Add react/no-useless-scu rule

This will check for `shouldComponentUpdate` defined in a class that
extends React.PureComponent.

README.md

@@ -102,6 +102,7 @@ Finally, enable all of the rules that you would like to use.  Use [our preset](#
 * [react/no-unescaped-entities](docs/rules/no-unescaped-entities.md): Prevent invalid characters from appearing in markup
 * [react/no-unknown-property](docs/rules/no-unknown-property.md): Prevent usage of unknown DOM property (fixable)
 * [react/no-unused-prop-types](docs/rules/no-unused-prop-types.md): Prevent definitions of unused prop types
+* [react/no-useless-scu](docs/rules/no-useless-scu.md): Prevent usage of `shouldComponentUpdate` when extending React.PureComponent
 * [react/prefer-es6-class](docs/rules/prefer-es6-class.md): Enforce ES5 or ES6 class for React Components
 * [react/prefer-stateless-function](docs/rules/prefer-stateless-function.md): Enforce stateless React Components to be written as a pure function
 * [react/prop-types](docs/rules/prop-types.md): Prevent missing props validation in a React component definition

docs/rules/no-useless-scu.md

@@ -0,0 +1,64 @@
+# Prevent usage of shouldComponentUpdate when extending React.PureComponent (react/no-useless-scu)
+
+Warns if you have `shouldComponentUpdate` defined when defining a component that extends React.PureComponent.
+While having `shouldComponentUpdate` will still work, it becomes pointless to extend PureComponent.
+
+## Rule Details
+
+The following patterns are considered warnings:
+
+```jsx
+class Foo extends React.PureComponent {
+  shouldComponentUpdate() {
+    return true;
+  }
+
+  render() {
+    return <div>Radical!</div>
+  }
+}
+
+function Bar() {
+  return class Baz extends React.PureComponent {
+    shouldComponentUpdate() {
+      return true;
+    }
+
+    render() {
+      return <div>Groovy!</div>
+    }
+  }
+}
+```
+
+The following patterns are not considered warnings:
+
+```jsx
+class Foo extends React.Component {
+  shouldComponentUpdate() {
+    return true;
+  }
+
+  render() {
+    return <div>Radical!</div>
+  }
+}
+
+function Bar() {
+  return class Baz extends React.Component {
+    shouldComponentUpdate() {
+      return true;
+    }
+
+    render() {
+      return <div>Groovy!</div>
+    }
+  }
+}
+
+class Qux extends React.PureComponent {
+  render() {
+    return <div>Tubular!</div>
+  }
+}
+```

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-useless-scu': require('./lib/rules/no-useless-scu')
 };
 
 function filterRules(rules, predicate) {

lib/rules/no-useless-scu.js

@@ -0,0 +1,94 @@
+/**
+ * @fileoverview Flag shouldComponentUpdate when extending PureComponent
+ */
+'use strict';
+
+var Components = require('../util/Components');
+
+function errorMessage(node) {
+  return node + ' does not need shouldComponentUpdate when extending React.PureComponent.';
+}
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Flag shouldComponentUpdate when extending PureComponent',
+      category: 'Possible Errors',
+      recommended: false
+    },
+    schema: []
+  },
+
+  create: Components.detect(function(context, components, utils) {
+
+    /**
+     * Get properties name
+     * @param {Object} node - Property.
+     * @returns {String} Property name.
+     */
+    function getPropertyName(node) {
+      // Special case for class properties
+      // (babel-eslint does not expose property name so we have to rely on tokens)
+      if (node.type === 'ClassProperty') {
+        var tokens = context.getFirstTokens(node, 2);
+        return tokens[1] && tokens[1].type === 'Identifier' ? tokens[1].value : tokens[0].value;
+      }
+
+      return node.key.name;
+    }
+
+    /**
+     * Get properties for a given AST node
+     * @param {ASTNode} node The AST node being checked.
+     * @returns {Array} Properties array.
+     */
+    function getComponentProperties(node) {
+      switch (node.type) {
+        case 'ClassExpression':
+        case 'ClassDeclaration':
+          return node.body.body;
+        default:
+          return [];
+      }
+    }
+
+    /**
+     * Checks for shouldComponentUpdate property
+     * @param {ASTNode} node The AST node being checked.
+     * @returns {Boolean} Whether or not the property exists.
+     */
+    function hasShouldComponentUpdate(node) {
+      var properties = getComponentProperties(node);
+      return properties.some(function(property) {
+        var name = getPropertyName(property);
+        return name === 'shouldComponentUpdate';
+      });
+    }
+
+    /**
+     * Checks for violation of rule
+     * @param {ASTNode} node The AST node being checked.
+     */
+    function checkForViolation(node) {
+      if (utils.isPureComponent(node)) {
+        var hasScu = hasShouldComponentUpdate(node);
+        if (hasScu) {
+          var className = node.id.name;
+          context.report({
+            node: node,
+            message: errorMessage(className)
+          });
+        }
+      }
+    }
+
+    return {
+      ClassDeclaration: checkForViolation,
+      ClassExpression: checkForViolation
+    };
+  })
+};

tests/lib/rules/no-useless-scu.js

@@ -0,0 +1,128 @@
+/**
+ * @fileoverview Tests for no-useless-scu
+ */
+
+'use strict';
+
+// -----------------------------------------------------------------------------
+// Requirements
+// -----------------------------------------------------------------------------
+
+var rule = require('../../../lib/rules/no-useless-scu');
+var RuleTester = require('eslint').RuleTester;
+
+var parserOptions = {
+  ecmaVersion: 6,
+  ecmaFeatures: {
+    experimentalObjectRestSpread: true,
+    jsx: true
+  }
+};
+
+function errorMessage(node) {
+  return node + ' does not need shouldComponentUpdate when extending React.PureComponent.';
+}
+
+// -----------------------------------------------------------------------------
+// Tests
+// -----------------------------------------------------------------------------
+
+var ruleTester = new RuleTester();
+ruleTester.run('no-useless-scu', rule, {
+  valid: [
+    {
+      code: [
+        'class Foo extends React.Component {',
+        '  shouldComponentUpdate() {',
+        '    return true;',
+        '  }',
+        '}'
+      ].join('\n'),
+      parserOptions: parserOptions
+    },
+    {
+      code: [
+        'class Foo extends React.Component {',
+        '  shouldComponentUpdate() {',
+        '    return true;',
+        '  }',
+        '}'
+      ].join('\n'),
+      parserOptions: parserOptions
+    },
+    {
+      code: [
+        'function Foo() {',
+        '  return class Bar extends React.Component {',
+        '    shouldComponentUpdate() {',
+        '      return true;',
+        '    }',
+        '  };',
+        '}'
+      ].join('\n'),
+      parserOptions: parserOptions
+    }
+  ],
+  invalid: [
+    {
+      code: [
+        'class Foo extends React.PureComponent {',
+        '  shouldComponentUpdate() {',
+        '    return true;',
+        '  }',
+        '}'
+      ].join('\n'),
+      errors: [{message: errorMessage('Foo')}],
+      parserOptions: parserOptions
+    },
+    {
+      code: [
+        'class Foo extends PureComponent {',
+        '  shouldComponentUpdate() {',
+        '    return true;',
+        '  }',
+        '}'
+      ].join('\n'),
+      errors: [{message: errorMessage('Foo')}],
+      parserOptions: parserOptions
+    },
+    {
+      code: [
+        'class Foo extends React.PureComponent {',
+        '  shouldComponentUpdate = () => {',
+        '    return true;',
+        '  }',
+        '}'
+      ].join('\n'),
+      errors: [{message: errorMessage('Foo')}],
+      parser: 'babel-eslint',
+      parserOptions: parserOptions
+    },
+    {
+      code: [
+        'function Foo() {',
+        '  return class Bar extends React.PureComponent {',
+        '    shouldComponentUpdate() {',
+        '      return true;',
+        '    }',
+        '  };',
+        '}'
+      ].join('\n'),
+      errors: [{message: errorMessage('Bar')}],
+      parserOptions: parserOptions
+    },
+    {
+      code: [
+        'function Foo() {',
+        '  return class Bar extends PureComponent {',
+        '    shouldComponentUpdate() {',
+        '      return true;',
+        '    }',
+        '  };',
+        '}'
+      ].join('\n'),
+      errors: [{message: errorMessage('Bar')}],
+      parserOptions: parserOptions
+    }
+  ]
+});