Add no-unused-state

This adds a new rule, react/no-unused-state, which discovers state fields in a React component and warns if any of them are never read. It was developed internally at Facebook by @rjbailey and has been in use throughout Facebook's JS for a couple months now. It was written against a modern version of node, so has been rebased on @jlharb's branch dropping support for node < 4.

It currently supports es2015 classes extending `React.Component`  (no support currently for `React.createClass()`) and can detect when state is as `this.state = {}`, assigning state in a property initializer, and when calling `this.setState()`.

.eslintrc

@@ -1,5 +1,6 @@
 {
     "env": {
+        "es6": true,
         "node": true
     },
     parserOptions: {

index.js

@@ -63,7 +63,8 @@ var allRules = {
   'no-comment-textnodes': require('./lib/rules/no-comment-textnodes'),
   'require-extension': require('./lib/rules/require-extension'),
   'wrap-multilines': require('./lib/rules/wrap-multilines'),
-  'jsx-tag-spacing': require('./lib/rules/jsx-tag-spacing')
+  'jsx-tag-spacing': require('./lib/rules/jsx-tag-spacing'),
+  'no-unused-state': require('./lib/rules/no-unused-state')
 };
 
 function filterRules(rules, predicate) {

lib/rules/no-unused-state.js

@@ -0,0 +1,299 @@
+/**
+ * @fileoverview  Attempts to discover all state fields in a React component and
+ * warn if any of them are never read.
+ *
+ * State field definitions are collected from `this.state = {}` assignments in
+ * the constructor, objects passed to `this.setState()`, and `state = {}` class
+ * property assignments.
+ */
+
+'use strict';
+
+// Descend through all wrapping TypeCastExpressions and return the expression
+// that was cast.
+function uncast(node) {
+  while (node.type === 'TypeCastExpression') {
+    node = node.expression;
+  }
+  return node;
+}
+
+// Return the name of an identifier or the string value of a literal. Useful
+// anywhere that a literal may be used as a key (e.g., member expressions,
+// method definitions, ObjectExpression property keys).
+function getName(node) {
+  node = uncast(node);
+  if (node.type === 'Identifier') {
+    return node.name;
+  } else if (node.type === 'Literal') {
+    return String(node.value);
+  }
+  return null;
+}
+
+function isMethodDefinitionWithName(node, name, isStatic) {
+  isStatic = isStatic || false;
+  return (
+    node.type === 'MethodDefinition' &&
+    node.static === isStatic &&
+    getName(node.key) === name
+  );
+}
+
+function isThisExpression(node) {
+  return uncast(node).type === 'ThisExpression';
+}
+
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Prevent definition of unused state fields',
+      category: 'Best Practices',
+      recommended: false
+    },
+    schema: []
+  },
+
+  create: function(context) {
+    // Non-null when we are inside a React component ClassDeclaration and we have
+    // not yet encountered any use of this.state which we have chosen not to
+    // analyze. If we encounter any such usage (like this.state being spread as
+    // JSX attributes), then this is again set to null.
+    let classInfo = null;
+
+    // Returns true if the given node is possibly a reference to `this.state`.
+    function isStateReference(node) {
+      node = uncast(node);
+
+      const isDirectStateReference =
+        node.type === 'MemberExpression' &&
+        isThisExpression(node.object) &&
+        node.property.name === 'state';
+
+      const isAliasedStateReference =
+        node.type === 'Identifier' &&
+        classInfo.aliases &&
+        classInfo.aliases.has(node.name);
+
+      return isDirectStateReference || isAliasedStateReference;
+    }
+
+    // Takes an ObjectExpression node and adds all named Property nodes to the
+    // current set of state fields.
+    function addStateFields(node) {
+      for (let prop of node.properties) {
+        if (prop.type === 'Property' && getName(prop.key) !== null) {
+          classInfo.stateFields.add(prop);
+        }
+      }
+    }
+
+    // Adds the name of the given node as a used state field if the node is an
+    // Identifier or a Literal. Other node types are ignored.
+    function addUsedStateField(node) {
+      let name = getName(node);
+      if (name) {
+        classInfo.usedStateFields.add(name);
+      }
+    }
+
+    // Records used state fields and new aliases for an ObjectPattern which
+    // destructures `this.state`.
+    function handleStateDestructuring(node) {
+      for (let prop of node.properties) {
+        if (prop.type === 'Property') {
+          addUsedStateField(prop.key);
+        } else if (
+          prop.type === 'ExperimentalRestProperty' &&
+          classInfo.aliases
+        ) {
+          classInfo.aliases.add(getName(prop.argument));
+        }
+      }
+    }
+
+    // Used to record used state fields and new aliases for both
+    // AssignmentExpressions and VariableDeclarators.
+    function handleAssignment(left, right) {
+      switch (left.type) {
+        case 'Identifier':
+          if (isStateReference(right) && classInfo.aliases) {
+            classInfo.aliases.add(left.name);
+          }
+          break;
+        case 'ObjectPattern':
+          if (isStateReference(right)) {
+            handleStateDestructuring(left);
+          } else if (isThisExpression(right) && classInfo.aliases) {
+            for (let prop of left.properties) {
+              if (prop.type === 'Property' && getName(prop.key) === 'state') {
+                let name = getName(prop.value);
+                if (name) {
+                  classInfo.aliases.add(name);
+                } else if (prop.value.type === 'ObjectPattern') {
+                  handleStateDestructuring(prop.value);
+                }
+              }
+            }
+          }
+          break;
+        default:
+          // pass
+      }
+    }
+
+    return {
+      ClassDeclaration(node) {
+        // Simple heuristic for determining whether we're in a React component.
+        const isReactComponent = node.body.body.some(
+          child => isMethodDefinitionWithName(child, 'render')
+        );
+
+        if (isReactComponent) {
+          classInfo = {
+            // Set of nodes where state fields were defined.
+            stateFields: new Set(),
+
+            // Set of names of state fields that we've seen used.
+            usedStateFields: new Set(),
+
+            // Names of local variables that may be pointing to this.state. To
+            // track this properly, we would need to keep track of all locals,
+            // shadowing, assignments, etc. To keep things simple, we only
+            // maintain one set of aliases per method and accept that it will
+            // produce some false negatives.
+            aliases: null
+          };
+        }
+      },
+
+      'ClassDeclaration:exit'() {
+        if (!classInfo) {
+          return;
+        }
+        // Report all unused state fields.
+        for (let node of classInfo.stateFields) {
+          let name = getName(node.key);
+          if (!classInfo.usedStateFields.has(name)) {
+            context.report(node, `Unused state field: '${name}'`);
+          }
+        }
+        classInfo = null;
+      },
+
+      CallExpression(node) {
+        if (!classInfo) {
+          return;
+        }
+        // If we're looking at a `this.setState({})` invocation, record all the
+        // properties as state fields.
+        if (
+          node.callee.type === 'MemberExpression' &&
+          isThisExpression(node.callee.object) &&
+          getName(node.callee.property) === 'setState' &&
+          node.arguments.length > 0 &&
+          node.arguments[0].type === 'ObjectExpression'
+        ) {
+          addStateFields(node.arguments[0]);
+        }
+      },
+
+      ClassProperty(node) {
+        if (!classInfo) {
+          return;
+        }
+        // If we see state being assigned as a class property using an object
+        // expression, record all the fields of that object as state fields.
+        if (
+          getName(node.key) === 'state' &&
+          !node.static &&
+          node.value &&
+          node.value.type === 'ObjectExpression'
+        ) {
+          addStateFields(node.value);
+        }
+      },
+
+      MethodDefinition() {
+        if (!classInfo) {
+          return;
+        }
+        // Create a new set for this.state aliases local to this method.
+        classInfo.aliases = new Set();
+      },
+
+      'MethodDefinition:exit'() {
+        if (!classInfo) {
+          return;
+        }
+        // Forget our set of local aliases.
+        classInfo.aliases = null;
+      },
+
+      AssignmentExpression(node) {
+        if (!classInfo) {
+          return;
+        }
+        // Check for assignments like `this.state = {}`
+        if (
+          node.left.type === 'MemberExpression' &&
+          isThisExpression(node.left.object) &&
+          getName(node.left.property) === 'state' &&
+          node.right.type === 'ObjectExpression'
+        ) {
+          // Find the nearest function expression containing this assignment.
+          let fn = node;
+          while (fn.type !== 'FunctionExpression' && fn.parent) {
+            fn = fn.parent;
+          }
+          // If the nearest containing function is the constructor, then we want
+          // to record all the assigned properties as state fields.
+          if (
+            fn.parent &&
+            fn.parent.type === 'MethodDefinition' &&
+            fn.parent.kind === 'constructor'
+          ) {
+            addStateFields(node.right);
+          }
+        } else {
+          // Check for assignments like `alias = this.state` and record the alias.
+          handleAssignment(node.left, node.right);
+        }
+      },
+
+      VariableDeclarator(node) {
+        if (!classInfo || !node.init) {
+          return;
+        }
+        handleAssignment(node.id, node.init);
+      },
+
+      MemberExpression(node) {
+        if (!classInfo) {
+          return;
+        }
+        if (isStateReference(node.object)) {
+          // If we see this.state[foo] access, give up.
+          if (node.computed && node.property.type !== 'Literal') {
+            classInfo = null;
+            return;
+          }
+          // Otherwise, record that we saw this property being accessed.
+          addUsedStateField(node.property);
+        }
+      },
+
+      JSXSpreadAttribute(node) {
+        if (classInfo && isStateReference(node.argument)) {
+          classInfo = null;
+        }
+      },
+
+      ExperimentalSpreadProperty(node) {
+        if (classInfo && isStateReference(node.argument)) {
+          classInfo = null;
+        }
+      }
+    };
+  }
+};