Rule no-in-array

packages/eslint-plugin/docs/rules/no-in-array.md

@@ -0,0 +1,37 @@
+---
+description: 'Disallow using in operator for arrays.'
+---
+
+> 🛑 This file is source code, not the primary documentation location! 🛑
+>
+> See **https://typescript-eslint.io/rules/no-in-array** for documentation.
+
+This rule bans using `in` operator for checking array members existence.
+
+## Rule Details
+
+Examples of code for this rule:
+
+### ❌ Incorrect
+
+```ts
+const arr = ['a', 'b', 'c'];
+
+if ('c' in arr) {
+  // ...
+}
+```
+
+### ✅ Correct
+
+```ts
+const arr = ['a', 'b', 'c'];
+
+if (arr.includes('a')) {
+  // ...
+}
+```
+
+## When Not To Use It
+
+When you want exactly iterate over array indexes.

packages/eslint-plugin/src/configs/all.ts

@@ -70,6 +70,7 @@ export = {
     '@typescript-eslint/no-for-in-array': 'error',
     'no-implied-eval': 'off',
     '@typescript-eslint/no-implied-eval': 'error',
+    '@typescript-eslint/no-in-array': 'error',
     '@typescript-eslint/no-inferrable-types': 'error',
     'no-invalid-this': 'off',
     '@typescript-eslint/no-invalid-this': 'error',

packages/eslint-plugin/src/configs/recommended-requiring-type-checking.ts

@@ -10,6 +10,7 @@ export = {
     '@typescript-eslint/no-for-in-array': 'error',
     'no-implied-eval': 'off',
     '@typescript-eslint/no-implied-eval': 'error',
+    '@typescript-eslint/no-in-array': 'error',
     '@typescript-eslint/no-misused-promises': 'error',
     '@typescript-eslint/no-unnecessary-type-assertion': 'error',
     '@typescript-eslint/no-unsafe-argument': 'error',

packages/eslint-plugin/src/rules/index.ts

@@ -48,6 +48,7 @@ import noFloatingPromises from './no-floating-promises';
 import noForInArray from './no-for-in-array';
 import noImplicitAnyCatch from './no-implicit-any-catch';
 import noImpliedEval from './no-implied-eval';
+import noInArray from './no-in-array';
 import noInferrableTypes from './no-inferrable-types';
 import noInvalidThis from './no-invalid-this';
 import noInvalidVoidType from './no-invalid-void-type';
@@ -179,6 +180,7 @@ export default {
   'no-for-in-array': noForInArray,
   'no-implicit-any-catch': noImplicitAnyCatch,
   'no-implied-eval': noImpliedEval,
+  'no-in-array': noInArray,
   'no-inferrable-types': noInferrableTypes,
   'no-invalid-this': noInvalidThis,
   'no-invalid-void-type': noInvalidVoidType,

packages/eslint-plugin/src/rules/no-in-array.ts

@@ -0,0 +1,46 @@
+import type { TSESTree } from '@typescript-eslint/utils';
+import * as ts from 'typescript';
+
+import * as util from '../util';
+
+export default util.createRule<[], 'inArrayViolation'>({
+  name: 'no-in-array',
+  meta: {
+    docs: {
+      description: 'Disallow using in operator for arrays',
+      recommended: 'error',
+      requiresTypeChecking: true,
+    },
+    messages: {
+      inArrayViolation:
+        "'in' operator for arrays is forbidden. Use array.indexOf or array.includes instead.",
+    },
+    schema: [],
+    type: 'problem',
+  },
+  defaultOptions: [],
+  create(context) {
+    return {
+      "BinaryExpression[operator='in']"(node: TSESTree.BinaryExpression): void {
+        const parserServices = util.getParserServices(context);
+        const checker = parserServices.program.getTypeChecker();
+        const originalNode = parserServices.esTreeNodeToTSNodeMap.get(node);
+
+        const type = util.getConstrainedTypeAtLocation(
+          checker,
+          originalNode.right,
+        );
+
+        if (
+          util.isTypeArrayTypeOrUnionOfArrayTypes(type, checker) ||
+          (type.flags & ts.TypeFlags.StringLike) !== 0
+        ) {
+          context.report({
+            node,
+            messageId: 'inArrayViolation',
+          });
+        }
+      },
+    };
+  },
+});

packages/eslint-plugin/tests/rules/no-in-array.test.ts

@@ -0,0 +1,40 @@
+import rule from '../../src/rules/no-in-array';
+import { getFixturesRootDir, RuleTester } from '../RuleTester';
+
+const rootDir = getFixturesRootDir();
+const ruleTester = new RuleTester({
+  parserOptions: {
+    ecmaVersion: 2015,
+    tsconfigRootDir: rootDir,
+    project: './tsconfig.json',
+  },
+  parser: '@typescript-eslint/parser',
+});
+
+ruleTester.run('no-in-array', rule, {
+  valid: [
+    {
+      code: `
+if (x in {}) {
+}
+      `,
+    },
+  ],
+  invalid: [
+    {
+      code: `
+if (x in ['z', 'y']) {
+}
+      `,
+      errors: [{ messageId: 'inArrayViolation' }],
+    },
+    {
+      code: `
+const arr = [5, 6, 7, 8];
+
+const has_6 = 6 in arr;
+      `,
+      errors: [{ messageId: 'inArrayViolation' }],
+    },
+  ],
+});