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# no-for-in-array

[Added in v0.1.4](https://github.com/web-infra-dev/rslint/releases/tag/v0.1.4)
## Configuration

| Preset                              | Configured Value |
| ----------------------------------- | ---------------- |
| ✅ ts.configs.recommendedTypeChecked | `"error"`        |
| ✅ ts.configs.strictTypeChecked      | `"error"`        |


```ts title=rslint.config.ts
import { defineConfig, ts } from '@rslint/core';

export default defineConfig([
  ts.configs.recommended,
  {
    rules: {
      '@typescript-eslint/no-for-in-array': 'error',
    },
  },
]);
```

## Rule Details

Disallows iterating over arrays with a `for-in` loop. Using `for-in` on arrays is problematic because it skips holes, returns indices as strings rather than numbers, and may visit inherited enumerable properties from the prototype chain. Use `for-of`, `Array.prototype.forEach`, or a standard `for` loop instead.

Examples of **incorrect** code for this rule:

```typescript
const arr = [1, 2, 3];
for (const index in arr) {
  console.log(index); // "0", "1", "2" (strings, not numbers)
}

for (const key in ['a', 'b', 'c']) {
  console.log(key);
}
```

Examples of **correct** code for this rule:

```typescript
const arr = [1, 2, 3];
for (const value of arr) {
  console.log(value);
}

arr.forEach((value, index) => {
  console.log(index, value);
});

for (let i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}
```

## Original Documentation

- [typescript-eslint: no-for-in-array](https://typescript-eslint.io/rules/no-for-in-array)
- [Source code](https://github.com/typescript-eslint/typescript-eslint/blob/v8.67.0/packages/eslint-plugin/src/rules/no-for-in-array.ts)