New: no-undefined rule.

README.md

@@ -28,6 +28,7 @@ npm install --save-dev eslint eslint-plugin-mysticatea
         "mysticatea/arrow-parens": "error",
         "mysticatea/block-scoped-var": "error",
         "mysticatea/no-literal-call": "error",
+        "mysticatea/no-undefined": "error",
         "mysticatea/no-use-ignored-vars": "error",
         "mysticatea/no-useless-rest-spread": "error",
         "arrow-parens": "off",
@@ -42,5 +43,6 @@ npm install --save-dev eslint eslint-plugin-mysticatea
 - [arrow-parens](docs/rules/arrow-parens.md) - Enforce parens of argument lists (excludes too redundant parens) (fixable).
 - [block-scoped-var](docs/rules/block-scoped-var.md) - The complete emulation of block-scoping for `var`.
 - [no-literal-call](docs/rules/no-literal-call.md) - Disallow a call of a literal.
+- [no-undefined](docs/rules/no-undefined.md) - disallow the declarations of `undefined`.
 - [no-use-ignored-vars](docs/rules/no-use-ignored-vars.md) - Disallow a use of ignored variables.
 - [no-useless-rest-spread](docs/rules/no-useless-rest-spread.md) - Disallow unnecessary rest/spread operators (fixable).

docs/rules/no-undefined.md

@@ -0,0 +1,35 @@
+# disallow the declarations of `undefined` (no-undefined)
+
+In JavaScript, `undefined` is not a keyword. So people can declare variables as `undefined`.
+
+This rule disallow such variables.
+
+## Rule Details
+
+Examples of **incorrect** code for this rule:
+
+```js
+/*eslint no-undefined: "error"*/
+
+var undefined;
+
+function undefined() {
+}
+
+function foo(undefined) {
+}
+```
+
+Examples of **correct** code for this rule:
+
+```js
+/*eslint no-undefined: "error"*/
+
+// allow a use of undefined.
+if (a === undefined) {
+}
+```
+
+## Options
+
+Nothing.

lib/rules/no-undefined.js

@@ -0,0 +1,65 @@
+/**
+ * @author Toru Nagashima
+ * @copyright 2016 Toru Nagashima. All rights reserved.
+ * See LICENSE file in root directory for full license.
+ */
+"use strict"
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+    meta: {
+        docs: {
+            description: "disallow the declarations of `undefined`",
+            category: "Best Practices",
+            recommended: false,
+        },
+        fixable: false,
+        schema: [],
+    },
+
+    create: function(context) {
+        /**
+         * Checks whether the given variable has `undefined` as its name or not.
+         *
+         * @param {escope.Variable} variable - The variable to check.
+         * @returns {boolean} `true` if the varaible is `undefined`.
+         */
+        function isUndefined(variable) {
+            return variable.name === "undefined"
+        }
+
+        /**
+         * Reports the given variable.
+         *
+         * @param {escope.Variable} variable - The variable to report.
+         * @returns {void}
+         */
+        function report(variable) {
+            variable.defs.forEach(function(def) {
+                var id = def.name
+
+                context.report({
+                    node: id,
+                    message: "Unexpected 'undefined'.",
+                })
+            })
+        }
+
+        return {
+            Program: function() {
+                var globalScope = context.getScope()
+                var scopes = [globalScope]
+                var scope = null
+
+                while ((scope = scopes.shift()) != null) {
+                    Array.prototype.push.apply(scopes, scope.childScopes)
+
+                    scope.variables.filter(isUndefined).forEach(report)
+                }
+            },
+        }
+    },
+}

tests/lib/rules/no-undefined.js

@@ -0,0 +1,29 @@
+/**
+ * @author Toru Nagashima
+ * @copyright 2016 Toru Nagashima. All rights reserved.
+ * See LICENSE file in root directory for full license.
+ */
+"use strict"
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+var rule = require("../../../lib/rules/no-undefined")
+var RuleTester = require("eslint").RuleTester
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+;(new RuleTester()).run("no-undefined", rule, {
+    valid: [
+        "if (a === undefined) {}",
+    ],
+    invalid: [
+        {code: "var undefined", errors: ["Unexpected 'undefined'."]},
+        {code: "function undefined() {}", errors: ["Unexpected 'undefined'."]},
+        {code: "function foo() { var undefined }", errors: ["Unexpected 'undefined'."]},
+        {code: "function foo(undefined) {}", errors: ["Unexpected 'undefined'."]},
+    ],
+})