Add state-in-constructor rule

docs/rules/state-in-constructor.md

@@ -0,0 +1,74 @@
+# Enforce state initialization style (react/state-in-constructor)
+
+This rule will enforce the state initialization style to be either in a constructor or with a class property.
+
+## Rule Options
+
+```js
+...
+"react/state-in-constructor": [<enabled>, <mode>]
+...
+```
+
+### `always` mode
+
+Will enforce the state initialization style to be in a constructor. This is the default mode.
+
+The following patterns are considered warnings:
+
+```jsx
+class Foo extends React.Component {
+  state = { bar: 0 }
+  render() {
+    return <div>Foo</div>
+  }
+}
+```
+
+The following patterns are **not** considered warnings:
+
+```jsx
+class Foo extends React.Component {
+  constructor(props) {
+    super(props)
+    this.state = { bar: 0 }
+  }
+  render() {
+    return <div>Foo</div>
+  }
+}
+```
+
+### `never` mode
+
+Will enforce the state initialization style to be with a class property.
+
+The following patterns are considered warnings:
+
+```jsx
+class Foo extends React.Component {
+  constructor(props) {
+    super(props)
+    this.state = { bar: 0 }
+  }
+  render() {
+    return <div>Foo</div>
+  }
+}
+```
+
+The following patterns are **not** considered warnings:
+
+```jsx
+class Foo extends React.Component {
+  state = { bar: 0 }
+  render() {
+    return <div>Foo</div>
+  }
+}
+```
+
+
+## When Not To Use It
+
+When the way a component state is being initialized doesn't matter.

index.js

@@ -78,6 +78,7 @@ const allRules = {
   'self-closing-comp': require('./lib/rules/self-closing-comp'),
   'sort-comp': require('./lib/rules/sort-comp'),
   'sort-prop-types': require('./lib/rules/sort-prop-types'),
+  'state-in-constructor': require('./lib/rules/state-in-constructor'),
   'style-prop-object': require('./lib/rules/style-prop-object'),
   'void-dom-elements-no-children': require('./lib/rules/void-dom-elements-no-children')
 };

lib/rules/state-in-constructor.js

@@ -0,0 +1,59 @@
+/**
+ * @fileoverview Enforce the state initialization style to be either in a constructor or with a class property
+ * @author Kanitkorn Sujautra
+ */
+'use strict';
+
+const Components = require('../util/Components');
+const docsUrl = require('../util/docsUrl');
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    docs: {
+      description: 'State initialization in an ES6 class component should be in a constructor',
+      category: 'Stylistic Issues',
+      recommended: false,
+      url: docsUrl('state-in-constructor')
+    },
+    schema: [{
+      enum: ['always', 'never']
+    }]
+  },
+
+  create: Components.detect((context, components, utils) => {
+    const option = context.options[0] || 'always';
+    return {
+      ClassProperty: function (node) {
+        if (
+          option === 'always' &&
+          !node.static &&
+          node.key.name === 'state' &&
+          utils.isES6Component(node.parent.parent)
+        ) {
+          context.report({
+            node,
+            message: 'State initialization should be in a constructor'
+          });
+        }
+      },
+      AssignmentExpression(node) {
+        if (
+          option === 'never' &&
+          node.left.object.type === 'ThisExpression' &&
+          node.left.property.name === 'state' &&
+          node.parent.parent.parent.parent.kind === 'constructor' &&
+          utils.isES6Component(node.parent.parent.parent.parent.parent.parent)
+        ) {
+          context.report({
+            node,
+            message: 'State initialization should be in a class property'
+          });
+        }
+      }
+    };
+  })
+};