typescript-eslint · JamesHenry · Feb 7, 2019 · Jan 28, 2019 · Jan 28, 2019 · Jan 28, 2019
diff --git a/packages/eslint-plugin/README.md b/packages/eslint-plugin/README.md
@@ -128,6 +128,7 @@ Then you should add `airbnb` (or `airbnb-base`) to your `extends` section of `.e
 | [`@typescript-eslint/no-empty-interface`](./docs/rules/no-empty-interface.md)                             | Disallow the declaration of empty interfaces (`no-empty-interface` from TSLint)                                                                     | :heavy_check_mark: |          |
 | [`@typescript-eslint/no-explicit-any`](./docs/rules/no-explicit-any.md)                                   | Disallow usage of the `any` type (`no-any` from TSLint)                                                                                             | :heavy_check_mark: |          |
 | [`@typescript-eslint/no-extraneous-class`](./docs/rules/no-extraneous-class.md)                           | Forbids the use of classes as namespaces (`no-unnecessary-class` from TSLint)                                                                       |                    |          |
+| [`@typescript-eslint/no-for-in-array`](./docs/rules/no-for-in-array.md)                                           | Disallow iterating over an array with a for-in loop (`no-for-in-array` from TSLint)                                                                 |                    |          |
 | [`@typescript-eslint/no-inferrable-types`](./docs/rules/no-inferrable-types.md)                           | Disallows explicit type declarations for variables or parameters initialized to a number, string, or boolean. (`no-inferrable-types` from TSLint)   | :heavy_check_mark: | :wrench: |
 | [`@typescript-eslint/no-misused-new`](./docs/rules/no-misused-new.md)                                     | Enforce valid definition of `new` and `constructor`. (`no-misused-new` from TSLint)                                                                 | :heavy_check_mark: |          |
 | [`@typescript-eslint/no-namespace`](./docs/rules/no-namespace.md)                                         | Disallow the use of custom TypeScript modules and namespaces (`no-namespace` from TSLint)                                                           | :heavy_check_mark: |          |

diff --git a/packages/eslint-plugin/ROADMAP.md b/packages/eslint-plugin/ROADMAP.md
@@ -1,10 +1,15 @@
 # Roadmap
 
-✅ (29) = done<br>
+<!--
+  When updating these totals, please make sure to subtract 1 from the total
+  count retrived via Ctrl/Cmd+F since this key shouldn’t count in the results.
+-->
+
+✅ (30) = done<br>
 🌟 (79) = in ESLint core<br>
 🔌 (33) = in another plugin<br>
 🌓 (16) = implementations differ or ESLint version is missing functionality<br>
-🛑 (68) = unimplemented
+🛑 (68) = unimplemented<br>
 
 ## TSLint rules
 
@@ -59,7 +64,7 @@
 | [`no-empty`]                         | 🌟  | [`no-empty`][no-empty]                                                |
 | [`no-eval`]                          | 🌟  | [`no-eval`][no-eval]                                                  |
 | [`no-floating-promises`]             | 🛑  | N/A ([relevant plugin][plugin:promise])                               |
-| [`no-for-in-array`]                  | 🛑  | N/A                                                                   |
+| [`no-for-in-array`]                  | ✅  | [`@typescript-eslint/no-for-in-array`]                                |
 | [`no-implicit-dependencies`]         | 🔌  | [`import/no-extraneous-dependencies`]                                 |
 | [`no-inferred-empty-object-type`]    | 🛑  | N/A                                                                   |
 | [`no-invalid-template-strings`]      | 🌟  | [`no-template-curly-in-string`][no-template-curly-in-string]          |
@@ -586,6 +591,7 @@ Relevant plugins: [`chai-expect-keywords`](https://github.com/gavinaiken/eslint-
 [`@typescript-eslint/member-delimiter-style`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/member-delimiter-style.md
 [`@typescript-eslint/prefer-interface`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/prefer-interface.md
 [`@typescript-eslint/no-array-constructor`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-array-constructor.md
+[`@typescript-eslint/no-for-in-array`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-for-in-array.md
 
 <!-- eslint-plugin-import -->
 

diff --git a/packages/eslint-plugin/docs/rules/no-for-in-array.md b/packages/eslint-plugin/docs/rules/no-for-in-array.md
@@ -0,0 +1,44 @@
+# Disallow iterating over an array with a for-in loop (no-for-in-array)
+
+This rule prohibits iterating over an array with a for-in loop.
+
+## Rule Details
+
+Rationale from TSLint:
+
+A for-in loop (`for (var k in o)`) iterates over the properties of an Object.
+While it is legal to use for-in loops with array types, it is not common.
+for-in will iterate over the indices of the array as strings, omitting any "holes" in
+the array.
+More common is to use for-of, which iterates over the values of an array.
+If you want to iterate over the indices, alternatives include:
+
+```js
+array.forEach((value, index) => { ... });
+for (const [index, value] of array.entries()) { ... }
+for (let i = 0; i < array.length; i++) { ... }
+```
+
+Examples of **incorrect** code for this rule:
+
+```js
+for (const x in [3, 4, 5]) {
+  console.log(x);
+}
+```
+
+Examples of **correct** code for this rule:
+
+```js
+for (const x in { a: 3, b: 4, c: 5 }) {
+  console.log(x);
+}
+```
+
+## When Not To Use It
+
+If you want to iterate through a loop using the indices in an array as strings, you can turn off this rule.
+
+## Related to
+
+- TSLint: ['no-for-in-array'](https://palantir.github.io/tslint/rules/no-for-in-array/)
diff --git a/packages/eslint-plugin/lib/rules/no-for-in-array.js b/packages/eslint-plugin/lib/rules/no-for-in-array.js
@@ -0,0 +1,56 @@
+/**
+ * @fileoverview Disallow iterating over an array with a for-in loop
+ * @author Benjamin Lichtman
+ */
+'use strict';
+const ts = require('typescript');
+const util = require('../util');
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+/**
+ * @type {import("eslint").Rule.RuleModule}
+ */
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Disallow iterating over an array with a for-in loop',
+      category: 'Functionality',
+      recommended: false,
+      extraDescription: [util.tslintRule('no-for-in-array')],
+      url: util.metaDocsUrl('no-for-in-array')
+    },
+    fixable: null,
+    messages: {
+      forInViolation:
+        'For-in loops over arrays are forbidden. Use for-of or array.forEach instead.'
+    },
+    schema: [],
+    type: 'problem'
+  },
+
+  create(context) {
+    return {
+      ForInStatement(node) {
+        const parserServices = util.getParserServices(context);
+        const checker = parserServices.program.getTypeChecker();
+        const originalNode = parserServices.esTreeNodeToTSNodeMap.get(node);
+
+        const type = checker.getTypeAtLocation(originalNode.expression);
+
+        if (
+          (typeof type.symbol !== 'undefined' &&
+            type.symbol.name === 'Array') ||
+          (type.flags & ts.TypeFlags.StringLike) !== 0
+        ) {
+          context.report({
+            node,
+            messageId: 'forInViolation'
+          });
+        }
+      }
+    };
+  }
+};
diff --git a/packages/eslint-plugin/lib/util.js b/packages/eslint-plugin/lib/util.js
@@ -109,7 +109,7 @@ exports.upperCaseFirst = str => str[0].toUpperCase() + str.slice(1);
 /**
  * Try to retrieve typescript parser service from context
  * @param {RuleContext} context Rule context
- * @returns {{esTreeNodeToTSNodeMap}|{program}|Object|*} parserServices
+ * @returns {{program: Program, esTreeNodeToTSNodeMap: NodeMap}} parserServices
  */
 exports.getParserServices = context => {
   if (

diff --git a/packages/eslint-plugin/tests/lib/rules/no-for-in-array.js b/packages/eslint-plugin/tests/lib/rules/no-for-in-array.js
@@ -0,0 +1,69 @@
+/**
+ * @fileoverview Disallow iterating over an array with a for-in loop
+ * @author Benjamin Lichtman
+ */
+'use strict';
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+const rule = require('../../../lib/rules/no-for-in-array'),
+  RuleTester = require('eslint').RuleTester,
+  path = require('path');
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+const rootDir = path.join(process.cwd(), 'tests/fixtures/');
+const parserOptions = {
+  ecmaVersion: 2015,
+  tsconfigRootDir: rootDir,
+  project: './tsconfig.json'
+};
+const ruleTester = new RuleTester({
+  parserOptions,
+  parser: '@typescript-eslint/parser'
+});
+
+ruleTester.run('no-for-in-array', rule, {
+  valid: [
+    `
+for (const x of [3, 4, 5]) {
+    console.log(x);
+}`,
+    `
+for (const x in { a: 1, b: 2, c: 3 }) {
+    console.log(x);
+}`
+  ],
+
+  invalid: [
+    {
+      code: `
+for (const x in [3, 4, 5]) {
+    console.log(x);
+}`,
+      errors: [
+        {
+          messageId: 'forInViolation',
+          type: 'ForInStatement'
+        }
+      ]
+    },
+    {
+      code: `
+const z = [3, 4, 5];
+for (const x in z) {
+    console.log(x);
+}`,
+      errors: [
+        {
+          messageId: 'forInViolation',
+          type: 'ForInStatement'
+        }
+      ]
+    }
+  ]
+});