diff --git a/src/renderers/shared/fiber/ReactChildFiber.js b/src/renderers/shared/fiber/ReactChildFiber.js
index ceb7dc345bb..79f80c9e408 100644
--- a/src/renderers/shared/fiber/ReactChildFiber.js
+++ b/src/renderers/shared/fiber/ReactChildFiber.js
@@ -73,9 +73,9 @@ const {
   Deletion,
 } = ReactTypeOfSideEffect;
 
-function coerceRef(current: ?Fiber, element: ReactElement) {
+function coerceRef(current: Fiber | null, element: ReactElement) {
   let mixedRef = element.ref;
-  if (mixedRef != null && typeof mixedRef !== 'function') {
+  if (mixedRef !== null && typeof mixedRef !== 'function') {
     if (element._owner) {
       const owner : ?(Fiber | ReactInstance) = (element._owner : any);
       let inst;
@@ -97,7 +97,7 @@ function coerceRef(current: ?Fiber, element: ReactElement) {
       );
       const stringRef = String(mixedRef);
       // Check if previous string ref matches new string ref
-      if (current && current.ref && current.ref._stringRef === stringRef) {
+      if (current !== null && current.ref !== null && current.ref._stringRef === stringRef) {
         return current.ref;
       }
       const ref = function(value) {
@@ -161,14 +161,14 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
       // When we're reconciling in place we have a work in progress copy. We
       // actually want the current copy. If there is no current copy, then we
       // don't need to track deletion side-effects.
-      if (!childToDelete.alternate) {
+      if (childToDelete.alternate === null) {
         return;
       }
       childToDelete = childToDelete.alternate;
     }
     // Deletions are added in reversed order so we add it to the front.
     const last = returnFiber.progressedLastDeletion;
-    if (last) {
+    if (last !== null) {
       last.nextEffect = childToDelete;
       returnFiber.progressedLastDeletion = childToDelete;
     } else {
@@ -182,7 +182,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function deleteRemainingChildren(
     returnFiber : Fiber,
-    currentFirstChild : ?Fiber
+    currentFirstChild : Fiber | null
   ) : null {
     if (!shouldTrackSideEffects) {
       // Noop.
@@ -192,7 +192,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     // TODO: For the shouldClone case, this could be micro-optimized a bit by
     // assuming that after the first child we've already added everything.
     let childToDelete = currentFirstChild;
-    while (childToDelete) {
+    while (childToDelete !== null) {
       deleteChild(returnFiber, childToDelete);
       childToDelete = childToDelete.sibling;
     }
@@ -209,7 +209,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     const existingChildren : Map<string | number, Fiber> = new Map();
 
     let existingChild = currentFirstChild;
-    while (existingChild) {
+    while (existingChild !== null) {
       if (existingChild.key !== null) {
         existingChildren.set(existingChild.key, existingChild);
       } else {
@@ -247,7 +247,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
       return lastPlacedIndex;
     }
     const current = newFiber.alternate;
-    if (current) {
+    if (current !== null) {
       const oldIndex = current.index;
       if (oldIndex < lastPlacedIndex) {
         // This is a move.
@@ -267,7 +267,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
   function placeSingleChild(newFiber : Fiber) : Fiber {
     // This is simpler for the single child case. We only need to do a
     // placement for inserting new children.
-    if (shouldTrackSideEffects && !newFiber.alternate) {
+    if (shouldTrackSideEffects && newFiber.alternate === null) {
       newFiber.effectTag = Placement;
     }
     return newFiber;
@@ -275,11 +275,11 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function updateTextNode(
     returnFiber : Fiber,
-    current : ?Fiber,
+    current : Fiber | null,
     textContent : string,
     priority : PriorityLevel
   ) {
-    if (current == null || current.tag !== HostText) {
+    if (current === null || current.tag !== HostText) {
       // Insert
       const created = createFiberFromText(textContent, priority);
       created.return = returnFiber;
@@ -295,11 +295,11 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function updateElement(
     returnFiber : Fiber,
-    current : ?Fiber,
+    current : Fiber | null,
     element : ReactElement,
     priority : PriorityLevel
   ) : Fiber {
-    if (current == null || current.type !== element.type) {
+    if (current === null || current.type !== element.type) {
       // Insert
       const created = createFiberFromElement(element, priority);
       created.ref = coerceRef(current, element);
@@ -321,12 +321,12 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function updateCoroutine(
     returnFiber : Fiber,
-    current : ?Fiber,
+    current : Fiber | null,
     coroutine : ReactCoroutine,
     priority : PriorityLevel
   ) : Fiber {
     // TODO: Should this also compare handler to determine whether to reuse?
-    if (current == null || current.tag !== CoroutineComponent) {
+    if (current === null || current.tag !== CoroutineComponent) {
       // Insert
       const created = createFiberFromCoroutine(coroutine, priority);
       created.return = returnFiber;
@@ -342,11 +342,11 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function updateYield(
     returnFiber : Fiber,
-    current : ?Fiber,
+    current : Fiber | null,
     yieldNode : ReactYield,
     priority : PriorityLevel
   ) : Fiber {
-    if (current == null || current.tag !== YieldComponent) {
+    if (current === null || current.tag !== YieldComponent) {
       // Insert
       const created = createFiberFromYield(yieldNode, priority);
       created.type = yieldNode.value;
@@ -363,12 +363,12 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function updatePortal(
     returnFiber : Fiber,
-    current : ?Fiber,
+    current : Fiber | null,
     portal : ReactPortal,
     priority : PriorityLevel
   ) : Fiber {
     if (
-      current == null ||
+      current === null ||
       current.tag !== HostPortal ||
       current.stateNode.containerInfo !== portal.containerInfo ||
       current.stateNode.implementation !== portal.implementation
@@ -388,11 +388,11 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function updateFragment(
     returnFiber : Fiber,
-    current : ?Fiber,
+    current : Fiber | null,
     fragment : Iterable<*>,
     priority : PriorityLevel
   ) : Fiber {
-    if (current == null || current.tag !== Fragment) {
+    if (current === null || current.tag !== Fragment) {
       // Insert
       const created = createFiberFromFragment(fragment, priority);
       created.return = returnFiber;
@@ -410,7 +410,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     returnFiber : Fiber,
     newChild : any,
     priority : PriorityLevel
-  ) : ?Fiber {
+  ) : Fiber | null {
     if (typeof newChild === 'string' || typeof newChild === 'number') {
       // Text nodes doesn't have keys. If the previous node is implicitly keyed
       // we can continue to replace it without aborting even if it is not a text
@@ -463,13 +463,13 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function updateSlot(
     returnFiber : Fiber,
-    oldFiber : ?Fiber,
+    oldFiber : Fiber | null,
     newChild : any,
     priority : PriorityLevel
-  ) : ?Fiber {
+  ) : Fiber | null {
     // Update the fiber if the keys match, otherwise return null.
 
-    const key = oldFiber ? oldFiber.key : null;
+    const key = oldFiber !== null ? oldFiber.key : null;
 
     if (typeof newChild === 'string' || typeof newChild === 'number') {
       // Text nodes doesn't have keys. If the previous node is implicitly keyed
@@ -540,7 +540,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     newIdx : number,
     newChild : any,
     priority : PriorityLevel
-  ) : ?Fiber {
+  ) : Fiber | null {
 
     if (typeof newChild === 'string' || typeof newChild === 'number') {
       // Text nodes doesn't have keys, so we neither have to check the old nor
@@ -596,7 +596,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     knownKeys : Set<string> | null
   ) : Set<string> | null {
     if (__DEV__) {
-      if (typeof child !== 'object' || child == null) {
+      if (typeof child !== 'object' || child === null) {
         return knownKeys;
       }
       switch (child.$$typeof) {
@@ -607,7 +607,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
           if (typeof key !== 'string') {
             break;
           }
-          if (knownKeys == null) {
+          if (knownKeys === null) {
             knownKeys = new Set();
             knownKeys.add(key);
             break;
@@ -634,9 +634,9 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function reconcileChildrenArray(
     returnFiber : Fiber,
-    currentFirstChild : ?Fiber,
+    currentFirstChild : Fiber | null,
     newChildren : Array<*>,
-    priority : PriorityLevel) : ?Fiber {
+    priority : PriorityLevel) : Fiber | null {
 
     // This algorithm can't optimize by searching from boths ends since we
     // don't have backpointers on fibers. I'm trying to see how far we can get
@@ -666,21 +666,19 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
       }
     }
 
-    let resultingFirstChild : ?Fiber = null;
-    let previousNewFiber : ?Fiber = null;
+    let resultingFirstChild : Fiber | null = null;
+    let previousNewFiber : Fiber | null = null;
 
     let oldFiber = currentFirstChild;
     let lastPlacedIndex = 0;
     let newIdx = 0;
     let nextOldFiber = null;
-    for (; oldFiber && newIdx < newChildren.length; newIdx++) {
-      if (oldFiber) {
-        if (oldFiber.index > newIdx) {
-          nextOldFiber = oldFiber;
-          oldFiber = null;
-        } else {
-          nextOldFiber = oldFiber.sibling;
-        }
+    for (; oldFiber !== null && newIdx < newChildren.length; newIdx++) {
+      if (oldFiber.index > newIdx) {
+        nextOldFiber = oldFiber;
+        oldFiber = null;
+      } else {
+        nextOldFiber = oldFiber.sibling;
       }
       const newFiber = updateSlot(
         returnFiber,
@@ -688,25 +686,25 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
         newChildren[newIdx],
         priority
       );
-      if (!newFiber) {
+      if (newFiber === null) {
         // TODO: This breaks on empty slots like null children. That's
         // unfortunate because it triggers the slow path all the time. We need
         // a better way to communicate whether this was a miss or null,
         // boolean, undefined, etc.
-        if (!oldFiber) {
+        if (oldFiber === null) {
           oldFiber = nextOldFiber;
         }
         break;
       }
       if (shouldTrackSideEffects) {
-        if (oldFiber && !newFiber.alternate) {
+        if (oldFiber && newFiber.alternate === null) {
           // We matched the slot, but we didn't reuse the existing fiber, so we
           // need to delete the existing child.
           deleteChild(returnFiber, oldFiber);
         }
       }
       lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
-      if (!previousNewFiber) {
+      if (previousNewFiber === null) {
         // TODO: Move out of the loop. This only happens for the first run.
         resultingFirstChild = newFiber;
       } else {
@@ -726,7 +724,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
       return resultingFirstChild;
     }
 
-    if (!oldFiber) {
+    if (oldFiber === null) {
       // If we don't have any more existing children we can choose a fast path
       // since the rest will all be insertions.
       for (; newIdx < newChildren.length; newIdx++) {
@@ -739,7 +737,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
           continue;
         }
         lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
-        if (!previousNewFiber) {
+        if (previousNewFiber === null) {
           // TODO: Move out of the loop. This only happens for the first run.
           resultingFirstChild = newFiber;
         } else {
@@ -764,7 +762,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
       );
       if (newFiber) {
         if (shouldTrackSideEffects) {
-          if (newFiber.alternate) {
+          if (newFiber.alternate !== null) {
             // The new fiber is a work in progress, but if there exists a
             // current, that means that we reused the fiber. We need to delete
             // it from the child list so that we don't add it to the deletion
@@ -775,7 +773,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
           }
         }
         lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
-        if (!previousNewFiber) {
+        if (previousNewFiber === null) {
           resultingFirstChild = newFiber;
         } else {
           previousNewFiber.sibling = newFiber;
@@ -795,9 +793,9 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function reconcileChildrenIterator(
     returnFiber : Fiber,
-    currentFirstChild : ?Fiber,
+    currentFirstChild : Fiber | null,
     newChildrenIterable : Iterable<*>,
-    priority : PriorityLevel) : ?Fiber {
+    priority : PriorityLevel) : Fiber | null {
 
     // This is the same implementation as reconcileChildrenArray(),
     // but using the iterator instead.
@@ -852,8 +850,8 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
       'An iterable object provided no iterator.',
     );
 
-    let resultingFirstChild : ?Fiber = null;
-    let previousNewFiber : ?Fiber = null;
+    let resultingFirstChild : Fiber | null = null;
+    let previousNewFiber : Fiber | null = null;
 
     let oldFiber = currentFirstChild;
     let lastPlacedIndex = 0;
@@ -861,14 +859,12 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     let nextOldFiber = null;
 
     let step = newChildren.next();
-    for (; oldFiber && !step.done; newIdx++, step = newChildren.next()) {
-      if (oldFiber) {
-        if (oldFiber.index > newIdx) {
-          nextOldFiber = oldFiber;
-          oldFiber = null;
-        } else {
-          nextOldFiber = oldFiber.sibling;
-        }
+    for (; oldFiber !== null && !step.done; newIdx++, step = newChildren.next()) {
+      if (oldFiber.index > newIdx) {
+        nextOldFiber = oldFiber;
+        oldFiber = null;
+      } else {
+        nextOldFiber = oldFiber.sibling;
       }
       const newFiber = updateSlot(
         returnFiber,
@@ -876,7 +872,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
         step.value,
         priority
       );
-      if (!newFiber) {
+      if (newFiber === null) {
         // TODO: This breaks on empty slots like null children. That's
         // unfortunate because it triggers the slow path all the time. We need
         // a better way to communicate whether this was a miss or null,
@@ -887,14 +883,14 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
         break;
       }
       if (shouldTrackSideEffects) {
-        if (oldFiber && !newFiber.alternate) {
+        if (oldFiber && newFiber.alternate === null) {
           // We matched the slot, but we didn't reuse the existing fiber, so we
           // need to delete the existing child.
           deleteChild(returnFiber, oldFiber);
         }
       }
       lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
-      if (!previousNewFiber) {
+      if (previousNewFiber === null) {
         // TODO: Move out of the loop. This only happens for the first run.
         resultingFirstChild = newFiber;
       } else {
@@ -914,7 +910,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
       return resultingFirstChild;
     }
 
-    if (!oldFiber) {
+    if (oldFiber === null) {
       // If we don't have any more existing children we can choose a fast path
       // since the rest will all be insertions.
       for (; !step.done; newIdx++, step = newChildren.next()) {
@@ -923,11 +919,11 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
           step.value,
           priority
         );
-        if (!newFiber) {
+        if (newFiber === null) {
           continue;
         }
         lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
-        if (!previousNewFiber) {
+        if (previousNewFiber === null) {
           // TODO: Move out of the loop. This only happens for the first run.
           resultingFirstChild = newFiber;
         } else {
@@ -950,9 +946,9 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
         step.value,
         priority
       );
-      if (newFiber) {
+      if (newFiber !== null) {
         if (shouldTrackSideEffects) {
-          if (newFiber.alternate) {
+          if (newFiber.alternate !== null) {
             // The new fiber is a work in progress, but if there exists a
             // current, that means that we reused the fiber. We need to delete
             // it from the child list so that we don't add it to the deletion
@@ -963,7 +959,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
           }
         }
         lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
-        if (!previousNewFiber) {
+        if (previousNewFiber === null) {
           resultingFirstChild = newFiber;
         } else {
           previousNewFiber.sibling = newFiber;
@@ -983,13 +979,13 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function reconcileSingleTextNode(
     returnFiber : Fiber,
-    currentFirstChild : ?Fiber,
+    currentFirstChild : Fiber | null,
     textContent : string,
     priority : PriorityLevel
   ) : Fiber {
     // There's no need to check for keys on text nodes since we don't have a
     // way to define them.
-    if (currentFirstChild && currentFirstChild.tag === HostText) {
+    if (currentFirstChild !== null && currentFirstChild.tag === HostText) {
       // We already have an existing node so let's just update it and delete
       // the rest.
       deleteRemainingChildren(returnFiber, currentFirstChild.sibling);
@@ -1008,13 +1004,13 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function reconcileSingleElement(
     returnFiber : Fiber,
-    currentFirstChild : ?Fiber,
+    currentFirstChild : Fiber | null,
     element : ReactElement,
     priority : PriorityLevel
   ) : Fiber {
     const key = element.key;
     let child = currentFirstChild;
-    while (child) {
+    while (child !== null) {
       // TODO: If key === null and child.key === null, then this only applies to
       // the first item in the list.
       if (child.key === key) {
@@ -1047,13 +1043,13 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function reconcileSingleCoroutine(
     returnFiber : Fiber,
-    currentFirstChild : ?Fiber,
+    currentFirstChild : Fiber | null,
     coroutine : ReactCoroutine,
     priority : PriorityLevel
   ) : Fiber {
     const key = coroutine.key;
     let child = currentFirstChild;
-    while (child) {
+    while (child !== null) {
       // TODO: If key === null and child.key === null, then this only applies to
       // the first item in the list.
       if (child.key === key) {
@@ -1080,13 +1076,13 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function reconcileSingleYield(
     returnFiber : Fiber,
-    currentFirstChild : ?Fiber,
+    currentFirstChild : Fiber | null,
     yieldNode : ReactYield,
     priority : PriorityLevel
   ) : Fiber {
     // There's no need to check for keys on yields since they're stateless.
     let child = currentFirstChild;
-    if (child) {
+    if (child !== null) {
       if (child.tag === YieldComponent) {
         deleteRemainingChildren(returnFiber, child.sibling);
         const existing = useFiber(child, priority);
@@ -1106,13 +1102,13 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
 
   function reconcileSinglePortal(
     returnFiber : Fiber,
-    currentFirstChild : ?Fiber,
+    currentFirstChild : Fiber | null,
     portal : ReactPortal,
     priority : PriorityLevel
   ) : Fiber {
     const key = portal.key;
     let child = currentFirstChild;
-    while (child) {
+    while (child !== null) {
       // TODO: If key === null and child.key === null, then this only applies to
       // the first item in the list.
       if (child.key === key) {
@@ -1146,10 +1142,10 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
   // children and the parent.
   function reconcileChildFibers(
     returnFiber : Fiber,
-    currentFirstChild : ?Fiber,
+    currentFirstChild : Fiber | null,
     newChild : any,
     priority : PriorityLevel
-  ) : ?Fiber {
+  ) : Fiber | null {
     // This function is not recursive.
     // If the top level item is an array, we treat it as a set of children,
     // not as a fragment. Nested arrays on the other hand will be treated as
@@ -1329,11 +1325,11 @@ exports.reconcileChildFibersInPlace = ChildReconciler(false, true);
 
 exports.mountChildFibersInPlace = ChildReconciler(false, false);
 
-exports.cloneChildFibers = function(current : ?Fiber, workInProgress : Fiber) : void {
+exports.cloneChildFibers = function(current : Fiber | null, workInProgress : Fiber) : void {
   if (!workInProgress.child) {
     return;
   }
-  if (current && workInProgress.child === current.child) {
+  if (current !== null && workInProgress.child === current.child) {
     // We use workInProgress.child since that lets Flow know that it can't be
     // null since we validated that already. However, as the line above suggests
     // they're actually the same thing.
@@ -1349,7 +1345,7 @@ exports.cloneChildFibers = function(current : ?Fiber, workInProgress : Fiber) :
     workInProgress.child = newChild;
 
     newChild.return = workInProgress;
-    while (currentChild.sibling) {
+    while (currentChild.sibling !== null) {
       currentChild = currentChild.sibling;
       newChild = newChild.sibling = cloneFiber(
         currentChild,
@@ -1365,7 +1361,7 @@ exports.cloneChildFibers = function(current : ?Fiber, workInProgress : Fiber) :
   // need to clone. We need to reset the return fiber though since we'll
   // traverse down into them.
   let child = workInProgress.child;
-  while (child) {
+  while (child !== null) {
     child.return = workInProgress;
     child = child.sibling;
   }
diff --git a/src/renderers/shared/fiber/ReactDebugCurrentFiber.js b/src/renderers/shared/fiber/ReactDebugCurrentFiber.js
index c1ce29ea1c4..d857e5f2dd3 100644
--- a/src/renderers/shared/fiber/ReactDebugCurrentFiber.js
+++ b/src/renderers/shared/fiber/ReactDebugCurrentFiber.js
@@ -22,7 +22,7 @@ if (__DEV__) {
 function getCurrentFiberOwnerName() : string | null {
   if (__DEV__) {
     const fiber = ReactDebugCurrentFiber.current;
-    if (fiber == null) {
+    if (fiber === null) {
       return null;
     }
     if (fiber._debugOwner != null) {
@@ -35,7 +35,7 @@ function getCurrentFiberOwnerName() : string | null {
 function getCurrentFiberStackAddendum() : string | null {
   if (__DEV__) {
     const fiber = ReactDebugCurrentFiber.current;
-    if (fiber == null) {
+    if (fiber === null) {
       return null;
     }
     // Safe because if current fiber exists, we are reconciling,
diff --git a/src/renderers/shared/fiber/ReactFiber.js b/src/renderers/shared/fiber/ReactFiber.js
index 6af37269e75..d7ecf743711 100644
--- a/src/renderers/shared/fiber/ReactFiber.js
+++ b/src/renderers/shared/fiber/ReactFiber.js
@@ -93,11 +93,11 @@ export type Fiber = {
   // This is effectively the parent, but there can be multiple parents (two)
   // so this is only the parent of the thing we're currently processing.
   // It is conceptually the same as the return address of a stack frame.
-  return: ?Fiber,
+  return: Fiber | null,
 
   // Singly Linked List Tree Structure.
-  child: ?Fiber,
-  sibling: ?Fiber,
+  child: Fiber | null,
+  sibling: Fiber | null,
   index: number,
 
   // The ref last used to attach this node.
@@ -119,13 +119,13 @@ export type Fiber = {
   effectTag: TypeOfSideEffect,
 
   // Singly linked list fast path to the next fiber with side-effects.
-  nextEffect: ?Fiber,
+  nextEffect: Fiber | null,
 
   // The first and last fiber with side-effect within this subtree. This allows
   // us to reuse a slice of the linked list when we reuse the work done within
   // this fiber.
-  firstEffect: ?Fiber,
-  lastEffect: ?Fiber,
+  firstEffect: Fiber | null,
+  lastEffect: Fiber | null,
 
   // This will be used to quickly determine if a subtree has no pending changes.
   pendingWorkPriority: PriorityLevel,
@@ -139,19 +139,19 @@ export type Fiber = {
   // priority, then we need to store the progressed work somewhere. This holds
   // the started child set until we need to get back to working on it. It may
   // or may not be the same as the "current" child.
-  progressedChild: ?Fiber,
+  progressedChild: Fiber | null,
 
   // When we reconcile children onto progressedChild it is possible that we have
   // to delete some child fibers. We need to keep track of this side-effects so
   // that if we continue later on, we have to include those effects. Deletions
   // are added in the reverse order from sibling pointers.
-  progressedFirstDeletion: ?Fiber,
-  progressedLastDeletion: ?Fiber,
+  progressedFirstDeletion: Fiber | null,
+  progressedLastDeletion: Fiber | null,
 
   // This is a pooled version of a Fiber. Every fiber that gets updated will
   // eventually have a pair. There are cases when we can clean up pairs to save
   // memory if we need to.
-  alternate: ?Fiber,
+  alternate: Fiber | null,
 
   // Conceptual aliases
   // workInProgress : Fiber ->  alternate The alternate used for reuse happens
@@ -249,7 +249,7 @@ exports.cloneFiber = function(fiber : Fiber, priorityLevel : PriorityLevel) : Fi
   // objects for things that are never updated. It also allow us to reclaim the
   // extra memory if needed.
   let alt = fiber.alternate;
-  if (alt) {
+  if (alt !== null) {
     // If we clone, then we do so from the "current" state. The current state
     // can't have any side-effects that are still valid so we reset just to be
     // sure.
diff --git a/src/renderers/shared/fiber/ReactFiberBeginWork.js b/src/renderers/shared/fiber/ReactFiberBeginWork.js
index 947ba6ed24e..ffe68983d4d 100644
--- a/src/renderers/shared/fiber/ReactFiberBeginWork.js
+++ b/src/renderers/shared/fiber/ReactFiberBeginWork.js
@@ -102,7 +102,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     // We now have clones. Let's store them as the currently progressed work.
     workInProgress.progressedChild = workInProgress.child;
     workInProgress.progressedPriority = priorityLevel;
-    if (current) {
+    if (current !== null) {
       // We also store it on the current. When the alternate swaps in we can
       // continue from this point.
       current.progressedChild = workInProgress.progressedChild;
@@ -131,7 +131,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     // At this point any memoization is no longer valid since we'll have changed
     // the children.
     workInProgress.memoizedProps = null;
-    if (!current) {
+    if (current === null) {
       // If this is a fresh new component that hasn't been rendered yet, we
       // won't update its child set by applying minimal side-effects. Instead,
       // we will add them all to the child before it gets rendered. That means
@@ -191,9 +191,9 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     return workInProgress.child;
   }
 
-  function markRef(current : ?Fiber, workInProgress : Fiber) {
+  function markRef(current : Fiber | null, workInProgress : Fiber) {
     const ref = workInProgress.ref;
-    if (ref && (!current || current.ref !== ref)) {
+    if (ref !== null && (!current || current.ref !== ref)) {
       // Schedule a Ref effect
       workInProgress.effectTag |= Ref;
     }
@@ -211,7 +211,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         nextProps = memoizedProps;
       }
     } else {
-      if (nextProps == null || memoizedProps === nextProps) {
+      if (nextProps === null || memoizedProps === nextProps) {
         return bailoutOnAlreadyFinishedWork(current, workInProgress);
       }
       // TODO: Disable this before release, since it is not part of the public API
@@ -240,14 +240,14 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     return workInProgress.child;
   }
 
-  function updateClassComponent(current : ?Fiber, workInProgress : Fiber, priorityLevel : PriorityLevel) {
+  function updateClassComponent(current : Fiber | null, workInProgress : Fiber, priorityLevel : PriorityLevel) {
     // Push context providers early to prevent context stack mismatches.
     // During mounting we don't know the child context yet as the instance doesn't exist.
     // We will invalidate the child context in finishClassComponent() right after rendering.
     const hasContext = pushContextProvider(workInProgress);
 
     let shouldUpdate;
-    if (!current) {
+    if (current === null) {
       if (!workInProgress.stateNode) {
         // In the initial pass we might need to construct the instance.
         constructClassInstance(workInProgress);
@@ -264,7 +264,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   }
 
   function finishClassComponent(
-    current : ?Fiber,
+    current : Fiber | null,
     workInProgress : Fiber,
     shouldUpdate : boolean,
     hasContext : boolean,
@@ -314,7 +314,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     pushHostContainer(workInProgress, root.containerInfo);
 
     const updateQueue = workInProgress.updateQueue;
-    if (updateQueue) {
+    if (updateQueue !== null) {
       const prevState = workInProgress.memoizedState;
       const state = beginUpdateQueue(workInProgress, updateQueue, null, prevState, null, priorityLevel);
       if (prevState === state) {
@@ -335,7 +335,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     pushHostContext(workInProgress);
 
     let nextProps = workInProgress.pendingProps;
-    const prevProps = current ? current.memoizedProps : null;
+    const prevProps = current !== null ? current.memoizedProps : null;
     const memoizedProps = workInProgress.memoizedProps;
     if (hasContextChanged()) {
       // Normally we can bail out on props equality but if context has changed
@@ -359,7 +359,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         // that is a bit tricky since workInProgress and current can have
         // different "hidden" settings.
         let child = workInProgress.progressedChild;
-        while (child) {
+        while (child !== null) {
           // To ensure that this subtree gets its priority reset, the children
           // need to be reset.
           child.pendingWorkPriority = OffscreenPriority;
@@ -409,16 +409,16 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       // Reconcile the children and stash them for later work.
       reconcileChildrenAtPriority(current, workInProgress, nextChildren, OffscreenPriority);
       memoizeProps(workInProgress, nextProps);
-      workInProgress.child = current ? current.child : null;
+      workInProgress.child = current !== null ? current.child : null;
 
-      if (!current) {
+      if (current === null) {
         // If this doesn't have a current we won't track it for placement
         // effects. However, when we come back around to this we have already
         // inserted the parent which means that we'll infact need to make this a
         // placement.
         // TODO: There has to be a better solution to this problem.
         let child = workInProgress.progressedChild;
-        while (child) {
+        while (child !== null) {
           child.effectTag = Placement;
           child = child.sibling;
         }
@@ -464,7 +464,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       value = fn(props, context);
     }
 
-    if (typeof value === 'object' && value && typeof value.render === 'function') {
+    if (typeof value === 'object' && value !== null && typeof value.render === 'function') {
       // Proceed under the assumption that this is a class instance
       workInProgress.tag = ClassComponent;
 
@@ -479,7 +479,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       // Proceed under the assumption that this is a functional component
       workInProgress.tag = FunctionalComponent;
       if (__DEV__) {
-        if (workInProgress.ref != null) {
+        if (workInProgress.ref !== null) {
           let info = '';
           const ownerName = ReactDebugCurrentFiber.getCurrentFiberOwnerName();
           if (ownerName) {
@@ -517,7 +517,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       if (nextCoroutine === null) {
         nextCoroutine = current && current.memoizedProps;
         invariant(
-          nextCoroutine != null,
+          nextCoroutine !== null,
           'We should always have pending or current props. This error is ' +
           'likely caused by a bug in React. Please file an issue.'
         );
@@ -538,7 +538,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     // At this point any memoization is no longer valid since we'll have changed
     // the children.
     workInProgress.memoizedProps = null;
-    if (!current) {
+    if (current === null) {
       workInProgress.stateNode = mountChildFibersInPlace(
         workInProgress,
         workInProgress.stateNode,
@@ -592,7 +592,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       return bailoutOnAlreadyFinishedWork(current, workInProgress);
     }
 
-    if (!current) {
+    if (current === null) {
       // Portals are special because we don't append the children during mount
       // but at commit. Therefore we need to track insertions which the normal
       // flow doesn't do during mount. This doesn't happen at the root because
@@ -632,7 +632,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   }
   */
 
-  function bailoutOnAlreadyFinishedWork(current, workInProgress : Fiber) : ?Fiber {
+  function bailoutOnAlreadyFinishedWork(current, workInProgress : Fiber) : Fiber | null {
     const priorityLevel = workInProgress.pendingWorkPriority;
     // TODO: We should ideally be able to bail out early if the children have no
     // more work to do. However, since we don't have a separation of this
@@ -687,7 +687,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     // is handled by beginUpdateQueue.
   }
 
-  function beginWork(current : ?Fiber, workInProgress : Fiber, priorityLevel : PriorityLevel) : ?Fiber {
+  function beginWork(current : Fiber | null, workInProgress : Fiber, priorityLevel : PriorityLevel) : Fiber | null {
     if (workInProgress.pendingWorkPriority === NoWork ||
         workInProgress.pendingWorkPriority > priorityLevel) {
       return bailoutOnLowPriority(current, workInProgress);
@@ -744,7 +744,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     }
   }
 
-  function beginFailedWork(current : ?Fiber, workInProgress : Fiber, priorityLevel : PriorityLevel) {
+  function beginFailedWork(current : Fiber | null, workInProgress : Fiber, priorityLevel : PriorityLevel) {
     invariant(
       workInProgress.tag === ClassComponent ||
       workInProgress.tag === HostRoot,
diff --git a/src/renderers/shared/fiber/ReactFiberClassComponent.js b/src/renderers/shared/fiber/ReactFiberClassComponent.js
index d135b23531c..94fbf67c21f 100644
--- a/src/renderers/shared/fiber/ReactFiberClassComponent.js
+++ b/src/renderers/shared/fiber/ReactFiberClassComponent.js
@@ -71,7 +71,7 @@ module.exports = function(
   };
 
   function checkShouldComponentUpdate(workInProgress, oldProps, newProps, oldState, newState, newContext) {
-    if (oldProps === null || (workInProgress.updateQueue && workInProgress.updateQueue.hasForceUpdate)) {
+    if (oldProps === null || (workInProgress.updateQueue !== null && workInProgress.updateQueue.hasForceUpdate)) {
       // If the workInProgress already has an Update effect, return true
       return true;
     }
@@ -208,10 +208,10 @@ module.exports = function(
     workInProgress.effectTag |= Update;
   }
 
-  function markUpdateIfAlreadyInProgress(current: ?Fiber, workInProgress : Fiber) {
+  function markUpdateIfAlreadyInProgress(current: Fiber | null, workInProgress : Fiber) {
     // If an update was already in progress, we should schedule an Update
     // effect even though we're bailing out, so that cWU/cDU are called.
-    if (current) {
+    if (current !== null) {
       if (workInProgress.memoizedProps !== current.memoizedProps ||
           workInProgress.memoizedState !== current.memoizedState) {
         markUpdate(workInProgress);
@@ -274,7 +274,7 @@ module.exports = function(
       // If we had additional state updates during this life-cycle, let's
       // process them now.
       const updateQueue = workInProgress.updateQueue;
-      if (updateQueue) {
+      if (updateQueue !== null) {
         instance.state = beginUpdateQueue(
           workInProgress,
           updateQueue,
@@ -343,7 +343,7 @@ module.exports = function(
     // They may be from componentWillMount() or from error boundary's setState()
     // during initial mounting.
     const newUpdateQueue = workInProgress.updateQueue;
-    if (newUpdateQueue) {
+    if (newUpdateQueue !== null) {
       newInstance.state = beginUpdateQueue(
         workInProgress,
         newUpdateQueue,
@@ -392,7 +392,7 @@ module.exports = function(
     const oldState = workInProgress.memoizedState;
     // TODO: Previous state can be null.
     let newState;
-    if (updateQueue) {
+    if (updateQueue !== null) {
       newState = beginUpdateQueue(
         workInProgress,
         updateQueue,
@@ -408,7 +408,7 @@ module.exports = function(
     if (oldProps === newProps &&
         oldState === newState &&
         !hasContextChanged() &&
-        !(updateQueue && updateQueue.hasForceUpdate)) {
+        !(updateQueue !== null && updateQueue.hasForceUpdate)) {
       markUpdateIfAlreadyInProgress(current, workInProgress);
       return false;
     }
diff --git a/src/renderers/shared/fiber/ReactFiberCommitWork.js b/src/renderers/shared/fiber/ReactFiberCommitWork.js
index 072c482355a..f1a9dffec57 100644
--- a/src/renderers/shared/fiber/ReactFiberCommitWork.js
+++ b/src/renderers/shared/fiber/ReactFiberCommitWork.js
@@ -38,7 +38,7 @@ var invariant = require('invariant');
 
 module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   config : HostConfig<T, P, I, TI, PI, C, CX, PL>,
-  captureError : (failedFiber : Fiber, error: Error) => ?Fiber
+  captureError : (failedFiber : Fiber, error: Error) => Fiber | null
 ) {
 
   const {
@@ -65,7 +65,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   function safelyDetachRef(current : Fiber) {
     try {
       const ref = current.ref;
-      if (ref) {
+      if (ref !== null) {
         ref(null);
       }
     } catch (error) {
@@ -74,10 +74,10 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   }
 
   // Only called during update. It's ok to throw.
-  function detachRefIfNeeded(current : ?Fiber, finishedWork : Fiber) {
+  function detachRefIfNeeded(current : Fiber | null, finishedWork : Fiber) {
     if (current) {
       const currentRef = current.ref;
-      if (currentRef && currentRef !== finishedWork.ref) {
+      if (currentRef !== null && currentRef !== finishedWork.ref) {
         currentRef(null);
       }
     }
@@ -85,7 +85,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
 
   function getHostParent(fiber : Fiber) : I | C {
     let parent = fiber.return;
-    while (parent) {
+    while (parent !== null) {
       switch (parent.tag) {
         case HostComponent:
           return parent.stateNode;
@@ -105,7 +105,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
 
   function getHostParentFiber(fiber : Fiber) : Fiber {
     let parent = fiber.return;
-    while (parent) {
+    while (parent !== null) {
       if (isHostParent(parent)) {
         return parent;
       }
@@ -134,8 +134,8 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     let node : Fiber = fiber;
     siblings: while (true) {
       // If we didn't find anything, let's try the next sibling.
-      while (!node.sibling) {
-        if (!node.return || isHostParent(node.return)) {
+      while (node.sibling === null) {
+        if (node.return === null || isHostParent(node.return)) {
           // If we pop out of the root or hit the parent the fiber we are the
           // last sibling.
           return null;
@@ -153,7 +153,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         }
         // If we don't have a child, try the siblings instead.
         // We also skip portals because they are not part of this host tree.
-        if (!node.child || node.tag === HostPortal) {
+        if (node.child === null || node.tag === HostPortal) {
           continue siblings;
         } else {
           node.child.return = node;
@@ -211,7 +211,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         // If the insertion itself is a portal, then we don't want to traverse
         // down its children. Instead, we'll get insertions from each child in
         // the portal directly.
-      } else if (node.child) {
+      } else if (node.child !== null) {
         node.child.return = node;
         node = node.child;
         continue;
@@ -219,8 +219,8 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       if (node === finishedWork) {
         return;
       }
-      while (!node.sibling) {
-        if (!node.return || node.return === finishedWork) {
+      while (node.sibling === null) {
+        if (node.return === null || node.return === finishedWork) {
           return;
         }
         node = node.return;
@@ -241,7 +241,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       commitUnmount(node);
       // Visit children because they may contain more composite or host nodes.
       // Skip portals because commitUnmount() currently visits them recursively.
-      if (node.child && node.tag !== HostPortal) {
+      if (node.child !== null && node.tag !== HostPortal) {
         node.child.return = node;
         node = node.child;
         continue;
@@ -249,8 +249,8 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       if (node === root) {
         return;
       }
-      while (!node.sibling) {
-        if (!node.return || node.return === root) {
+      while (node.sibling === null) {
+        if (node.return === null || node.return === root) {
           return;
         }
         node = node.return;
@@ -276,7 +276,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         // We will reassign it back when we pop the portal on the way up.
         parent = node.stateNode.containerInfo;
         // Visit children because portals might contain host components.
-        if (node.child) {
+        if (node.child !== null) {
           node.child.return = node;
           node = node.child;
           continue;
@@ -284,7 +284,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       } else {
         commitUnmount(node);
         // Visit children because we may find more host components below.
-        if (node.child) {
+        if (node.child !== null) {
           node.child.return = node;
           node = node.child;
           continue;
@@ -293,8 +293,8 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       if (node === current) {
         return;
       }
-      while (!node.sibling) {
-        if (!node.return || node.return === current) {
+      while (node.sibling === null) {
+        if (node.return === null || node.return === current) {
           return;
         }
         node = node.return;
@@ -364,7 +364,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     }
   }
 
-  function commitWork(current : ?Fiber, finishedWork : Fiber) : void {
+  function commitWork(current : Fiber | null, finishedWork : Fiber) : void {
     switch (finishedWork.tag) {
       case ClassComponent: {
         detachRefIfNeeded(current, finishedWork);
@@ -372,7 +372,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       }
       case HostComponent: {
         const instance : I = finishedWork.stateNode;
-        if (instance != null && current) {
+        if (instance != null && current !== null) {
           // Commit the work prepared earlier.
           const newProps = finishedWork.memoizedProps;
           const oldProps = current.memoizedProps;
@@ -380,7 +380,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
           // TODO: Type the updateQueue to be specific to host components.
           const updatePayload : null | PL = (finishedWork.updateQueue : any);
           finishedWork.updateQueue = null;
-          if (updatePayload) {
+          if (updatePayload !== null) {
             commitUpdate(instance, updatePayload, type, oldProps, newProps, finishedWork);
           }
         }
@@ -389,7 +389,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       }
       case HostText: {
         invariant(
-          finishedWork.stateNode !== null && current != null,
+          finishedWork.stateNode !== null && current !== null,
           'This should only be done during updates. This error is likely ' +
           'caused by a bug in React. Please file an issue.'
         );
@@ -415,12 +415,12 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     }
   }
 
-  function commitLifeCycles(current : ?Fiber, finishedWork : Fiber) : void {
+  function commitLifeCycles(current : Fiber | null, finishedWork : Fiber) : void {
     switch (finishedWork.tag) {
       case ClassComponent: {
         const instance = finishedWork.stateNode;
         if (finishedWork.effectTag & Update) {
-          if (!current) {
+          if (current === null) {
             if (typeof instance.componentDidMount === 'function') {
               instance.componentDidMount();
             }
@@ -432,14 +432,14 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
             }
           }
         }
-        if ((finishedWork.effectTag & Callback) && finishedWork.updateQueue) {
+        if ((finishedWork.effectTag & Callback) && finishedWork.updateQueue !== null) {
           commitCallbacks(finishedWork, finishedWork.updateQueue, instance);
         }
         return;
       }
       case HostRoot: {
         const updateQueue = finishedWork.updateQueue;
-        if (updateQueue) {
+        if (updateQueue !== null) {
           const instance = finishedWork.child && finishedWork.child.stateNode;
           commitCallbacks(finishedWork, updateQueue, instance);
         }
@@ -453,7 +453,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         // These effects should only be committed when components are first mounted,
         // aka when there is no current/alternate.
         if (
-          !current &&
+          current === null &&
           finishedWork.effectTag & Update
         ) {
           const type = finishedWork.type;
@@ -486,7 +486,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       return;
     }
     const ref = finishedWork.ref;
-    if (ref) {
+    if (ref !== null) {
       const instance = getPublicInstance(finishedWork.stateNode);
       ref(instance);
     }
diff --git a/src/renderers/shared/fiber/ReactFiberCompleteWork.js b/src/renderers/shared/fiber/ReactFiberCompleteWork.js
index a8c8137f9a5..c8a8888d605 100644
--- a/src/renderers/shared/fiber/ReactFiberCompleteWork.js
+++ b/src/renderers/shared/fiber/ReactFiberCompleteWork.js
@@ -71,7 +71,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     // We now have clones. Let's store them as the currently progressed work.
     workInProgress.progressedChild = workInProgress.child;
     workInProgress.progressedPriority = priorityLevel;
-    if (current) {
+    if (current !== null) {
       // We also store it on the current. When the alternate swaps in we can
       // continue from this point.
       current.progressedChild = workInProgress.progressedChild;
@@ -90,7 +90,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     if (node) {
       node.return = workInProgress;
     }
-    while (node) {
+    while (node !== null) {
       if (node.tag === HostComponent || node.tag === HostText ||
           node.tag === HostPortal) {
         invariant(
@@ -99,13 +99,13 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         );
       } else if (node.tag === YieldComponent) {
         yields.push(node.type);
-      } else if (node.child) {
+      } else if (node.child !== null) {
         node.child.return = node;
         node = node.child;
         continue;
       }
-      while (!node.sibling) {
-        if (!node.return || node.return === workInProgress) {
+      while (node.sibling === null) {
+        if (node.return === null || node.return === workInProgress) {
           return;
         }
         node = node.return;
@@ -115,7 +115,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     }
   }
 
-  function moveCoroutineToHandlerPhase(current : ?Fiber, workInProgress : Fiber) {
+  function moveCoroutineToHandlerPhase(current : Fiber | null, workInProgress : Fiber) {
     var coroutine = (workInProgress.memoizedProps : ?ReactCoroutine);
     invariant(
       coroutine,
@@ -140,7 +140,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     var props = coroutine.props;
     var nextChildren = fn(props, yields);
 
-    var currentFirstChild = current ? current.child : null;
+    var currentFirstChild = current !== null ? current.child : null;
     // Inherit the priority of the returnFiber.
     const priority = workInProgress.pendingWorkPriority;
     workInProgress.child = reconcileChildFibers(
@@ -157,22 +157,22 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     // We only have the top Fiber that was created but we need recurse down its
     // children to find all the terminal nodes.
     let node = workInProgress.child;
-    while (node) {
+    while (node !== null) {
       if (node.tag === HostComponent || node.tag === HostText) {
         appendInitialChild(parent, node.stateNode);
       } else if (node.tag === HostPortal) {
         // If we have a portal child, then we don't want to traverse
         // down its children. Instead, we'll get insertions from each child in
         // the portal directly.
-      } else if (node.child) {
+      } else if (node.child !== null) {
         node = node.child;
         continue;
       }
       if (node === workInProgress) {
         return;
       }
-      while (!node.sibling) {
-        if (!node.return || node.return === workInProgress) {
+      while (node.sibling === null) {
+        if (node.return === null || node.return === workInProgress) {
           return;
         }
         node = node.return;
@@ -181,7 +181,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     }
   }
 
-  function completeWork(current : ?Fiber, workInProgress : Fiber) : ?Fiber {
+  function completeWork(current : Fiber | null, workInProgress : Fiber) : Fiber | null {
     if (__DEV__) {
       ReactDebugCurrentFiber.current = workInProgress;
     }
@@ -208,7 +208,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         const rootContainerInstance = getRootHostContainer();
         const type = workInProgress.type;
         const newProps = workInProgress.memoizedProps;
-        if (current && workInProgress.stateNode != null) {
+        if (current !== null && workInProgress.stateNode != null) {
           // If we have an alternate, that means this is an update and we need to
           // schedule a side-effect to do the updates.
           const oldProps = current.memoizedProps;
@@ -268,7 +268,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
           }
 
           workInProgress.stateNode = instance;
-          if (workInProgress.ref) {
+          if (workInProgress.ref !== null) {
             // If there is a ref on a host node we need to schedule a callback
             workInProgress.effectTag |= Ref;
           }
diff --git a/src/renderers/shared/fiber/ReactFiberContext.js b/src/renderers/shared/fiber/ReactFiberContext.js
index 322affe797d..8b7383458b2 100644
--- a/src/renderers/shared/fiber/ReactFiberContext.js
+++ b/src/renderers/shared/fiber/ReactFiberContext.js
@@ -149,7 +149,7 @@ function processChildContext(fiber : Fiber, parentContext : Object, isReconcilin
   if (typeof instance.getChildContext !== 'function') {
     if (__DEV__) {
       const componentName = getComponentName(fiber);
-      
+
       if (!warnedAboutMissingGetChildContext[componentName]) {
         warnedAboutMissingGetChildContext[componentName] = true;
         warning(
diff --git a/src/renderers/shared/fiber/ReactFiberHostContext.js b/src/renderers/shared/fiber/ReactFiberHostContext.js
index 8abbc538d74..86c130c7cdd 100644
--- a/src/renderers/shared/fiber/ReactFiberHostContext.js
+++ b/src/renderers/shared/fiber/ReactFiberHostContext.js
@@ -44,14 +44,14 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     getRootHostContext,
   } = config;
 
-  let contextStackCursor : StackCursor<?CX> = createCursor((null: ?CX));
-  let contextFiberStackCursor : StackCursor<?Fiber> = createCursor((null: ?Fiber));
-  let rootInstanceStackCursor : StackCursor<?C> = createCursor((null: ?C));
+  let contextStackCursor : StackCursor<CX | null> = createCursor((null: ?CX));
+  let contextFiberStackCursor : StackCursor<Fiber | null> = createCursor((null: Fiber | null));
+  let rootInstanceStackCursor : StackCursor<C | null> = createCursor((null: ?C));
 
   function getRootHostContainer() : C {
     const rootInstance = rootInstanceStackCursor.current;
     invariant(
-      rootInstance != null,
+      rootInstance !== null,
       'Expected root container to exist. This error is likely caused by a ' +
       'bug in React. Please file an issue.'
     );
@@ -95,7 +95,9 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       'a bug in React. Please file an issue.'
     );
 
-    const context = contextStackCursor.current || emptyObject;
+    const context = contextStackCursor.current !== null ?
+      contextStackCursor.current :
+      emptyObject;
     const nextContext = getChildHostContext(context, fiber.type, rootInstance);
 
     // Don't push this Fiber's context unless it's unique.
diff --git a/src/renderers/shared/fiber/ReactFiberReconciler.js b/src/renderers/shared/fiber/ReactFiberReconciler.js
index ab298f6b6ae..1a429daf0a7 100644
--- a/src/renderers/shared/fiber/ReactFiberReconciler.js
+++ b/src/renderers/shared/fiber/ReactFiberReconciler.js
@@ -206,7 +206,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
 
     findHostInstance(fiber : Fiber) : I | TI | null {
       const hostFiber = findCurrentHostFiber(fiber);
-      if (!hostFiber) {
+      if (hostFiber === null) {
         return null;
       }
       return hostFiber.stateNode;
diff --git a/src/renderers/shared/fiber/ReactFiberRoot.js b/src/renderers/shared/fiber/ReactFiberRoot.js
index 3bb55ff52d9..f93dff9ba3d 100644
--- a/src/renderers/shared/fiber/ReactFiberRoot.js
+++ b/src/renderers/shared/fiber/ReactFiberRoot.js
@@ -24,10 +24,10 @@ export type FiberRoot = {
   // Determines if this root has already been added to the schedule for work.
   isScheduled: boolean,
   // The work schedule is a linked list.
-  nextScheduledRoot: ?FiberRoot,
+  nextScheduledRoot: FiberRoot | null,
   // Top context object, used by renderSubtreeIntoContainer
-  context: ?Object,
-  pendingContext: ?Object,
+  context: Object | null,
+  pendingContext: Object | null,
 };
 
 exports.createFiberRoot = function(containerInfo : any) : FiberRoot {
diff --git a/src/renderers/shared/fiber/ReactFiberScheduler.js b/src/renderers/shared/fiber/ReactFiberScheduler.js
index d070cf4de40..e88126f6960 100644
--- a/src/renderers/shared/fiber/ReactFiberScheduler.js
+++ b/src/renderers/shared/fiber/ReactFiberScheduler.js
@@ -22,7 +22,7 @@ export type CapturedError = {
   componentStack : string,
   error : Error,
   errorBoundaryFound : boolean,
-  errorBoundaryName : ?string,
+  errorBoundaryName : string | null,
   willRetry : boolean,
 };
 
@@ -134,17 +134,17 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
   let isBatchingUpdates : boolean = false;
 
   // The next work in progress fiber that we're currently working on.
-  let nextUnitOfWork : ?Fiber = null;
+  let nextUnitOfWork : Fiber | null = null;
   let nextPriorityLevel : PriorityLevel = NoWork;
 
   // The next fiber with an effect that we're currently committing.
-  let nextEffect : ?Fiber = null;
+  let nextEffect : Fiber | null = null;
 
-  let pendingCommit : ?Fiber = null;
+  let pendingCommit : Fiber | null = null;
 
   // Linked list of roots with scheduled work on them.
-  let nextScheduledRoot : ?FiberRoot = null;
-  let lastScheduledRoot : ?FiberRoot = null;
+  let nextScheduledRoot : FiberRoot | null = null;
+  let lastScheduledRoot : FiberRoot | null = null;
 
   // Keep track of which host environment callbacks are scheduled.
   let isAnimationCallbackScheduled : boolean = false;
@@ -193,7 +193,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
   // the work loop.
   function findNextUnitOfWork() {
     // Clear out roots with no more work on them, or if they have uncaught errors
-    while (nextScheduledRoot && nextScheduledRoot.current.pendingWorkPriority === NoWork) {
+    while (nextScheduledRoot !== null && nextScheduledRoot.current.pendingWorkPriority === NoWork) {
       // Unschedule this root.
       nextScheduledRoot.isScheduled = false;
       // Read the next pointer now.
@@ -215,7 +215,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     let root = nextScheduledRoot;
     let highestPriorityRoot = null;
     let highestPriorityLevel = NoWork;
-    while (root) {
+    while (root !== null) {
       if (root.current.pendingWorkPriority !== NoWork && (
           highestPriorityLevel === NoWork ||
           highestPriorityLevel > root.current.pendingWorkPriority)) {
@@ -225,7 +225,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       // We didn't find anything to do in this root, so let's try the next one.
       root = root.nextScheduledRoot;
     }
-    if (highestPriorityRoot) {
+    if (highestPriorityRoot !== null) {
       nextPriorityLevel = highestPriorityLevel;
       priorityContext = nextPriorityLevel;
 
@@ -247,7 +247,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
   }
 
   function commitAllHostEffects() {
-    while (nextEffect) {
+    while (nextEffect !== null) {
       if (__DEV__) {
         ReactDebugCurrentFiber.current = nextEffect;
       }
@@ -306,7 +306,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
   }
 
   function commitAllLifeCycles() {
-    while (nextEffect) {
+    while (nextEffect !== null) {
       const current = nextEffect.alternate;
       // Use Task priority for lifecycle updates
       if (nextEffect.effectTag & (Update | Callback)) {
@@ -359,7 +359,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       // the root has an effect, we need to add it to the end of the list. The
       // resulting list is the set that would belong to the root's parent, if
       // it had one; that is, all the effects in the tree including the root.
-      if (finishedWork.lastEffect) {
+      if (finishedWork.lastEffect !== null) {
         finishedWork.lastEffect.nextEffect = finishedWork;
         firstEffect = finishedWork.firstEffect;
       } else {
@@ -376,18 +376,18 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     // The first pass performs all the host insertions, updates, deletions and
     // ref unmounts.
     nextEffect = firstEffect;
-    while (nextEffect) {
+    while (nextEffect !== null) {
       try {
         commitAllHostEffects(finishedWork);
       } catch (error) {
         invariant(
-          nextEffect != null,
+          nextEffect !== null,
           'Should have next effect. This error is likely caused by a bug ' +
           'in React. Please file an issue.'
         );
         captureError(nextEffect, error);
         // Clean-up
-        if (nextEffect) {
+        if (nextEffect !== null) {
           nextEffect = nextEffect.nextEffect;
         }
       }
@@ -406,17 +406,17 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     // and deletions in the entire tree have already been invoked.
     // This pass also triggers any renderer-specific initial effects.
     nextEffect = firstEffect;
-    while (nextEffect) {
+    while (nextEffect !== null) {
       try {
         commitAllLifeCycles(finishedWork, nextEffect);
       } catch (error) {
         invariant(
-          nextEffect != null,
+          nextEffect !== null,
           'Should have next effect. This error is likely caused by a bug ' +
           'in React. Please file an issue.'
         );
         captureError(nextEffect, error);
-        if (nextEffect) {
+        if (nextEffect !== null) {
           nextEffect = nextEffect.nextEffect;
         }
       }
@@ -446,7 +446,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     // Check for pending update priority. This is usually null so it shouldn't
     // be a perf issue.
     const queue = workInProgress.updateQueue;
-    if (queue) {
+    if (queue !== null) {
       newPriority = getPendingPriority(queue);
     }
 
@@ -456,7 +456,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     // Either it is the same as child, or it just bailed out because it choose
     // not to do the work.
     let child = workInProgress.progressedChild;
-    while (child) {
+    while (child !== null) {
       // Ensure that remaining work priority bubbles up.
       if (child.pendingWorkPriority !== NoWork &&
           (newPriority === NoWork ||
@@ -468,7 +468,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     workInProgress.pendingWorkPriority = newPriority;
   }
 
-  function completeUnitOfWork(workInProgress : Fiber) : ?Fiber {
+  function completeUnitOfWork(workInProgress : Fiber) : Fiber | null {
     while (true) {
       // The current, flushed, state of this fiber is the alternate.
       // Ideally nothing should rely on this, but relying on it here
@@ -482,7 +482,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
 
       resetWorkPriority(workInProgress);
 
-      if (next) {
+      if (next !== null) {
         if (__DEV__ && ReactFiberInstrumentation.debugTool) {
           ReactFiberInstrumentation.debugTool.onCompleteWork(workInProgress);
         }
@@ -491,15 +491,15 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
         return next;
       }
 
-      if (returnFiber) {
+      if (returnFiber !== null) {
         // Append all the effects of the subtree and this fiber onto the effect
         // list of the parent. The completion order of the children affects the
         // side-effect order.
-        if (!returnFiber.firstEffect) {
+        if (returnFiber.firstEffect === null) {
           returnFiber.firstEffect = workInProgress.firstEffect;
         }
-        if (workInProgress.lastEffect) {
-          if (returnFiber.lastEffect) {
+        if (workInProgress.lastEffect !== null) {
+          if (returnFiber.lastEffect !== null) {
             returnFiber.lastEffect.nextEffect = workInProgress.firstEffect;
           }
           returnFiber.lastEffect = workInProgress.lastEffect;
@@ -512,7 +512,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
         // reusing children we'll schedule this effect onto itself since we're
         // at the end.
         if (workInProgress.effectTag !== NoEffect) {
-          if (returnFiber.lastEffect) {
+          if (returnFiber.lastEffect !== null) {
             returnFiber.lastEffect.nextEffect = workInProgress;
           } else {
             returnFiber.firstEffect = workInProgress;
@@ -525,10 +525,10 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
         ReactFiberInstrumentation.debugTool.onCompleteWork(workInProgress);
       }
 
-      if (siblingFiber) {
+      if (siblingFiber !== null) {
         // If there is more work to do in this returnFiber, do that next.
         return siblingFiber;
-      } else if (returnFiber) {
+      } else if (returnFiber !== null) {
         // If there's no more work in this returnFiber. Complete the returnFiber.
         workInProgress = returnFiber;
         continue;
@@ -548,7 +548,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     }
   }
 
-  function performUnitOfWork(workInProgress : Fiber) : ?Fiber {
+  function performUnitOfWork(workInProgress : Fiber) : Fiber | null {
     // The current, flushed, state of this fiber is the alternate.
     // Ideally nothing should rely on this, but relying on it here
     // means that we don't need an additional field on the work in
@@ -561,7 +561,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       ReactFiberInstrumentation.debugTool.onBeginWork(workInProgress);
     }
 
-    if (!next) {
+    if (next === null) {
       // If this doesn't spawn new work, complete the current work.
       next = completeUnitOfWork(workInProgress);
     }
@@ -574,7 +574,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     return next;
   }
 
-  function performFailedUnitOfWork(workInProgress : Fiber) : ?Fiber {
+  function performFailedUnitOfWork(workInProgress : Fiber) : Fiber | null {
 
     // The current, flushed, state of this fiber is the alternate.
     // Ideally nothing should rely on this, but relying on it here
@@ -588,7 +588,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       ReactFiberInstrumentation.debugTool.onBeginWork(workInProgress);
     }
 
-    if (!next) {
+    if (next === null) {
       // If this doesn't spawn new work, complete the current work.
       next = completeUnitOfWork(workInProgress);
     }
@@ -610,16 +610,16 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
 
   function performAnimationWork() {
     isAnimationCallbackScheduled = false;
-    performWork(AnimationPriority);
+    performWork(AnimationPriority, null);
   }
 
   function clearErrors() {
-    if (!nextUnitOfWork) {
+    if (nextUnitOfWork === null) {
       nextUnitOfWork = findNextUnitOfWork();
     }
     // Keep performing work until there are no more errors
-    while (capturedErrors && capturedErrors.size &&
-           nextUnitOfWork &&
+    while (capturedErrors !== null && capturedErrors.size &&
+           nextUnitOfWork !== null &&
            nextPriorityLevel !== NoWork &&
            nextPriorityLevel <= TaskPriority) {
       if (hasCapturedError(nextUnitOfWork)) {
@@ -628,7 +628,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       } else {
         nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
       }
-      if (!nextUnitOfWork) {
+      if (nextUnitOfWork === null) {
         // If performUnitOfWork returns null, that means we just comitted
         // a root. Normally we'd need to clear any errors that were scheduled
         // during the commit phase. But we're already clearing errors, so
@@ -642,16 +642,16 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     // Clear any errors.
     clearErrors();
 
-    if (!nextUnitOfWork) {
+    if (nextUnitOfWork === null) {
       nextUnitOfWork = findNextUnitOfWork();
     }
 
     let hostRootTimeMarker;
     if (
       ReactFeatureFlags.logTopLevelRenders &&
-      nextUnitOfWork &&
+      nextUnitOfWork !== null &&
       nextUnitOfWork.tag === HostRoot &&
-      nextUnitOfWork.child
+      nextUnitOfWork.child !== null
     ) {
       const componentName = getComponentName(nextUnitOfWork.child) || '';
       hostRootTimeMarker = 'React update: ' + componentName;
@@ -660,17 +660,17 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
 
     // If there's a deadline, and we're not performing Task work, perform work
     // using this loop that checks the deadline on every iteration.
-    if (deadline && priorityLevel > TaskPriority) {
+    if (deadline !== null && priorityLevel > TaskPriority) {
       // The deferred work loop will run until there's no time left in
       // the current frame.
-      while (nextUnitOfWork && !deadlineHasExpired) {
+      while (nextUnitOfWork !== null && !deadlineHasExpired) {
         if (deadline.timeRemaining() > timeHeuristicForUnitOfWork) {
           nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
           // In a deferred work batch, iff nextUnitOfWork returns null, we just
           // completed a root and a pendingCommit exists. Logically, we could
           // omit either of the checks in the following condition, but we need
           // both to satisfy Flow.
-          if (!nextUnitOfWork && pendingCommit) {
+          if (nextUnitOfWork === null && pendingCommit !== null) {
             // If we have time, we should commit the work now.
             if (deadline.timeRemaining() > timeHeuristicForUnitOfWork) {
               commitAllWork(pendingCommit);
@@ -690,11 +690,11 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       // If there's no deadline, or if we're performing Task work, use this loop
       // that doesn't check how much time is remaining. It will keep running
       // until we run out of work at this priority level.
-      while (nextUnitOfWork &&
+      while (nextUnitOfWork !== null &&
              nextPriorityLevel !== NoWork &&
              nextPriorityLevel <= priorityLevel) {
         nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
-        if (!nextUnitOfWork) {
+        if (nextUnitOfWork === null) {
           nextUnitOfWork = findNextUnitOfWork();
           // performUnitOfWork returned null, which means we just comitted a
           // root. Clear any errors that were scheduled during the commit phase.
@@ -725,14 +725,14 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     // deferred batch.
     while (priorityLevel !== NoWork && !fatalError) {
       invariant(
-        deadline || (priorityLevel < HighPriority),
+        deadline !== null || (priorityLevel < HighPriority),
         'Cannot perform deferred work without a deadline. This error is ' +
         'likely caused by a bug in React. Please file an issue.'
       );
 
       // Before starting any work, check to see if there are any pending
       // commits from the previous frame.
-      if (pendingCommit && !deadlineHasExpired) {
+      if (pendingCommit !== null && !deadlineHasExpired) {
         commitAllWork(pendingCommit);
       }
 
@@ -746,7 +746,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       } catch (error) {
         // We caught an error during either the begin or complete phases.
         const failedWork = nextUnitOfWork;
-        if (failedWork) {
+        if (failedWork !== null) {
           // Reset the priority context to its value before reconcilation.
           priorityContext = priorityContextBeforeReconciliation;
 
@@ -754,7 +754,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
           // error boundary, the nearest host container acts as one. If
           // captureError returns null, the error was intentionally ignored.
           const maybeBoundary = captureError(failedWork, error);
-          if (maybeBoundary) {
+          if (maybeBoundary !== null) {
             const boundary = maybeBoundary;
 
             // Complete the boundary as if it rendered null. This will unmount
@@ -774,7 +774,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
           }
           // Continue performing work
           continue;
-        } else if (!fatalError) {
+        } else if (fatalError === null) {
           // There is no current unit of work. This is a worst-case scenario
           // and should only be possible if there's a bug in the renderer, e.g.
           // inside resetAfterCommit.
@@ -830,13 +830,13 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     failedBoundaries = null;
 
     // It's safe to throw any unhandled errors.
-    if (errorToThrow) {
+    if (errorToThrow !== null) {
       throw errorToThrow;
     }
   }
 
   // Returns the boundary that captured the error, or null if the error is ignored
-  function captureError(failedWork : Fiber, error : Error) : ?Fiber {
+  function captureError(failedWork : Fiber, error : Error) : Fiber | null {
     // It is no longer valid because we exited the user code.
     ReactCurrentOwner.current = null;
     if (__DEV__) {
@@ -846,12 +846,12 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     nextUnitOfWork = null;
 
     // Search for the nearest error boundary.
-    let boundary : ?Fiber = null;
+    let boundary : Fiber | null = null;
 
     // Passed to logCapturedError()
     let errorBoundaryFound : boolean = false;
     let willRetry : boolean = false;
-    let errorBoundaryName : ?string = null;
+    let errorBoundaryName : string | null = null;
 
     // Host containers are a special case. If the failed work itself is a host
     // container, then it acts as its own boundary. In all other cases, we
@@ -867,7 +867,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       }
     } else {
       let node = failedWork.return;
-      while (node && !boundary) {
+      while (node !== null && boundary === null) {
         if (node.tag === ClassComponent) {
           const instance = node.stateNode;
           if (typeof instance.unstable_handleError === 'function') {
@@ -897,9 +897,9 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
           // this boundary already captured an error during this commit.
           // This case exists because multiple errors can be thrown during
           // a single commit without interruption.
-          if (commitPhaseBoundaries && (
+          if (commitPhaseBoundaries !== null && (
             commitPhaseBoundaries.has(node) ||
-            (node.alternate) && commitPhaseBoundaries.has(node.alternate)
+            (node.alternate !== null && commitPhaseBoundaries.has(node.alternate))
           )) {
             // If so, we should ignore this error.
             return null;
@@ -914,10 +914,10 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       }
     }
 
-    if (boundary) {
+    if (boundary !== null) {
       // Add to the collection of failed boundaries. This lets us know that
       // subsequent errors in this subtree should propagate to the next boundary.
-      if (!failedBoundaries) {
+      if (failedBoundaries === null) {
         failedBoundaries = new Set();
       }
       failedBoundaries.add(boundary);
@@ -933,7 +933,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       // map of errors and their component stack location keyed by the boundaries
       // that capture them. We mostly use this Map as a Set; it's a Map only to
       // avoid adding a field to Fiber to store the error.
-      if (!capturedErrors) {
+      if (capturedErrors === null) {
         capturedErrors = new Map();
       }
       capturedErrors.set(boundary, {
@@ -948,7 +948,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       // If we're in the commit phase, defer scheduling an update on the
       // boundary until after the commit is complete
       if (isCommitting) {
-        if (!commitPhaseBoundaries) {
+        if (commitPhaseBoundaries === null) {
           commitPhaseBoundaries = new Set();
         }
         commitPhaseBoundaries.add(boundary);
@@ -957,7 +957,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
         scheduleErrorRecovery(boundary);
       }
       return boundary;
-    } else if (!firstUncaughtError) {
+    } else if (firstUncaughtError === null) {
       // If no boundary is found, we'll need to throw the error
       firstUncaughtError = error;
     }
@@ -968,8 +968,8 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     // TODO: capturedErrors should store the boundary instance, to avoid needing
     // to check the alternate.
     return Boolean(
-      capturedErrors &&
-      (capturedErrors.has(fiber) || (fiber.alternate && capturedErrors.has(fiber.alternate)))
+      capturedErrors !== null &&
+      (capturedErrors.has(fiber) || (fiber.alternate !== null && capturedErrors.has(fiber.alternate)))
     );
   }
 
@@ -977,18 +977,18 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     // TODO: failedBoundaries should store the boundary instance, to avoid
     // needing to check the alternate.
     return Boolean(
-      failedBoundaries &&
-      (failedBoundaries.has(fiber) || (fiber.alternate && failedBoundaries.has(fiber.alternate)))
+      failedBoundaries !== null &&
+      (failedBoundaries.has(fiber) || (fiber.alternate !== null && failedBoundaries.has(fiber.alternate)))
     );
   }
 
   function commitErrorHandling(effectfulFiber : Fiber) {
     let capturedError;
-    if (capturedErrors) {
+    if (capturedErrors !== null) {
       capturedError = capturedErrors.get(effectfulFiber);
       capturedErrors.delete(effectfulFiber);
-      if (!capturedError) {
-        if (effectfulFiber.alternate) {
+      if (capturedError == null) {
+        if (effectfulFiber.alternate !== null) {
           effectfulFiber = effectfulFiber.alternate;
           capturedError = capturedErrors.get(effectfulFiber);
           capturedErrors.delete(effectfulFiber);
@@ -1002,19 +1002,13 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       'bug in React. Please file an issue.'
     );
 
-    let error;
-
-    // Conditional required to satisfy Flow
-    if (capturedError) {
-      error = capturedError.error;
-
-      try {
-        logCapturedError(capturedError);
-      } catch (e) {
-        // Prevent cycle if logCapturedError() throws.
-        // A cycle may still occur if logCapturedError renders a component that throws.
-        console.error(e);
-      }
+    const error = capturedError.error;
+    try {
+      logCapturedError(capturedError);
+    } catch (e) {
+      // Prevent cycle if logCapturedError() throws.
+      // A cycle may still occur if logCapturedError renders a component that throws.
+      console.error(e);
     }
 
     switch (effectfulFiber.tag) {
@@ -1025,7 +1019,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
         instance.unstable_handleError(error);
         return;
       case HostRoot:
-        if (!firstUncaughtError) {
+        if (firstUncaughtError === null) {
           // If this is the host container, we treat it as a no-op error
           // boundary. We'll throw the first uncaught error once it's safe to
           // do so, at the end of the batch.
@@ -1043,7 +1037,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
 
   function unwindContexts(from : Fiber, to: Fiber) {
     let node = from;
-    while (node && (node !== to) && (node.alternate !== to)) {
+    while (node !== null && (node !== to) && (node.alternate !== to)) {
       switch (node.tag) {
         case ClassComponent:
           popContextProvider(node);
@@ -1091,7 +1085,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
 
     let node = fiber;
     let shouldContinue = true;
-    while (node && shouldContinue) {
+    while (node !== null && shouldContinue) {
       // Walk the parent path to the root and update each node's priority. Once
       // we reach a node whose priority matches (and whose alternate's priority
       // matches) we can exit safely knowing that the rest of the path is correct.
@@ -1102,7 +1096,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
         shouldContinue = true;
         node.pendingWorkPriority = priorityLevel;
       }
-      if (node.alternate) {
+      if (node.alternate !== null) {
         if (node.alternate.pendingWorkPriority === NoWork ||
             node.alternate.pendingWorkPriority > priorityLevel) {
           // Priority did not match. Update and keep going.
@@ -1110,7 +1104,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
           node.alternate.pendingWorkPriority = priorityLevel;
         }
       }
-      if (!node.return) {
+      if (node.return === null) {
         if (node.tag === HostRoot) {
           const root : FiberRoot = (node.stateNode : any);
           scheduleRoot(root, priorityLevel);
@@ -1118,7 +1112,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
           // schedule a callback to perform work later.
           switch (priorityLevel) {
             case SynchronousPriority:
-              performWork(SynchronousPriority);
+              performWork(SynchronousPriority, null);
               return;
             case TaskPriority:
               // TODO: If we're not already performing work, schedule a
@@ -1175,7 +1169,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
       // If we're not already inside a batch, we need to flush any task work
       // that was created by the user-provided function.
       if (!isPerformingWork && !isBatchingUpdates) {
-        performWork(TaskPriority);
+        performWork(TaskPriority, null);
       }
     }
   }