Add docs for no-direct-mutation-state

Author: petersendidit

README.md

@@ -62,6 +62,7 @@ Finally, enable all of the rules that you would like to use.
     "react/no-danger": 1,
     "react/no-did-mount-set-state": 1,
     "react/no-did-update-set-state": 1,
+    "react/no-direct-mutation-state": 1,
     "react/no-multi-comp": 1,
     "react/no-set-state": 1,
     "react/no-unknown-property": 1,
@@ -94,6 +95,7 @@ Finally, enable all of the rules that you would like to use.
 * [no-danger](docs/rules/no-danger.md): Prevent usage of dangerous JSX properties
 * [no-did-mount-set-state](docs/rules/no-did-mount-set-state.md): Prevent usage of `setState` in `componentDidMount`
 * [no-did-update-set-state](docs/rules/no-did-update-set-state.md): Prevent usage of `setState` in `componentDidUpdate`
+* [no-direct-mutation-state](docs/rules/no-direct-mutation-state.md): Prevent direct mutation of `this.state`
 * [no-multi-comp](docs/rules/no-multi-comp.md): Prevent multiple component definition per file
 * [no-set-state](docs/rules/no-set-state.md): Prevent usage of `setState`
 * [no-unknown-property](docs/rules/no-unknown-property.md): Prevent usage of unknown DOM property

docs/rules/no-direct-mutation-state.md

@@ -0,0 +1,37 @@
+# Prevent direct mutation of this.state (no-direct-mutation-state)
+
+NEVER mutate `this.state` directly, as calling `setState()` afterwards may replace
+the mutation you made. Treat `this.state` as if it were immutable.
+
+## Rule Details
+
+This rule is aimed to forbid the use of mutating `this.state` directly.
+
+The following patterns are considered warnings:
+
+```js
+var Hello = React.createClass({
+  componentDidMount: function() {
+    this.state.name = this.props.name.toUpperCase();
+  },
+  render: function() {
+    return <div>Hello {this.state.name}</div>;
+  }
+});
+```
+
+
+The following patterns are not considered warnings:
+
+```js
+var Hello = React.createClass({
+  componentDidMount: function() {
+    this.setState({
+      name: this.props.name.toUpperCase();
+    });
+  },
+  render: function() {
+    return <div>Hello {this.state.name}</div>;
+  }
+});
+```

lib/rules/no-direct-mutation-state.js

@@ -17,7 +17,11 @@ module.exports = function(context) {
   return {
 
     AssignmentExpression: function(node) {
-      var item = node.left.object;
+      var item;
+      if (!node.left || !node.left.object || !node.left.object.object) {
+        return;
+      }
+      item = node.left.object;
       while (item.object.property) {
         item = item.object;
       }

tests/lib/rules/no-direct-mutation-state.js

@@ -44,6 +44,14 @@ ruleTester.run('no-direct-mutation-state', rule, {
     ecmaFeatures: {
       jsx: true
     }
+  }, {
+    code: [
+      'var Hello = "foo";',
+      'module.exports = {};'
+    ].join('\n'),
+    ecmaFeatures: {
+      jsx: true
+    }
   }],
 
   invalid: [{