log = [];
 
     class Spy extends React.Component {
       componentWillMount() {
@@ -372,10 +372,10 @@ describe('ReactMultiChild', () => {
 
     // These are reference-unequal so they will be swapped even if they have
     // matching keys
-    var SpyA = props => <Spy {...props} />;
-    var SpyB = props => <Spy {...props} />;
+    const SpyA = props => <Spy {...props} />;
+    const SpyB = props => <Spy {...props} />;
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(
       <div>
         <SpyA key="one" name="oneA" />
diff --git a/packages/react-dom/src/__tests__/ReactMultiChildReconcile-test.js b/packages/react-dom/src/__tests__/ReactMultiChildReconcile-test.js
index 18242db86888e..4b74b3087e2cf 100644
--- a/packages/react-dom/src/__tests__/ReactMultiChildReconcile-test.js
+++ b/packages/react-dom/src/__tests__/ReactMultiChildReconcile-test.js
@@ -9,12 +9,12 @@
 
 'use strict';
 
-var React = require('react');
-var ReactDOM = require('react-dom');
+const React = require('react');
+const ReactDOM = require('react-dom');
 
-var stripEmptyValues = function(obj) {
-  var ret = {};
-  for (var name in obj) {
+const stripEmptyValues = function(obj) {
+  const ret = {};
+  for (const name in obj) {
     if (!obj.hasOwnProperty(name)) {
       continue;
     }
@@ -25,7 +25,7 @@ var stripEmptyValues = function(obj) {
   return ret;
 };
 
-var idCounter = 123;
+let idCounter = 123;
 
 /**
  * Contains internal static internal state in order to test that updates to
@@ -66,8 +66,8 @@ class FriendsStatusDisplay extends React.Component {
    * `this._renderedChildren` (surprisingly).
    */
   getOriginalKeys() {
-    var originalKeys = [];
-    for (var key in this.props.usernameToStatus) {
+    const originalKeys = [];
+    for (const key in this.props.usernameToStatus) {
       if (this.props.usernameToStatus[key]) {
         originalKeys.push(key);
       }
@@ -80,10 +80,10 @@ class FriendsStatusDisplay extends React.Component {
    * `this.props.usernameToStatus`.
    */
   getStatusDisplays() {
-    var res = {};
-    var originalKeys = this.getOriginalKeys();
-    for (var i = 0; i < originalKeys.length; i++) {
-      var key = originalKeys[i];
+    const res = {};
+    const originalKeys = this.getOriginalKeys();
+    for (let i = 0; i < originalKeys.length; i++) {
+      const key = originalKeys[i];
       res[key] = this.refs[key];
     }
     return res;
@@ -97,9 +97,9 @@ class FriendsStatusDisplay extends React.Component {
    * it to be more declarative before making ref resolution indeterministic.
    */
   verifyPreviousRefsResolved(flushedKey) {
-    var originalKeys = this.getOriginalKeys();
-    for (var i = 0; i < originalKeys.length; i++) {
-      var key = originalKeys[i];
+    const originalKeys = this.getOriginalKeys();
+    for (let i = 0; i < originalKeys.length; i++) {
+      const key = originalKeys[i];
       if (key === flushedKey) {
         // We are only interested in children up to the current key.
         return;
@@ -109,9 +109,9 @@ class FriendsStatusDisplay extends React.Component {
   }
 
   render() {
-    var children = [];
-    for (var key in this.props.usernameToStatus) {
-      var status = this.props.usernameToStatus[key];
+    const children = [];
+    for (const key in this.props.usernameToStatus) {
+      const status = this.props.usernameToStatus[key];
       children.push(
         !status ? null : (
           <StatusDisplay
@@ -124,7 +124,7 @@ class FriendsStatusDisplay extends React.Component {
         ),
       );
     }
-    var childrenToRender = this.props.prepareChildren(children);
+    const childrenToRender = this.props.prepareChildren(children);
     return <div>{childrenToRender}</div>;
   }
 }
@@ -142,9 +142,9 @@ function getInternalStateByUserName(statusDisplays) {
  * message as well as the order of them.
  */
 function verifyStatuses(statusDisplays, props) {
-  var nonEmptyStatusDisplays = stripEmptyValues(statusDisplays);
-  var nonEmptyStatusProps = stripEmptyValues(props.usernameToStatus);
-  var username;
+  const nonEmptyStatusDisplays = stripEmptyValues(statusDisplays);
+  const nonEmptyStatusProps = stripEmptyValues(props.usernameToStatus);
+  let username;
   expect(Object.keys(nonEmptyStatusDisplays).length).toEqual(
     Object.keys(nonEmptyStatusProps).length,
   );
@@ -179,7 +179,7 @@ function verifyStatuses(statusDisplays, props) {
  * movements.
  */
 function verifyStatesPreserved(lastInternalStates, statusDisplays) {
-  var key;
+  let key;
   for (key in statusDisplays) {
     if (!statusDisplays.hasOwnProperty(key)) {
       continue;
@@ -197,37 +197,37 @@ function verifyStatesPreserved(lastInternalStates, statusDisplays) {
  * accurately reflects what is in the DOM.
  */
 function verifyDomOrderingAccurate(outerContainer, statusDisplays) {
-  var containerNode = outerContainer.firstChild;
-  var statusDisplayNodes = containerNode.childNodes;
-  var orderedDomKeys = [];
-  for (var i = 0; i < statusDisplayNodes.length; i++) {
-    var contentKey = statusDisplayNodes[i].textContent;
+  const containerNode = outerContainer.firstChild;
+  const statusDisplayNodes = containerNode.childNodes;
+  const orderedDomKeys = [];
+  for (let i = 0; i < statusDisplayNodes.length; i++) {
+    const contentKey = statusDisplayNodes[i].textContent;
     orderedDomKeys.push(contentKey);
   }
 
-  var orderedLogicalKeys = [];
-  var username;
+  const orderedLogicalKeys = [];
+  let username;
   for (username in statusDisplays) {
     if (!statusDisplays.hasOwnProperty(username)) {
       continue;
     }
-    var statusDisplay = statusDisplays[username];
+    const statusDisplay = statusDisplays[username];
     orderedLogicalKeys.push(statusDisplay.props.contentKey);
   }
   expect(orderedDomKeys).toEqual(orderedLogicalKeys);
 }
 
 function testPropsSequenceWithPreparedChildren(sequence, prepareChildren) {
-  var container = document.createElement('div');
-  var parentInstance = ReactDOM.render(
+  const container = document.createElement('div');
+  const parentInstance = ReactDOM.render(
     <FriendsStatusDisplay {...sequence[0]} prepareChildren={prepareChildren} />,
     container,
   );
-  var statusDisplays = parentInstance.getStatusDisplays();
-  var lastInternalStates = getInternalStateByUserName(statusDisplays);
+  let statusDisplays = parentInstance.getStatusDisplays();
+  let lastInternalStates = getInternalStateByUserName(statusDisplays);
   verifyStatuses(statusDisplays, sequence[0]);
 
-  for (var i = 1; i < sequence.length; i++) {
+  for (let i = 1; i < sequence.length; i++) {
     ReactDOM.render(
       <FriendsStatusDisplay
         {...sequence[i]}
@@ -270,22 +270,22 @@ describe('ReactMultiChildReconcile', () => {
 
   it('should reset internal state if removed then readded in an array', () => {
     // Test basics.
-    var props = {
+    const props = {
       usernameToStatus: {
         jcw: 'jcwStatus',
       },
     };
 
-    var container = document.createElement('div');
-    var parentInstance = ReactDOM.render(
+    const container = document.createElement('div');
+    const parentInstance = ReactDOM.render(
       <FriendsStatusDisplay
         {...props}
         prepareChildren={prepareChildrenArray}
       />,
       container,
     );
-    var statusDisplays = parentInstance.getStatusDisplays();
-    var startingInternalState = statusDisplays.jcw.getInternalState();
+    let statusDisplays = parentInstance.getStatusDisplays();
+    const startingInternalState = statusDisplays.jcw.getInternalState();
 
     // Now remove the child.
     ReactDOM.render(
@@ -312,22 +312,22 @@ describe('ReactMultiChildReconcile', () => {
 
   it('should reset internal state if removed then readded in an iterable', () => {
     // Test basics.
-    var props = {
+    const props = {
       usernameToStatus: {
         jcw: 'jcwStatus',
       },
     };
 
-    var container = document.createElement('div');
-    var parentInstance = ReactDOM.render(
+    const container = document.createElement('div');
+    const parentInstance = ReactDOM.render(
       <FriendsStatusDisplay
         {...props}
         prepareChildren={prepareChildrenIterable}
       />,
       container,
     );
-    var statusDisplays = parentInstance.getStatusDisplays();
-    var startingInternalState = statusDisplays.jcw.getInternalState();
+    let statusDisplays = parentInstance.getStatusDisplays();
+    const startingInternalState = statusDisplays.jcw.getInternalState();
 
     // Now remove the child.
     ReactDOM.render(
@@ -354,7 +354,7 @@ describe('ReactMultiChildReconcile', () => {
 
   it('should create unique identity', () => {
     // Test basics.
-    var usernameToStatus = {
+    const usernameToStatus = {
       jcw: 'jcwStatus',
       awalke: 'awalkeStatus',
       bob: 'bobStatus',
@@ -364,7 +364,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should preserve order if children order has not changed', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -382,7 +382,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should transition from zero to one children correctly', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {usernameToStatus: {}},
       {
         usernameToStatus: {
@@ -394,7 +394,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should transition from one to zero children correctly', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           first: 'firstStatus',
@@ -450,7 +450,7 @@ describe('ReactMultiChildReconcile', () => {
    * of `FriendsStatusDisplay`, nothing related to React or these test cases.
    */
   it('should remove nulled out children at the beginning', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -468,7 +468,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should remove nulled out children at the end', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -486,7 +486,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should reverse the order of two children', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           userOne: 'userOneStatus',
@@ -504,7 +504,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should reverse the order of more than two children', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           userOne: 'userOneStatus',
@@ -524,7 +524,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should cycle order correctly', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           userOne: 'userOneStatus',
@@ -571,7 +571,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should cycle order correctly in the other direction', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           userOne: 'userOneStatus',
@@ -618,7 +618,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should remove nulled out children and ignore new null children', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -637,7 +637,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should remove nulled out children and reorder remaining', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -658,7 +658,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should append children to the end', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -677,7 +677,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should append multiple children to the end', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -697,7 +697,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should prepend children to the beginning', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -716,7 +716,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should prepend multiple children to the beginning', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -736,7 +736,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should not prepend an empty child to the beginning', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -755,7 +755,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should not append an empty child to the end', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -774,7 +774,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should not insert empty children in the middle', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -795,7 +795,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should insert one new child in the middle', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -814,7 +814,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should insert multiple new truthy children in the middle', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
@@ -835,7 +835,7 @@ describe('ReactMultiChildReconcile', () => {
   });
 
   it('should insert non-empty children in middle where nulls were', () => {
-    var PROPS_SEQUENCE = [
+    const PROPS_SEQUENCE = [
       {
         usernameToStatus: {
           jcw: 'jcwStatus',
diff --git a/packages/react-dom/src/__tests__/ReactMultiChildText-test.js b/packages/react-dom/src/__tests__/ReactMultiChildText-test.js
index 18bedccbd4f67..1900cda4e4992 100644
--- a/packages/react-dom/src/__tests__/ReactMultiChildText-test.js
+++ b/packages/react-dom/src/__tests__/ReactMultiChildText-test.js
@@ -9,21 +9,21 @@
 
 'use strict';
 
-var React = require('react');
-var ReactDOM = require('react-dom');
-var ReactTestUtils = require('react-dom/test-utils');
+const React = require('react');
+const ReactDOM = require('react-dom');
+const ReactTestUtils = require('react-dom/test-utils');
 
 // Helpers
-var testAllPermutations = function(testCases) {
-  for (var i = 0; i < testCases.length; i += 2) {
-    var renderWithChildren = testCases[i];
-    var expectedResultAfterRender = testCases[i + 1];
+const testAllPermutations = function(testCases) {
+  for (let i = 0; i < testCases.length; i += 2) {
+    const renderWithChildren = testCases[i];
+    const expectedResultAfterRender = testCases[i + 1];
 
-    for (var j = 0; j < testCases.length; j += 2) {
-      var updateWithChildren = testCases[j];
-      var expectedResultAfterUpdate = testCases[j + 1];
+    for (let j = 0; j < testCases.length; j += 2) {
+      const updateWithChildren = testCases[j];
+      const expectedResultAfterUpdate = testCases[j + 1];
 
-      var container = document.createElement('div');
+      const container = document.createElement('div');
       ReactDOM.render(<div>{renderWithChildren}</div>, container);
       expectChildren(container, expectedResultAfterRender);
 
@@ -33,9 +33,9 @@ var testAllPermutations = function(testCases) {
   }
 };
 
-var expectChildren = function(container, children) {
-  var outerNode = container.firstChild;
-  var textNode;
+const expectChildren = function(container, children) {
+  const outerNode = container.firstChild;
+  let textNode;
   if (typeof children === 'string') {
     textNode = outerNode.firstChild;
 
@@ -47,10 +47,10 @@ var expectChildren = function(container, children) {
       expect(textNode.data).toBe('' + children);
     }
   } else {
-    var mountIndex = 0;
+    let mountIndex = 0;
 
-    for (var i = 0; i < children.length; i++) {
-      var child = children[i];
+    for (let i = 0; i < children.length; i++) {
+      const child = children[i];
 
       if (typeof child === 'string') {
         textNode = outerNode.childNodes[mountIndex];
@@ -58,7 +58,7 @@ var expectChildren = function(container, children) {
         expect(textNode.data).toBe('' + child);
         mountIndex++;
       } else {
-        var elementDOMNode = outerNode.childNodes[mountIndex];
+        const elementDOMNode = outerNode.childNodes[mountIndex];
         expect(elementDOMNode.tagName).toBe('DIV');
         mountIndex++;
       }
diff --git a/packages/react-dom/src/__tests__/ReactRenderDocument-test.js b/packages/react-dom/src/__tests__/ReactRenderDocument-test.js
index 0f0c62df7eb35..3ff4c0d7b2894 100644
--- a/packages/react-dom/src/__tests__/ReactRenderDocument-test.js
+++ b/packages/react-dom/src/__tests__/ReactRenderDocument-test.js
@@ -9,12 +9,12 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
-var ReactDOMServer;
+let React;
+let ReactDOM;
+let ReactDOMServer;
 
 function getTestDocument(markup) {
-  var doc = document.implementation.createHTMLDocument('');
+  const doc = document.implementation.createHTMLDocument('');
   doc.open();
   doc.write(
     markup ||
@@ -60,9 +60,9 @@ describe('rendering React components at document', () => {
         }
       }
 
-      var markup = ReactDOMServer.renderToString(<Root hello="world" />);
-      var testDocument = getTestDocument(markup);
-      var body = testDocument.body;
+      const markup = ReactDOMServer.renderToString(<Root hello="world" />);
+      const testDocument = getTestDocument(markup);
+      const body = testDocument.body;
 
       ReactDOM.render(<Root hello="world" />, testDocument);
       expect(testDocument.body.innerHTML).toBe('Hello world');
@@ -89,8 +89,8 @@ describe('rendering React components at document', () => {
         }
       }
 
-      var markup = ReactDOMServer.renderToString(<Root />);
-      var testDocument = getTestDocument(markup);
+      const markup = ReactDOMServer.renderToString(<Root />);
+      const testDocument = getTestDocument(markup);
       ReactDOM.render(<Root />, testDocument);
       expect(testDocument.body.innerHTML).toBe('Hello world');
 
@@ -129,8 +129,8 @@ describe('rendering React components at document', () => {
         }
       }
 
-      var markup = ReactDOMServer.renderToString(<Component />);
-      var testDocument = getTestDocument(markup);
+      const markup = ReactDOMServer.renderToString(<Component />);
+      const testDocument = getTestDocument(markup);
 
       ReactDOM.render(<Component />, testDocument);
       expect(testDocument.body.innerHTML).toBe('Hello world');
@@ -157,10 +157,10 @@ describe('rendering React components at document', () => {
         }
       }
 
-      var markup = ReactDOMServer.renderToString(
+      const markup = ReactDOMServer.renderToString(
         <Component text="Hello world" />,
       );
-      var testDocument = getTestDocument(markup);
+      const testDocument = getTestDocument(markup);
 
       ReactDOM.render(<Component text="Hello world" />, testDocument);
 
@@ -192,10 +192,10 @@ describe('rendering React components at document', () => {
         }
       }
 
-      var markup = ReactDOMServer.renderToString(
+      const markup = ReactDOMServer.renderToString(
         <Component text="Goodbye world" />,
       );
-      var testDocument = getTestDocument(markup);
+      const testDocument = getTestDocument(markup);
 
       spyOnDev(console, 'warn');
       spyOnDev(console, 'error');
@@ -216,7 +216,7 @@ describe('rendering React components at document', () => {
     });
 
     it('should throw on full document render w/ no markup', () => {
-      var testDocument = getTestDocument();
+      const testDocument = getTestDocument();
 
       class Component extends React.Component {
         render() {
@@ -240,7 +240,7 @@ describe('rendering React components at document', () => {
 
     it('supports findDOMNode on full-page components', () => {
       spyOnDev(console, 'warn');
-      var tree = (
+      const tree = (
         <html>
           <head>
             <title>Hello World</title>
@@ -249,9 +249,9 @@ describe('rendering React components at document', () => {
         </html>
       );
 
-      var markup = ReactDOMServer.renderToString(tree);
-      var testDocument = getTestDocument(markup);
-      var component = ReactDOM.render(tree, testDocument);
+      const markup = ReactDOMServer.renderToString(tree);
+      const testDocument = getTestDocument(markup);
+      const component = ReactDOM.render(tree, testDocument);
       expect(testDocument.body.innerHTML).toBe('Hello world');
       expect(ReactDOM.findDOMNode(component).tagName).toBe('HTML');
       expectDeprecationWarningWithFiber();
@@ -273,9 +273,9 @@ describe('rendering React components at document', () => {
         }
       }
 
-      var markup = ReactDOMServer.renderToString(<Root hello="world" />);
-      var testDocument = getTestDocument(markup);
-      var body = testDocument.body;
+      const markup = ReactDOMServer.renderToString(<Root hello="world" />);
+      const testDocument = getTestDocument(markup);
+      const body = testDocument.body;
 
       ReactDOM.hydrate(<Root hello="world" />, testDocument);
       expect(testDocument.body.innerHTML).toBe('Hello world');
@@ -300,8 +300,8 @@ describe('rendering React components at document', () => {
         }
       }
 
-      var markup = ReactDOMServer.renderToString(<Root />);
-      var testDocument = getTestDocument(markup);
+      const markup = ReactDOMServer.renderToString(<Root />);
+      const testDocument = getTestDocument(markup);
       ReactDOM.hydrate(<Root />, testDocument);
       expect(testDocument.body.innerHTML).toBe('Hello world');
 
@@ -337,8 +337,8 @@ describe('rendering React components at document', () => {
         }
       }
 
-      var markup = ReactDOMServer.renderToString(<Component />);
-      var testDocument = getTestDocument(markup);
+      const markup = ReactDOMServer.renderToString(<Component />);
+      const testDocument = getTestDocument(markup);
 
       ReactDOM.hydrate(<Component />, testDocument);
 
@@ -364,10 +364,10 @@ describe('rendering React components at document', () => {
         }
       }
 
-      var markup = ReactDOMServer.renderToString(
+      const markup = ReactDOMServer.renderToString(
         <Component text="Hello world" />,
       );
-      var testDocument = getTestDocument(markup);
+      const testDocument = getTestDocument(markup);
 
       ReactDOM.hydrate(<Component text="Hello world" />, testDocument);
 
@@ -402,10 +402,10 @@ describe('rendering React components at document', () => {
         }
       }
 
-      var markup = ReactDOMServer.renderToString(
+      const markup = ReactDOMServer.renderToString(
         <Component text="Goodbye world" />,
       );
-      var testDocument = getTestDocument(markup);
+      const testDocument = getTestDocument(markup);
 
       spyOnDev(console, 'error');
       ReactDOM.hydrate(<Component text="Hello world" />, testDocument);
@@ -420,7 +420,7 @@ describe('rendering React components at document', () => {
 
     it('should render w/ no markup to full document', () => {
       spyOnDev(console, 'error');
-      var testDocument = getTestDocument();
+      const testDocument = getTestDocument();
 
       class Component extends React.Component {
         render() {
@@ -447,7 +447,7 @@ describe('rendering React components at document', () => {
     });
 
     it('supports findDOMNode on full-page components', () => {
-      var tree = (
+      const tree = (
         <html>
           <head>
             <title>Hello World</title>
@@ -456,9 +456,9 @@ describe('rendering React components at document', () => {
         </html>
       );
 
-      var markup = ReactDOMServer.renderToString(tree);
-      var testDocument = getTestDocument(markup);
-      var component = ReactDOM.hydrate(tree, testDocument);
+      const markup = ReactDOMServer.renderToString(tree);
+      const testDocument = getTestDocument(markup);
+      const component = ReactDOM.hydrate(tree, testDocument);
       expect(testDocument.body.innerHTML).toBe('Hello world');
       expect(ReactDOM.findDOMNode(component).tagName).toBe('HTML');
     });
diff --git a/packages/react-dom/src/__tests__/ReactServerRendering-test.js b/packages/react-dom/src/__tests__/ReactServerRendering-test.js
index 9bb4c6129444e..a65fea1c8b110 100644
--- a/packages/react-dom/src/__tests__/ReactServerRendering-test.js
+++ b/packages/react-dom/src/__tests__/ReactServerRendering-test.js
@@ -9,12 +9,12 @@
 
 'use strict';
 
-var React;
-var ReactCallReturn;
-var ReactDOM;
-var ReactDOMServer;
-var ReactTestUtils;
-var PropTypes;
+let React;
+let ReactCallReturn;
+let ReactDOM;
+let ReactDOMServer;
+let ReactTestUtils;
+let PropTypes;
 
 function normalizeCodeLocInfo(str) {
   return str && str.replace(/\(at .+?:\d+\)/g, '(at **)');
@@ -33,14 +33,14 @@ describe('ReactDOMServer', () => {
 
   describe('renderToString', () => {
     it('should generate simple markup', () => {
-      var response = ReactDOMServer.renderToString(<span>hello world</span>);
+      const response = ReactDOMServer.renderToString(<span>hello world</span>);
       expect(response).toMatch(
         new RegExp('<span data-reactroot=""' + '>hello world</span>'),
       );
     });
 
     it('should generate simple markup for self-closing tags', () => {
-      var response = ReactDOMServer.renderToString(<img />);
+      const response = ReactDOMServer.renderToString(<img />);
       expect(response).toMatch(new RegExp('<img data-reactroot=""' + '/>'));
     });
 
@@ -51,7 +51,7 @@ describe('ReactDOMServer', () => {
         }
       }
 
-      var response = ReactDOMServer.renderToString(<NullComponent />);
+      const response = ReactDOMServer.renderToString(<NullComponent />);
       expect(response).toBe('');
     });
 
@@ -74,7 +74,7 @@ describe('ReactDOMServer', () => {
         }
       }
 
-      var response = ReactDOMServer.renderToString(<Parent />);
+      const response = ReactDOMServer.renderToString(<Parent />);
       expect(response).toMatch(
         new RegExp(
           '<div ' +
@@ -92,7 +92,7 @@ describe('ReactDOMServer', () => {
 
     it('should only execute certain lifecycle methods', () => {
       function runTest() {
-        var lifecycle = [];
+        const lifecycle = [];
 
         class TestComponent extends React.Component {
           constructor(props) {
@@ -135,7 +135,7 @@ describe('ReactDOMServer', () => {
           }
         }
 
-        var response = ReactDOMServer.renderToString(<TestComponent />);
+        const response = ReactDOMServer.renderToString(<TestComponent />);
 
         expect(response).toMatch(
           new RegExp(
@@ -161,8 +161,8 @@ describe('ReactDOMServer', () => {
       spyOnDev(console, 'warn');
       spyOnDev(console, 'error');
 
-      var mountCount = 0;
-      var numClicks = 0;
+      let mountCount = 0;
+      let numClicks = 0;
 
       class TestComponent extends React.Component {
         componentDidMount() {
@@ -182,10 +182,10 @@ describe('ReactDOMServer', () => {
         }
       }
 
-      var element = document.createElement('div');
+      const element = document.createElement('div');
       ReactDOM.render(<TestComponent />, element);
 
-      var lastMarkup = element.innerHTML;
+      let lastMarkup = element.innerHTML;
 
       // Exercise the update path. Markup should not change,
       // but some lifecycle methods should be run again.
@@ -210,7 +210,7 @@ describe('ReactDOMServer', () => {
       lastMarkup = ReactDOMServer.renderToString(<TestComponent name="x" />);
       element.innerHTML = lastMarkup;
 
-      var instance = ReactDOM.render(<TestComponent name="x" />, element);
+      let instance = ReactDOM.render(<TestComponent name="x" />, element);
       expect(mountCount).toEqual(3);
       if (__DEV__) {
         expect(console.warn.calls.count()).toBe(1);
@@ -259,8 +259,8 @@ describe('ReactDOMServer', () => {
     it('should have the correct mounting behavior (new hydrate API)', () => {
       spyOnDev(console, 'error');
 
-      var mountCount = 0;
-      var numClicks = 0;
+      let mountCount = 0;
+      let numClicks = 0;
 
       class TestComponent extends React.Component {
         componentDidMount() {
@@ -280,10 +280,10 @@ describe('ReactDOMServer', () => {
         }
       }
 
-      var element = document.createElement('div');
+      const element = document.createElement('div');
       ReactDOM.render(<TestComponent />, element);
 
-      var lastMarkup = element.innerHTML;
+      let lastMarkup = element.innerHTML;
 
       // Exercise the update path. Markup should not change,
       // but some lifecycle methods should be run again.
@@ -308,7 +308,7 @@ describe('ReactDOMServer', () => {
       lastMarkup = ReactDOMServer.renderToString(<TestComponent name="x" />);
       element.innerHTML = lastMarkup;
 
-      var instance = ReactDOM.hydrate(<TestComponent name="x" />, element);
+      let instance = ReactDOM.hydrate(<TestComponent name="x" />, element);
       expect(mountCount).toEqual(3);
       expect(element.innerHTML).toBe(lastMarkup);
 
@@ -341,7 +341,7 @@ describe('ReactDOMServer', () => {
     // want it to call focus() when hydrating because this can mess up existing
     // focus before the JS has loaded.
     it('should emit autofocus on the server but not focus() when hydrating', () => {
-      var element = document.createElement('div');
+      const element = document.createElement('div');
       element.innerHTML = ReactDOMServer.renderToString(
         <input autoFocus={true} />,
       );
@@ -358,7 +358,7 @@ describe('ReactDOMServer', () => {
     });
 
     it('should not focus on either server or client with autofocus={false}', () => {
-      var element = document.createElement('div');
+      const element = document.createElement('div');
       element.innerHTML = ReactDOMServer.renderToString(
         <input autoFocus={false} />,
       );
@@ -414,7 +414,7 @@ describe('ReactDOMServer', () => {
         }
       }
 
-      var response = ReactDOMServer.renderToStaticMarkup(<TestComponent />);
+      const response = ReactDOMServer.renderToStaticMarkup(<TestComponent />);
 
       expect(response).toBe('<span><div>inner text</div></span>');
     });
@@ -430,7 +430,7 @@ describe('ReactDOMServer', () => {
         }
       }
 
-      var response = ReactDOMServer.renderToStaticMarkup(<TestComponent />);
+      const response = ReactDOMServer.renderToStaticMarkup(<TestComponent />);
 
       expect(response).toBe('<span>hello world</span>');
     });
@@ -442,14 +442,14 @@ describe('ReactDOMServer', () => {
         }
       }
 
-      var response = ReactDOMServer.renderToStaticMarkup(<TestComponent />);
+      const response = ReactDOMServer.renderToStaticMarkup(<TestComponent />);
 
       expect(response).toBe('');
     });
 
     it('should only execute certain lifecycle methods', () => {
       function runTest() {
-        var lifecycle = [];
+        const lifecycle = [];
 
         class TestComponent extends React.Component {
           constructor(props) {
@@ -492,7 +492,7 @@ describe('ReactDOMServer', () => {
           }
         }
 
-        var response = ReactDOMServer.renderToStaticMarkup(<TestComponent />);
+        const response = ReactDOMServer.renderToStaticMarkup(<TestComponent />);
 
         expect(response).toBe('<span>Component name: TestComponent</span>');
         expect(lifecycle).toEqual([
@@ -523,7 +523,7 @@ describe('ReactDOMServer', () => {
           return <div>{this.state.text}</div>;
         }
       }
-      var markup = ReactDOMServer.renderToString(<Component />);
+      const markup = ReactDOMServer.renderToString(<Component />);
       expect(markup).toContain('hello, world');
     });
 
@@ -542,7 +542,7 @@ describe('ReactDOMServer', () => {
           return <div>{this.state.text}</div>;
         }
       }
-      var markup = ReactDOMServer.renderToString(<Component />);
+      const markup = ReactDOMServer.renderToString(<Component />);
       expect(markup).toContain('hello, world');
     });
 
@@ -557,7 +557,7 @@ describe('ReactDOMServer', () => {
         }
       }
 
-      var markup = ReactDOMServer.renderToString(
+      const markup = ReactDOMServer.renderToString(
         <Component text="hello, world" />,
       );
       expect(markup).toContain('hello, world');
@@ -594,7 +594,7 @@ describe('ReactDOMServer', () => {
         text: PropTypes.string,
       };
 
-      var markup = ReactDOMServer.renderToString(
+      const markup = ReactDOMServer.renderToString(
         <ContextProvider>
           <Component />
         </ContextProvider>,
@@ -661,7 +661,7 @@ describe('ReactDOMServer', () => {
       );
     }
 
-    var markup = ReactDOMServer.renderToStaticMarkup(<Foo />);
+    const markup = ReactDOMServer.renderToStaticMarkup(<Foo />);
     expect(markup).toBe('<div>hello</div>');
     jest.runOnlyPendingTimers();
     if (__DEV__) {
@@ -694,7 +694,7 @@ describe('ReactDOMServer', () => {
           'This is a no-op.\n\nPlease check the code for the Baz component.',
       );
     }
-    var markup = ReactDOMServer.renderToStaticMarkup(<Baz />);
+    const markup = ReactDOMServer.renderToStaticMarkup(<Baz />);
     expect(markup).toBe('<div></div>');
   });
 
@@ -776,7 +776,7 @@ describe('ReactDOMServer', () => {
   });
 
   it('should throw rendering portals on the server', () => {
-    var div = document.createElement('div');
+    const div = document.createElement('div');
     expect(() => {
       ReactDOMServer.renderToString(
         <div>{ReactDOM.createPortal(<div />, div)}</div>,
@@ -788,7 +788,7 @@ describe('ReactDOMServer', () => {
   });
 
   it('should throw rendering call/return on the server', () => {
-    var div = document.createElement('div');
+    const div = document.createElement('div');
     expect(() => {
       ReactDOMServer.renderToString(
         <div>{ReactCallReturn.unstable_createReturn(42)}</div>,
diff --git a/packages/react-dom/src/__tests__/ReactServerRenderingBrowser-test.js b/packages/react-dom/src/__tests__/ReactServerRenderingBrowser-test.js
index 3dbd4b2a8aa45..f4f54a91d283c 100644
--- a/packages/react-dom/src/__tests__/ReactServerRenderingBrowser-test.js
+++ b/packages/react-dom/src/__tests__/ReactServerRenderingBrowser-test.js
@@ -9,9 +9,9 @@
 
 'use strict';
 
-var React;
-var ReactDOMServer;
-var ReactDOMServerBrowser;
+let React;
+let ReactDOMServer;
+let ReactDOMServerBrowser;
 
 describe('ReactServerRenderingBrowser', () => {
   beforeEach(() => {
diff --git a/packages/react-dom/src/__tests__/ReactStatelessComponent-test.js b/packages/react-dom/src/__tests__/ReactStatelessComponent-test.js
index d9ec8cc6f4c57..9f4cce3e1fde8 100644
--- a/packages/react-dom/src/__tests__/ReactStatelessComponent-test.js
+++ b/packages/react-dom/src/__tests__/ReactStatelessComponent-test.js
@@ -9,10 +9,10 @@
 
 'use strict';
 
-var PropTypes;
-var React;
-var ReactDOM;
-var ReactTestUtils;
+let PropTypes;
+let React;
+let ReactDOM;
+let ReactTestUtils;
 
 function StatelessComponent(props) {
   return <div>{props.name}</div>;
@@ -32,7 +32,7 @@ describe('ReactStatelessComponent', () => {
   });
 
   it('should render stateless component', () => {
-    var el = document.createElement('div');
+    const el = document.createElement('div');
     ReactDOM.render(<StatelessComponent name="A" />, el);
 
     expect(el.textContent).toBe('A');
@@ -45,7 +45,7 @@ describe('ReactStatelessComponent', () => {
       }
     }
 
-    var el = document.createElement('div');
+    const el = document.createElement('div');
     ReactDOM.render(<Parent name="A" />, el);
     expect(el.textContent).toBe('A');
 
@@ -54,7 +54,7 @@ describe('ReactStatelessComponent', () => {
   });
 
   it('should unmount stateless component', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
     ReactDOM.render(<StatelessComponent name="A" />, container);
     expect(container.textContent).toBe('A');
@@ -92,7 +92,7 @@ describe('ReactStatelessComponent', () => {
       }
     }
 
-    var el = document.createElement('div');
+    const el = document.createElement('div');
     ReactDOM.render(<GrandParent test="test" />, el);
 
     expect(el.textContent).toBe('test');
@@ -112,7 +112,7 @@ describe('ReactStatelessComponent', () => {
       foo: PropTypes.string,
     };
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
     ReactDOM.render(<StatelessComponentWithChildContext name="A" />, container);
 
@@ -376,7 +376,7 @@ describe('ReactStatelessComponent', () => {
       return <div />;
     }
 
-    var comp = ReactTestUtils.renderIntoDocument(<Child />);
+    const comp = ReactTestUtils.renderIntoDocument(<Child />);
     expect(comp).toBe(null);
   });
 
@@ -437,13 +437,13 @@ describe('ReactStatelessComponent', () => {
     }
     Child.contextTypes = {lang: PropTypes.string};
 
-    var el = document.createElement('div');
+    const el = document.createElement('div');
     ReactDOM.render(<Parent />, el);
     expect(el.textContent).toBe('en');
   });
 
   it('should work with arrow functions', () => {
-    var Child = function() {
+    let Child = function() {
       return <div />;
     };
     // Will create a new bound function without a prototype, much like a native
@@ -454,7 +454,7 @@ describe('ReactStatelessComponent', () => {
   });
 
   it('should allow simple functions to return null', () => {
-    var Child = function() {
+    const Child = function() {
       return null;
     };
     expect(() => ReactTestUtils.renderIntoDocument(<Child />)).not.toThrow();
diff --git a/packages/react-dom/src/__tests__/ReactTestUtils-test.js b/packages/react-dom/src/__tests__/ReactTestUtils-test.js
index 7dfd063f35bc2..7fad8d2d748a3 100644
--- a/packages/react-dom/src/__tests__/ReactTestUtils-test.js
+++ b/packages/react-dom/src/__tests__/ReactTestUtils-test.js
@@ -16,7 +16,7 @@ let ReactDOMServer;
 let ReactTestUtils;
 
 function getTestDocument(markup) {
-  var doc = document.implementation.createHTMLDocument('');
+  const doc = document.implementation.createHTMLDocument('');
   doc.open();
   doc.write(
     markup ||
@@ -47,7 +47,7 @@ describe('ReactTestUtils', () => {
     // Patch it up so it returns its children.
     ReactTestUtils.mockComponent(MockedComponent);
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<MockedComponent>Hello</MockedComponent>, container);
     expect(container.textContent).toBe('Hello');
   });
diff --git a/packages/react-dom/src/__tests__/ReactTreeTraversal-test.js b/packages/react-dom/src/__tests__/ReactTreeTraversal-test.js
index 9acb9314fd192..a3d7701068657 100644
--- a/packages/react-dom/src/__tests__/ReactTreeTraversal-test.js
+++ b/packages/react-dom/src/__tests__/ReactTreeTraversal-test.js
@@ -9,8 +9,8 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
+let React;
+let ReactDOM;
 
 const ChildComponent = ({id, eventHandler}) => (
   <div
@@ -63,10 +63,10 @@ const ParentComponent = ({eventHandler}) => (
 );
 
 describe('ReactTreeTraversal', () => {
-  var mockFn = jest.fn();
-  var container;
-  var outerNode1;
-  var outerNode2;
+  const mockFn = jest.fn();
+  let container;
+  let outerNode1;
+  let outerNode2;
 
   beforeEach(() => {
     React = require('react');
@@ -103,7 +103,7 @@ describe('ReactTreeTraversal', () => {
     });
 
     it('should traverse two phase across component boundary', () => {
-      var expectedCalls = [
+      const expectedCalls = [
         ['P', 'captured', 'click'],
         ['P_P1', 'captured', 'click'],
         ['P_P1_C1__DIV', 'captured', 'click'],
@@ -115,7 +115,7 @@ describe('ReactTreeTraversal', () => {
         ['P', 'bubbled', 'click'],
       ];
 
-      var node = document.getElementById('P_P1_C1__DIV_1');
+      const node = document.getElementById('P_P1_C1__DIV_1');
       node.dispatchEvent(
         new MouseEvent('click', {bubbles: true, cancelable: true}),
       );
@@ -124,12 +124,12 @@ describe('ReactTreeTraversal', () => {
     });
 
     it('should traverse two phase at shallowest node', () => {
-      var node = document.getElementById('P');
+      const node = document.getElementById('P');
       node.dispatchEvent(
         new MouseEvent('click', {bubbles: true, cancelable: true}),
       );
 
-      var expectedCalls = [
+      const expectedCalls = [
         ['P', 'captured', 'click'],
         ['P', 'bubbled', 'click'],
       ];
@@ -151,8 +151,8 @@ describe('ReactTreeTraversal', () => {
     });
 
     it('should not traverse if enter/leave the same node', () => {
-      var leaveNode = document.getElementById('P_P1_C1__DIV_1');
-      var enterNode = document.getElementById('P_P1_C1__DIV_1');
+      const leaveNode = document.getElementById('P_P1_C1__DIV_1');
+      const enterNode = document.getElementById('P_P1_C1__DIV_1');
 
       leaveNode.dispatchEvent(
         new MouseEvent('mouseout', {
@@ -166,10 +166,10 @@ describe('ReactTreeTraversal', () => {
     });
 
     it('should traverse enter/leave to sibling - avoids parent', () => {
-      var leaveNode = document.getElementById('P_P1_C1__DIV_1');
-      var enterNode = document.getElementById('P_P1_C1__DIV_2');
+      const leaveNode = document.getElementById('P_P1_C1__DIV_1');
+      const enterNode = document.getElementById('P_P1_C1__DIV_2');
 
-      var expectedCalls = [
+      const expectedCalls = [
         ['P_P1_C1__DIV_1', 'mouseleave'],
         // enter/leave shouldn't fire anything on the parent
         ['P_P1_C1__DIV_2', 'mouseenter'],
@@ -187,10 +187,10 @@ describe('ReactTreeTraversal', () => {
     });
 
     it('should traverse enter/leave to parent - avoids parent', () => {
-      var leaveNode = document.getElementById('P_P1_C1__DIV_1');
-      var enterNode = document.getElementById('P_P1_C1__DIV');
+      const leaveNode = document.getElementById('P_P1_C1__DIV_1');
+      const enterNode = document.getElementById('P_P1_C1__DIV');
 
-      var expectedCalls = [['P_P1_C1__DIV_1', 'mouseleave']];
+      const expectedCalls = [['P_P1_C1__DIV_1', 'mouseleave']];
 
       leaveNode.dispatchEvent(
         new MouseEvent('mouseout', {
@@ -204,9 +204,9 @@ describe('ReactTreeTraversal', () => {
     });
 
     it('should enter from the window', () => {
-      var enterNode = document.getElementById('P_P1_C1__DIV');
+      const enterNode = document.getElementById('P_P1_C1__DIV');
 
-      var expectedCalls = [
+      const expectedCalls = [
         ['P', 'mouseenter'],
         ['P_P1', 'mouseenter'],
         ['P_P1_C1__DIV', 'mouseenter'],
@@ -224,9 +224,9 @@ describe('ReactTreeTraversal', () => {
     });
 
     it('should enter from the window to the shallowest', () => {
-      var enterNode = document.getElementById('P');
+      const enterNode = document.getElementById('P');
 
-      var expectedCalls = [['P', 'mouseenter']];
+      const expectedCalls = [['P', 'mouseenter']];
 
       outerNode1.dispatchEvent(
         new MouseEvent('mouseout', {
@@ -240,9 +240,9 @@ describe('ReactTreeTraversal', () => {
     });
 
     it('should leave to the window', () => {
-      var leaveNode = document.getElementById('P_P1_C1__DIV');
+      const leaveNode = document.getElementById('P_P1_C1__DIV');
 
-      var expectedCalls = [
+      const expectedCalls = [
         ['P_P1_C1__DIV', 'mouseleave'],
         ['P_P1', 'mouseleave'],
         ['P', 'mouseleave'],
@@ -260,9 +260,9 @@ describe('ReactTreeTraversal', () => {
     });
 
     it('should leave to the window from the shallowest', () => {
-      var leaveNode = document.getElementById('P');
+      const leaveNode = document.getElementById('P');
 
-      var expectedCalls = [['P', 'mouseleave']];
+      const expectedCalls = [['P', 'mouseleave']];
 
       leaveNode.dispatchEvent(
         new MouseEvent('mouseout', {
diff --git a/packages/react-dom/src/__tests__/ReactUpdates-test.js b/packages/react-dom/src/__tests__/ReactUpdates-test.js
index 8bca250ab1291..b033550641855 100644
--- a/packages/react-dom/src/__tests__/ReactUpdates-test.js
+++ b/packages/react-dom/src/__tests__/ReactUpdates-test.js
@@ -9,9 +9,9 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
-var ReactTestUtils;
+let React;
+let ReactDOM;
+let ReactTestUtils;
 
 describe('ReactUpdates', () => {
   beforeEach(() => {
@@ -21,7 +21,7 @@ describe('ReactUpdates', () => {
   });
 
   it('should batch state when updating state twice', () => {
-    var updateCount = 0;
+    let updateCount = 0;
 
     class Component extends React.Component {
       state = {x: 0};
@@ -35,7 +35,7 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var instance = ReactTestUtils.renderIntoDocument(<Component />);
+    const instance = ReactTestUtils.renderIntoDocument(<Component />);
     expect(instance.state.x).toBe(0);
 
     ReactDOM.unstable_batchedUpdates(function() {
@@ -50,7 +50,7 @@ describe('ReactUpdates', () => {
   });
 
   it('should batch state when updating two different state keys', () => {
-    var updateCount = 0;
+    let updateCount = 0;
 
     class Component extends React.Component {
       state = {x: 0, y: 0};
@@ -68,7 +68,7 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var instance = ReactTestUtils.renderIntoDocument(<Component />);
+    const instance = ReactTestUtils.renderIntoDocument(<Component />);
     expect(instance.state.x).toBe(0);
     expect(instance.state.y).toBe(0);
 
@@ -86,7 +86,7 @@ describe('ReactUpdates', () => {
   });
 
   it('should batch state and props together', () => {
-    var updateCount = 0;
+    let updateCount = 0;
 
     class Component extends React.Component {
       state = {y: 0};
@@ -104,8 +104,8 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var instance = ReactDOM.render(<Component x={0} />, container);
+    const container = document.createElement('div');
+    const instance = ReactDOM.render(<Component x={0} />, container);
     expect(instance.props.x).toBe(0);
     expect(instance.state.y).toBe(0);
 
@@ -123,7 +123,7 @@ describe('ReactUpdates', () => {
   });
 
   it('should batch parent/child state updates together', () => {
-    var parentUpdateCount = 0;
+    let parentUpdateCount = 0;
 
     class Parent extends React.Component {
       state = {x: 0};
@@ -141,7 +141,7 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var childUpdateCount = 0;
+    let childUpdateCount = 0;
 
     class Child extends React.Component {
       state = {y: 0};
@@ -155,8 +155,8 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var instance = ReactTestUtils.renderIntoDocument(<Parent />);
-    var child = instance.refs.child;
+    const instance = ReactTestUtils.renderIntoDocument(<Parent />);
+    const child = instance.refs.child;
     expect(instance.state.x).toBe(0);
     expect(child.state.y).toBe(0);
 
@@ -176,7 +176,7 @@ describe('ReactUpdates', () => {
   });
 
   it('should batch child/parent state updates together', () => {
-    var parentUpdateCount = 0;
+    let parentUpdateCount = 0;
 
     class Parent extends React.Component {
       state = {x: 0};
@@ -194,7 +194,7 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var childUpdateCount = 0;
+    let childUpdateCount = 0;
 
     class Child extends React.Component {
       state = {y: 0};
@@ -208,8 +208,8 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var instance = ReactTestUtils.renderIntoDocument(<Parent />);
-    var child = instance.refs.child;
+    const instance = ReactTestUtils.renderIntoDocument(<Parent />);
+    const child = instance.refs.child;
     expect(instance.state.x).toBe(0);
     expect(child.state.y).toBe(0);
 
@@ -231,7 +231,7 @@ describe('ReactUpdates', () => {
   });
 
   it('should support chained state updates', () => {
-    var updateCount = 0;
+    let updateCount = 0;
 
     class Component extends React.Component {
       state = {x: 0};
@@ -245,10 +245,10 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var instance = ReactTestUtils.renderIntoDocument(<Component />);
+    const instance = ReactTestUtils.renderIntoDocument(<Component />);
     expect(instance.state.x).toBe(0);
 
-    var innerCallbackRun = false;
+    let innerCallbackRun = false;
     ReactDOM.unstable_batchedUpdates(function() {
       instance.setState({x: 1}, function() {
         instance.setState({x: 2}, function() {
@@ -270,8 +270,8 @@ describe('ReactUpdates', () => {
   });
 
   it('should batch forceUpdate together', () => {
-    var shouldUpdateCount = 0;
-    var updateCount = 0;
+    let shouldUpdateCount = 0;
+    let updateCount = 0;
 
     class Component extends React.Component {
       state = {x: 0};
@@ -289,10 +289,10 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var instance = ReactTestUtils.renderIntoDocument(<Component />);
+    const instance = ReactTestUtils.renderIntoDocument(<Component />);
     expect(instance.state.x).toBe(0);
 
-    var callbacksRun = 0;
+    let callbacksRun = 0;
     ReactDOM.unstable_batchedUpdates(function() {
       instance.setState({x: 1}, function() {
         callbacksRun++;
@@ -312,8 +312,8 @@ describe('ReactUpdates', () => {
   });
 
   it('should update children even if parent blocks updates', () => {
-    var parentRenderCount = 0;
-    var childRenderCount = 0;
+    let parentRenderCount = 0;
+    let childRenderCount = 0;
 
     class Parent extends React.Component {
       shouldComponentUpdate() {
@@ -336,7 +336,7 @@ describe('ReactUpdates', () => {
     expect(parentRenderCount).toBe(0);
     expect(childRenderCount).toBe(0);
 
-    var instance = <Parent />;
+    let instance = <Parent />;
     instance = ReactTestUtils.renderIntoDocument(instance);
 
     expect(parentRenderCount).toBe(1);
@@ -358,8 +358,8 @@ describe('ReactUpdates', () => {
   });
 
   it('should not reconcile children passed via props', () => {
-    var numMiddleRenders = 0;
-    var numBottomRenders = 0;
+    let numMiddleRenders = 0;
+    let numBottomRenders = 0;
 
     class Top extends React.Component {
       render() {
@@ -395,10 +395,10 @@ describe('ReactUpdates', () => {
   });
 
   it('should flow updates correctly', () => {
-    var willUpdates = [];
-    var didUpdates = [];
+    let willUpdates = [];
+    let didUpdates = [];
 
-    var UpdateLoggingMixin = {
+    const UpdateLoggingMixin = {
       componentWillUpdate: function() {
         willUpdates.push(this.constructor.displayName);
       },
@@ -424,7 +424,7 @@ describe('ReactUpdates', () => {
     class Switcher extends React.Component {
       state = {tabKey: 'hello'};
       render() {
-        var child = this.props.children;
+        const child = this.props.children;
 
         return (
           <Box ref="box">
@@ -452,11 +452,11 @@ describe('ReactUpdates', () => {
     }
     Object.assign(App.prototype, UpdateLoggingMixin);
 
-    var root = <App />;
+    let root = <App />;
     root = ReactTestUtils.renderIntoDocument(root);
 
     function expectUpdates(desiredWillUpdates, desiredDidUpdates) {
-      var i;
+      let i;
       for (i = 0; i < desiredWillUpdates; i++) {
         expect(willUpdates).toContain(desiredWillUpdates[i]);
       }
@@ -472,7 +472,7 @@ describe('ReactUpdates', () => {
     }
 
     function testUpdates(components, desiredWillUpdates, desiredDidUpdates) {
-      var i;
+      let i;
 
       ReactDOM.unstable_batchedUpdates(function() {
         for (i = 0; i < components.length; i++) {
@@ -519,12 +519,12 @@ describe('ReactUpdates', () => {
     // componentDidUpdate handlers is called, B's DOM should already have been
     // updated.
 
-    var bContainer = document.createElement('div');
+    const bContainer = document.createElement('div');
 
-    var a;
-    var b;
+    let a;
+    let b;
 
-    var aUpdated = false;
+    let aUpdated = false;
 
     class A extends React.Component {
       state = {x: 0};
@@ -535,7 +535,7 @@ describe('ReactUpdates', () => {
       }
 
       render() {
-        var portal = null;
+        let portal = null;
         // If we're using Fiber, we use Portals instead to achieve this.
         portal = ReactDOM.createPortal(<B ref={n => (b = n)} />, bContainer);
         return (
@@ -565,7 +565,7 @@ describe('ReactUpdates', () => {
   });
 
   it('should flush updates in the correct order', () => {
-    var updates = [];
+    const updates = [];
 
     class Outer extends React.Component {
       state = {x: 0};
@@ -580,7 +580,7 @@ describe('ReactUpdates', () => {
       }
 
       componentDidUpdate() {
-        var x = this.state.x;
+        const x = this.state.x;
         updates.push('Outer-didUpdate-' + x);
         updates.push('Inner-setState-' + x);
         this.refs.inner.setState({x: x}, function() {
@@ -602,7 +602,7 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var instance = ReactTestUtils.renderIntoDocument(<Outer />);
+    const instance = ReactTestUtils.renderIntoDocument(<Outer />);
 
     updates.push('Outer-setState-1');
     instance.setState({x: 1}, function() {
@@ -646,8 +646,8 @@ describe('ReactUpdates', () => {
   });
 
   it('should flush updates in the correct order across roots', () => {
-    var instances = [];
-    var updates = [];
+    const instances = [];
+    const updates = [];
 
     class MockComponent extends React.Component {
       render() {
@@ -728,8 +728,8 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var x;
-    var y;
+    let x;
+    let y;
 
     x = ReactTestUtils.renderIntoDocument(<X />);
     y = ReactTestUtils.renderIntoDocument(<Y />);
@@ -741,7 +741,7 @@ describe('ReactUpdates', () => {
 
   it('should queue updates from during mount', () => {
     // See https://github.com/facebook/react/issues/1353
-    var a;
+    let a;
 
     class A extends React.Component {
       state = {x: 0};
@@ -779,13 +779,13 @@ describe('ReactUpdates', () => {
   });
 
   it('calls componentWillReceiveProps setState callback properly', () => {
-    var callbackCount = 0;
+    let callbackCount = 0;
 
     class A extends React.Component {
       state = {x: this.props.x};
 
       componentWillReceiveProps(nextProps) {
-        var newX = nextProps.x;
+        const newX = nextProps.x;
         this.setState({x: newX}, function() {
           // State should have updated by the time this callback gets called
           expect(this.state.x).toBe(newX);
@@ -798,15 +798,15 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<A x={1} />, container);
     ReactDOM.render(<A x={2} />, container);
     expect(callbackCount).toBe(1);
   });
 
   it('does not call render after a component as been deleted', () => {
-    var renderCount = 0;
-    var componentB = null;
+    let renderCount = 0;
+    let componentB = null;
 
     class B extends React.Component {
       state = {updates: 0};
@@ -829,7 +829,7 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var component = ReactTestUtils.renderIntoDocument(<A />);
+    const component = ReactTestUtils.renderIntoDocument(<A />);
 
     ReactDOM.unstable_batchedUpdates(function() {
       // B will have scheduled an update but the batching should ensure that its
@@ -857,7 +857,7 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var component = ReactTestUtils.renderIntoDocument(<A />);
+    let component = ReactTestUtils.renderIntoDocument(<A />);
 
     expect(() => component.setState({}, 'no')).toThrowError(
       'Invalid argument passed as callback. Expected a function. Instead ' +
@@ -910,7 +910,7 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var component = ReactTestUtils.renderIntoDocument(<A />);
+    let component = ReactTestUtils.renderIntoDocument(<A />);
 
     expect(() => component.forceUpdate('no')).toThrowError(
       'Invalid argument passed as callback. Expected a function. Instead ' +
@@ -958,9 +958,9 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var renderCount = 0;
-    var postRenderCount = 0;
-    var once = false;
+    let renderCount = 0;
+    let postRenderCount = 0;
+    let once = false;
 
     class Child extends React.Component {
       state = {updated: false};
@@ -989,8 +989,8 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var parent = ReactTestUtils.renderIntoDocument(<Parent />);
-    var child = parent.getChild();
+    const parent = ReactTestUtils.renderIntoDocument(<Parent />);
+    const child = parent.getChild();
     ReactDOM.unstable_batchedUpdates(function() {
       parent.forceUpdate();
       child.forceUpdate();
@@ -998,7 +998,7 @@ describe('ReactUpdates', () => {
   });
 
   it('does not update one component twice in a batch (#6371)', () => {
-    var callbacks = [];
+    let callbacks = [];
     function emitChange() {
       callbacks.forEach(c => c());
     }
@@ -1048,14 +1048,14 @@ describe('ReactUpdates', () => {
   });
 
   it('unstable_batchedUpdates should return value from a callback', () => {
-    var result = ReactDOM.unstable_batchedUpdates(function() {
+    const result = ReactDOM.unstable_batchedUpdates(function() {
       return 42;
     });
     expect(result).toEqual(42);
   });
 
   it('unmounts and remounts a root in the same batch', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<span>a</span>, container);
     ReactDOM.unstable_batchedUpdates(function() {
       ReactDOM.unmountComponentAtNode(container);
@@ -1065,7 +1065,7 @@ describe('ReactUpdates', () => {
   });
 
   it('handles reentrant mounting in synchronous mode', () => {
-    var mounts = 0;
+    let mounts = 0;
     class Editor extends React.Component {
       render() {
         return <div>{this.props.text}</div>;
@@ -1079,7 +1079,7 @@ describe('ReactUpdates', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     function render() {
       ReactDOM.render(
         <Editor
@@ -1093,7 +1093,7 @@ describe('ReactUpdates', () => {
       );
     }
 
-    var props = {text: 'hello', rendered: false};
+    let props = {text: 'hello', rendered: false};
     render();
     props = {...props, text: 'goodbye'};
     render();
@@ -1102,8 +1102,8 @@ describe('ReactUpdates', () => {
   });
 
   it('mounts and unmounts are sync even in a batch', () => {
-    var ops = [];
-    var container = document.createElement('div');
+    const ops = [];
+    const container = document.createElement('div');
     ReactDOM.unstable_batchedUpdates(() => {
       ReactDOM.render(<div>Hello</div>, container);
       ops.push(container.textContent);
diff --git a/packages/react-dom/src/__tests__/TapEventPlugin-test.internal.js b/packages/react-dom/src/__tests__/TapEventPlugin-test.internal.js
index ae5274d52767f..3abf048e04279 100644
--- a/packages/react-dom/src/__tests__/TapEventPlugin-test.internal.js
+++ b/packages/react-dom/src/__tests__/TapEventPlugin-test.internal.js
@@ -9,25 +9,25 @@
 
 'use strict';
 
-var EventPluginHub;
-var React;
-var ReactDOM;
-var ReactTestUtils;
-var TapEventPlugin;
-
-var tapMoveThreshold;
-var idCallOrder;
-var recordID = function(id) {
+let EventPluginHub;
+let React;
+let ReactDOM;
+let ReactTestUtils;
+let TapEventPlugin;
+
+let tapMoveThreshold;
+let idCallOrder;
+const recordID = function(id) {
   idCallOrder.push(id);
 };
-var LISTENER = jest.fn();
-var ON_TOUCH_TAP_KEY = 'onTouchTap';
+const LISTENER = jest.fn();
+const ON_TOUCH_TAP_KEY = 'onTouchTap';
 
-var GRANDPARENT;
-var PARENT;
-var CHILD;
+let GRANDPARENT;
+let PARENT;
+let CHILD;
 
-var putListener;
+let putListener;
 
 describe('TapEventPlugin', () => {
   beforeEach(() => {
@@ -40,11 +40,11 @@ describe('TapEventPlugin', () => {
     ReactTestUtils = require('react-dom/test-utils');
     TapEventPlugin = require('react-dom/src/events/TapEventPlugin').default;
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var GRANDPARENT_PROPS = {};
-    var PARENT_PROPS = {};
-    var CHILD_PROPS = {};
+    const GRANDPARENT_PROPS = {};
+    const PARENT_PROPS = {};
+    const CHILD_PROPS = {};
 
     function Child(props) {
       return <div ref={c => (CHILD = c)} {...props} />;
diff --git a/packages/react-dom/src/__tests__/escapeTextForBrowser-test.js b/packages/react-dom/src/__tests__/escapeTextForBrowser-test.js
index a76cacb7fdf65..ab9b82f95036c 100644
--- a/packages/react-dom/src/__tests__/escapeTextForBrowser-test.js
+++ b/packages/react-dom/src/__tests__/escapeTextForBrowser-test.js
@@ -9,8 +9,8 @@
 
 'use strict';
 
-var React;
-var ReactDOMServer;
+let React;
+let ReactDOMServer;
 
 describe('escapeTextForBrowser', () => {
   beforeEach(() => {
@@ -20,42 +20,42 @@ describe('escapeTextForBrowser', () => {
   });
 
   it('ampersand is escaped when passed as text content', () => {
-    var response = ReactDOMServer.renderToString(<span>{'&'}</span>);
+    const response = ReactDOMServer.renderToString(<span>{'&'}</span>);
     expect(response).toMatch('<span data-reactroot="">&amp;</span>');
   });
 
   it('double quote is escaped when passed as text content', () => {
-    var response = ReactDOMServer.renderToString(<span>{'"'}</span>);
+    const response = ReactDOMServer.renderToString(<span>{'"'}</span>);
     expect(response).toMatch('<span data-reactroot="">&quot;</span>');
   });
 
   it('single quote is escaped when passed as text content', () => {
-    var response = ReactDOMServer.renderToString(<span>{"'"}</span>);
+    const response = ReactDOMServer.renderToString(<span>{"'"}</span>);
     expect(response).toMatch('<span data-reactroot="">&#x27;</span>');
   });
 
   it('greater than entity is escaped when passed as text content', () => {
-    var response = ReactDOMServer.renderToString(<span>{'>'}</span>);
+    const response = ReactDOMServer.renderToString(<span>{'>'}</span>);
     expect(response).toMatch('<span data-reactroot="">&gt;</span>');
   });
 
   it('lower than entity is escaped when passed as text content', () => {
-    var response = ReactDOMServer.renderToString(<span>{'<'}</span>);
+    const response = ReactDOMServer.renderToString(<span>{'<'}</span>);
     expect(response).toMatch('<span data-reactroot="">&lt;</span>');
   });
 
   it('number is correctly passed as text content', () => {
-    var response = ReactDOMServer.renderToString(<span>{42}</span>);
+    const response = ReactDOMServer.renderToString(<span>{42}</span>);
     expect(response).toMatch('<span data-reactroot="">42</span>');
   });
 
   it('number is escaped to string when passed as text content', () => {
-    var response = ReactDOMServer.renderToString(<img data-attr={42} />);
+    const response = ReactDOMServer.renderToString(<img data-attr={42} />);
     expect(response).toMatch('<img data-attr="42" data-reactroot=""/>');
   });
 
   it('escape text content representing a script tag', () => {
-    var response = ReactDOMServer.renderToString(
+    const response = ReactDOMServer.renderToString(
       <span>{'<script type=\'\' src=""></script>'}</span>,
     );
     expect(response).toMatch(
diff --git a/packages/react-dom/src/__tests__/findDOMNode-test.js b/packages/react-dom/src/__tests__/findDOMNode-test.js
index 42b98838b0481..f1c015e5c5da9 100644
--- a/packages/react-dom/src/__tests__/findDOMNode-test.js
+++ b/packages/react-dom/src/__tests__/findDOMNode-test.js
@@ -9,9 +9,9 @@
 
 'use strict';
 
-var React = require('react');
-var ReactDOM = require('react-dom');
-var ReactTestUtils = require('react-dom/test-utils');
+const React = require('react');
+const ReactDOM = require('react-dom');
+const ReactTestUtils = require('react-dom/test-utils');
 
 describe('findDOMNode', () => {
   it('findDOMNode should return null if passed null', () => {
@@ -29,9 +29,9 @@ describe('findDOMNode', () => {
       }
     }
 
-    var myNode = ReactTestUtils.renderIntoDocument(<MyNode />);
-    var myDiv = ReactDOM.findDOMNode(myNode);
-    var mySameDiv = ReactDOM.findDOMNode(myDiv);
+    const myNode = ReactTestUtils.renderIntoDocument(<MyNode />);
+    const myDiv = ReactDOM.findDOMNode(myNode);
+    const mySameDiv = ReactDOM.findDOMNode(myDiv);
     expect(myDiv.tagName).toBe('DIV');
     expect(mySameDiv).toBe(myDiv);
   });
@@ -49,16 +49,16 @@ describe('findDOMNode', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
 
-    var myNodeA = ReactDOM.render(<MyNode />, container);
-    var a = ReactDOM.findDOMNode(myNodeA);
+    const myNodeA = ReactDOM.render(<MyNode />, container);
+    const a = ReactDOM.findDOMNode(myNodeA);
     expect(a).toBe(null);
 
-    var myNodeB = ReactDOM.render(<MyNode flag={true} />, container);
+    const myNodeB = ReactDOM.render(<MyNode flag={true} />, container);
     expect(myNodeA === myNodeB).toBe(true);
 
-    var b = ReactDOM.findDOMNode(myNodeB);
+    const b = ReactDOM.findDOMNode(myNodeB);
     expect(b.tagName).toBe('SPAN');
   });
 
@@ -77,8 +77,8 @@ describe('findDOMNode', () => {
       }
     }
 
-    var container = document.createElement('div');
-    var inst = ReactDOM.render(<Foo />, container);
+    const container = document.createElement('div');
+    const inst = ReactDOM.render(<Foo />, container);
     ReactDOM.unmountComponentAtNode(container);
 
     expect(() => ReactDOM.findDOMNode(inst)).toThrowError(
diff --git a/packages/react-dom/src/__tests__/multiple-copies-of-react-test.js b/packages/react-dom/src/__tests__/multiple-copies-of-react-test.js
index ee1cc2032d7d4..47bbcbbdaf20f 100644
--- a/packages/react-dom/src/__tests__/multiple-copies-of-react-test.js
+++ b/packages/react-dom/src/__tests__/multiple-copies-of-react-test.js
@@ -10,7 +10,7 @@
 'use strict';
 
 let React = require('react');
-var ReactTestUtils = require('react-dom/test-utils');
+const ReactTestUtils = require('react-dom/test-utils');
 
 class TextWithStringRef extends React.Component {
   render() {
diff --git a/packages/react-dom/src/__tests__/quoteAttributeValueForBrowser-test.js b/packages/react-dom/src/__tests__/quoteAttributeValueForBrowser-test.js
index 1a0db78037f2d..08c8257d4dcee 100644
--- a/packages/react-dom/src/__tests__/quoteAttributeValueForBrowser-test.js
+++ b/packages/react-dom/src/__tests__/quoteAttributeValueForBrowser-test.js
@@ -9,8 +9,8 @@
 
 'use strict';
 
-var React;
-var ReactDOMServer;
+let React;
+let ReactDOMServer;
 
 describe('quoteAttributeValueForBrowser', () => {
   beforeEach(() => {
@@ -20,50 +20,50 @@ describe('quoteAttributeValueForBrowser', () => {
   });
 
   it('ampersand is escaped inside attributes', () => {
-    var response = ReactDOMServer.renderToString(<img data-attr="&" />);
+    const response = ReactDOMServer.renderToString(<img data-attr="&" />);
     expect(response).toMatch('<img data-attr="&amp;" data-reactroot=""/>');
   });
 
   it('double quote is escaped inside attributes', () => {
-    var response = ReactDOMServer.renderToString(<img data-attr={'"'} />);
+    const response = ReactDOMServer.renderToString(<img data-attr={'"'} />);
     expect(response).toMatch('<img data-attr="&quot;" data-reactroot=""/>');
   });
 
   it('single quote is escaped inside attributes', () => {
-    var response = ReactDOMServer.renderToString(<img data-attr="'" />);
+    const response = ReactDOMServer.renderToString(<img data-attr="'" />);
     expect(response).toMatch('<img data-attr="&#x27;" data-reactroot=""/>');
   });
 
   it('greater than entity is escaped inside attributes', () => {
-    var response = ReactDOMServer.renderToString(<img data-attr=">" />);
+    const response = ReactDOMServer.renderToString(<img data-attr=">" />);
     expect(response).toMatch('<img data-attr="&gt;" data-reactroot=""/>');
   });
 
   it('lower than entity is escaped inside attributes', () => {
-    var response = ReactDOMServer.renderToString(<img data-attr="<" />);
+    const response = ReactDOMServer.renderToString(<img data-attr="<" />);
     expect(response).toMatch('<img data-attr="&lt;" data-reactroot=""/>');
   });
 
   it('number is escaped to string inside attributes', () => {
-    var response = ReactDOMServer.renderToString(<img data-attr={42} />);
+    const response = ReactDOMServer.renderToString(<img data-attr={42} />);
     expect(response).toMatch('<img data-attr="42" data-reactroot=""/>');
   });
 
   it('object is passed to a string inside attributes', () => {
-    var sampleObject = {
+    const sampleObject = {
       toString: function() {
         return 'ponys';
       },
     };
 
-    var response = ReactDOMServer.renderToString(
+    const response = ReactDOMServer.renderToString(
       <img data-attr={sampleObject} />,
     );
     expect(response).toMatch('<img data-attr="ponys" data-reactroot=""/>');
   });
 
   it('script tag is escaped inside attributes', () => {
-    var response = ReactDOMServer.renderToString(
+    const response = ReactDOMServer.renderToString(
       <img data-attr={'<script type=\'\' src=""></script>'} />,
     );
     expect(response).toMatch(
diff --git a/packages/react-dom/src/__tests__/refs-destruction-test.js b/packages/react-dom/src/__tests__/refs-destruction-test.js
index 3d2bd06a57ee8..bbb24c97e7026 100644
--- a/packages/react-dom/src/__tests__/refs-destruction-test.js
+++ b/packages/react-dom/src/__tests__/refs-destruction-test.js
@@ -9,11 +9,11 @@
 
 'use strict';
 
-var React;
-var ReactDOM;
-var ReactTestUtils;
+let React;
+let ReactDOM;
+let ReactTestUtils;
 
-var TestComponent;
+let TestComponent;
 
 describe('refs-destruction', () => {
   beforeEach(() => {
@@ -37,8 +37,8 @@ describe('refs-destruction', () => {
   });
 
   it('should remove refs when destroying the parent', () => {
-    var container = document.createElement('div');
-    var testInstance = ReactDOM.render(<TestComponent />, container);
+    const container = document.createElement('div');
+    const testInstance = ReactDOM.render(<TestComponent />, container);
     expect(ReactTestUtils.isDOMComponent(testInstance.refs.theInnerDiv)).toBe(
       true,
     );
@@ -54,8 +54,8 @@ describe('refs-destruction', () => {
   });
 
   it('should remove refs when destroying the child', () => {
-    var container = document.createElement('div');
-    var testInstance = ReactDOM.render(<TestComponent />, container);
+    const container = document.createElement('div');
+    const testInstance = ReactDOM.render(<TestComponent />, container);
     expect(ReactTestUtils.isDOMComponent(testInstance.refs.theInnerDiv)).toBe(
       true,
     );
@@ -83,7 +83,7 @@ describe('refs-destruction', () => {
       }
 
       componentWillUnmount() {
-        var self = this;
+        const self = this;
         // some async animation
         setTimeout(function() {
           expect(function() {
@@ -114,7 +114,7 @@ describe('refs-destruction', () => {
       }
     }
 
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     ReactDOM.render(<App />, container);
     ReactDOM.render(<App hidden={true} />, container);
     jest.runAllTimers();
diff --git a/packages/react-dom/src/__tests__/refs-test.js b/packages/react-dom/src/__tests__/refs-test.js
index 268706c686435..c9b32b1e45609 100644
--- a/packages/react-dom/src/__tests__/refs-test.js
+++ b/packages/react-dom/src/__tests__/refs-test.js
@@ -9,8 +9,8 @@
 
 'use strict';
 
-var React = require('react');
-var ReactTestUtils = require('react-dom/test-utils');
+let React = require('react');
+let ReactTestUtils = require('react-dom/test-utils');
 
 /**
  * Counts clicks and has a renders an item for each click. Each item rendered
@@ -28,8 +28,8 @@ class ClickCounter extends React.Component {
   };
 
   render() {
-    var children = [];
-    var i;
+    const children = [];
+    let i;
     for (i = 0; i < this.state.count; i++) {
       children.push(
         <div
@@ -84,23 +84,23 @@ class TestRefsComponent extends React.Component {
 /**
  * Render a TestRefsComponent and ensure that the main refs are wired up.
  */
-var renderTestRefsComponent = function() {
-  var testRefsComponent = ReactTestUtils.renderIntoDocument(
+const renderTestRefsComponent = function() {
+  const testRefsComponent = ReactTestUtils.renderIntoDocument(
     <TestRefsComponent />,
   );
   expect(testRefsComponent instanceof TestRefsComponent).toBe(true);
 
-  var generalContainer = testRefsComponent.refs.myContainer;
+  const generalContainer = testRefsComponent.refs.myContainer;
   expect(generalContainer instanceof GeneralContainerComponent).toBe(true);
 
-  var counter = testRefsComponent.refs.myCounter;
+  const counter = testRefsComponent.refs.myCounter;
   expect(counter instanceof ClickCounter).toBe(true);
 
   return testRefsComponent;
 };
 
-var expectClickLogsLengthToBe = function(instance, length) {
-  var clickLogs = ReactTestUtils.scryRenderedDOMComponentsWithClass(
+const expectClickLogsLengthToBe = function(instance, length) {
+  const clickLogs = ReactTestUtils.scryRenderedDOMComponentsWithClass(
     instance,
     'clickLogDiv',
   );
@@ -120,8 +120,8 @@ describe('reactiverefs', () => {
    * perspective of the injected ClickCounter component.
    */
   it('Should increase refs with an increase in divs', () => {
-    var testRefsComponent = renderTestRefsComponent();
-    var clickIncrementer = ReactTestUtils.findRenderedDOMComponentWithClass(
+    const testRefsComponent = renderTestRefsComponent();
+    const clickIncrementer = ReactTestUtils.findRenderedDOMComponentWithClass(
       testRefsComponent,
       'clickIncrementer',
     );
@@ -178,7 +178,7 @@ describe('ref swapping', () => {
       };
 
       render() {
-        var count = this.state.count;
+        const count = this.state.count;
         /**
          * What we have here, is three divs with refs (div1/2/3), but a single
          * moving cursor ref `hopRef` that "hops" around the three. We'll call the
@@ -206,17 +206,17 @@ describe('ref swapping', () => {
   });
 
   it('Allow refs to hop around children correctly', () => {
-    var refHopsAround = ReactTestUtils.renderIntoDocument(<RefHopsAround />);
+    const refHopsAround = ReactTestUtils.renderIntoDocument(<RefHopsAround />);
 
-    var firstDiv = ReactTestUtils.findRenderedDOMComponentWithClass(
+    const firstDiv = ReactTestUtils.findRenderedDOMComponentWithClass(
       refHopsAround,
       'first',
     );
-    var secondDiv = ReactTestUtils.findRenderedDOMComponentWithClass(
+    const secondDiv = ReactTestUtils.findRenderedDOMComponentWithClass(
       refHopsAround,
       'second',
     );
-    var thirdDiv = ReactTestUtils.findRenderedDOMComponentWithClass(
+    const thirdDiv = ReactTestUtils.findRenderedDOMComponentWithClass(
       refHopsAround,
       'third',
     );
@@ -252,12 +252,12 @@ describe('ref swapping', () => {
       }
     }
 
-    var instance = ReactTestUtils.renderIntoDocument(<Component />);
+    const instance = ReactTestUtils.renderIntoDocument(<Component />);
     expect(!!instance.refs).toBe(true);
   });
 
   it('ref called correctly for stateless component', () => {
-    var refCalled = 0;
+    let refCalled = 0;
     function Inner(props) {
       return <a ref={props.saveA} />;
     }
@@ -293,13 +293,13 @@ describe('ref swapping', () => {
 
 describe('root level refs', () => {
   it('attaches and detaches root refs', () => {
-    var ReactDOM = require('react-dom');
-    var inst = null;
+    const ReactDOM = require('react-dom');
+    let inst = null;
 
     // host node
-    var ref = jest.fn(value => (inst = value));
-    var container = document.createElement('div');
-    var result = ReactDOM.render(<div ref={ref} />, container);
+    let ref = jest.fn(value => (inst = value));
+    const container = document.createElement('div');
+    let result = ReactDOM.render(<div ref={ref} />, container);
     expect(ref).toHaveBeenCalledTimes(1);
     expect(ref.mock.calls[0][0]).toBeInstanceOf(HTMLDivElement);
     expect(result).toBe(ref.mock.calls[0][0]);
@@ -336,8 +336,8 @@ describe('root level refs', () => {
     // fragment
     inst = null;
     ref = jest.fn(value => (inst = value));
-    var divInst = null;
-    var ref2 = jest.fn(value => (divInst = value));
+    let divInst = null;
+    const ref2 = jest.fn(value => (divInst = value));
     result = ReactDOM.render(
       [
         <Comp ref={ref} key="a" />,
diff --git a/packages/react-dom/src/__tests__/renderSubtreeIntoContainer-test.js b/packages/react-dom/src/__tests__/renderSubtreeIntoContainer-test.js
index dd7cf60fe4944..6fe992f4f2611 100644
--- a/packages/react-dom/src/__tests__/renderSubtreeIntoContainer-test.js
+++ b/packages/react-dom/src/__tests__/renderSubtreeIntoContainer-test.js
@@ -9,16 +9,16 @@
 
 'use strict';
 
-var React = require('react');
-var PropTypes = require('prop-types');
-var ReactDOM = require('react-dom');
-var ReactTestUtils = require('react-dom/test-utils');
-var renderSubtreeIntoContainer = require('react-dom')
+const React = require('react');
+const PropTypes = require('prop-types');
+const ReactDOM = require('react-dom');
+const ReactTestUtils = require('react-dom/test-utils');
+const renderSubtreeIntoContainer = require('react-dom')
   .unstable_renderSubtreeIntoContainer;
 
 describe('renderSubtreeIntoContainer', () => {
   it('should pass context when rendering subtree elsewhere', () => {
-    var portal = document.createElement('div');
+    const portal = document.createElement('div');
 
     class Component extends React.Component {
       static contextTypes = {
@@ -59,7 +59,7 @@ describe('renderSubtreeIntoContainer', () => {
   });
 
   it('should throw if parentComponent is invalid', () => {
-    var portal = document.createElement('div');
+    const portal = document.createElement('div');
 
     class Component extends React.Component {
       static contextTypes = {
@@ -98,9 +98,9 @@ describe('renderSubtreeIntoContainer', () => {
   });
 
   it('should update context if it changes due to setState', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     document.body.appendChild(container);
-    var portal = document.createElement('div');
+    const portal = document.createElement('div');
 
     class Component extends React.Component {
       static contextTypes = {
@@ -143,16 +143,16 @@ describe('renderSubtreeIntoContainer', () => {
       }
     }
 
-    var instance = ReactDOM.render(<Parent />, container);
+    const instance = ReactDOM.render(<Parent />, container);
     expect(portal.firstChild.innerHTML).toBe('initial-initial');
     instance.setState({bar: 'changed'});
     expect(portal.firstChild.innerHTML).toBe('changed-changed');
   });
 
   it('should update context if it changes due to re-render', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     document.body.appendChild(container);
-    var portal = document.createElement('div');
+    const portal = document.createElement('div');
 
     class Component extends React.Component {
       static contextTypes = {
@@ -198,9 +198,9 @@ describe('renderSubtreeIntoContainer', () => {
   });
 
   it('should render portal with non-context-provider parent', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     document.body.appendChild(container);
-    var portal = document.createElement('div');
+    const portal = document.createElement('div');
 
     class Parent extends React.Component {
       render() {
@@ -217,9 +217,9 @@ describe('renderSubtreeIntoContainer', () => {
   });
 
   it('should get context through non-context-provider parent', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     document.body.appendChild(container);
-    var portal = document.createElement('div');
+    const portal = document.createElement('div');
 
     class Parent extends React.Component {
       render() {
@@ -256,10 +256,10 @@ describe('renderSubtreeIntoContainer', () => {
   });
 
   it('should get context through middle non-context-provider layer', () => {
-    var container = document.createElement('div');
+    const container = document.createElement('div');
     document.body.appendChild(container);
-    var portal1 = document.createElement('div');
-    var portal2 = document.createElement('div');
+    const portal1 = document.createElement('div');
+    const portal2 = document.createElement('div');
 
     class Parent extends React.Component {
       render() {
diff --git a/packages/react-dom/src/__tests__/utils/ReactDOMServerIntegrationTestUtils.js b/packages/react-dom/src/__tests__/utils/ReactDOMServerIntegrationTestUtils.js
index aff17a4dd76c5..04af042f088a1 100644
--- a/packages/react-dom/src/__tests__/utils/ReactDOMServerIntegrationTestUtils.js
+++ b/packages/react-dom/src/__tests__/utils/ReactDOMServerIntegrationTestUtils.js
@@ -57,7 +57,7 @@ module.exports = function(initModules) {
       );
       if (console.error.calls.count() > 0) {
         console.log(`We saw these warnings:`);
-        for (var i = 0; i < console.error.calls.count(); i++) {
+        for (let i = 0; i < console.error.calls.count(); i++) {
           console.log(console.error.calls.argsFor(i)[0]);
         }
       }
@@ -97,7 +97,7 @@ module.exports = function(initModules) {
   // Does not render on client or perform client-side revival.
   async function serverRender(reactElement, errorCount = 0) {
     const markup = await renderIntoString(reactElement, errorCount);
-    var domElement = document.createElement('div');
+    const domElement = document.createElement('div');
     domElement.innerHTML = markup;
     return domElement.firstChild;
   }
@@ -133,7 +133,7 @@ module.exports = function(initModules) {
   // Does not render on client or perform client-side revival.
   async function streamRender(reactElement, errorCount = 0) {
     const markup = await renderIntoStream(reactElement, errorCount);
-    var domElement = document.createElement('div');
+    const domElement = document.createElement('div');
     domElement.innerHTML = markup;
     return domElement.firstChild;
   }
@@ -147,7 +147,7 @@ module.exports = function(initModules) {
     const markup = await renderIntoString(element, errorCount);
     resetModules();
 
-    var domElement = document.createElement('div');
+    const domElement = document.createElement('div');
     domElement.innerHTML = markup;
     let serverNode = domElement.firstChild;
 
@@ -178,13 +178,13 @@ module.exports = function(initModules) {
 
   const clientRenderOnBadMarkup = async (element, errorCount = 0) => {
     // First we render the top of bad mark up.
-    var domElement = document.createElement('div');
+    const domElement = document.createElement('div');
     domElement.innerHTML =
       '<div id="badIdWhichWillCauseMismatch" data-reactroot="" data-reactid="1"></div>';
     await renderIntoDom(element, domElement, true, errorCount + 1);
 
     // This gives us the resulting text content.
-    var hydratedTextContent = domElement.textContent;
+    const hydratedTextContent = domElement.textContent;
 
     // Next we render the element into a clean DOM node client side.
     const cleanDomElement = document.createElement('div');
diff --git a/packages/react-dom/src/__tests__/validateDOMNesting-test.internal.js b/packages/react-dom/src/__tests__/validateDOMNesting-test.internal.js
index f1a37c953d454..d68e6ea5a8d5b 100644
--- a/packages/react-dom/src/__tests__/validateDOMNesting-test.internal.js
+++ b/packages/react-dom/src/__tests__/validateDOMNesting-test.internal.js
@@ -9,10 +9,10 @@
 
 'use strict';
 
-var validateDOMNesting;
+let validateDOMNesting;
 
 // https://html.spec.whatwg.org/multipage/syntax.html#special
-var specialTags = [
+const specialTags = [
   'address',
   'applet',
   'area',
@@ -99,7 +99,7 @@ var specialTags = [
 ];
 
 // https://html.spec.whatwg.org/multipage/syntax.html#formatting
-var formattingTags = [
+const formattingTags = [
   'a',
   'b',
   'big',
@@ -117,8 +117,8 @@ var formattingTags = [
 ];
 
 function isTagStackValid(stack) {
-  var ancestorInfo = null;
-  for (var i = 0; i < stack.length; i++) {
+  let ancestorInfo = null;
+  for (let i = 0; i < stack.length; i++) {
     if (!validateDOMNesting.isTagValidInContext(stack[i], ancestorInfo)) {
       return false;
     }
@@ -143,7 +143,7 @@ describe('validateDOMNesting', () => {
     if (__DEV__) {
       // With renderToString (for example), we don't know where we're mounting the
       // tag so we must err on the side of leniency.
-      var allTags = [].concat(specialTags, formattingTags, ['mysterytag']);
+      const allTags = [].concat(specialTags, formattingTags, ['mysterytag']);
       allTags.forEach(function(tag) {
         expect(validateDOMNesting.isTagValidInContext(tag, null)).toBe(true);
       });