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# no-loop-func

[Added in v0.5.0](https://github.com/web-infra-dev/rslint/releases/tag/v0.5.0)
## Configuration


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

export default defineConfig([
  js.configs.recommended,
  {
    rules: {
      'no-loop-func': 'error',
    },
  },
]);
```

Disallow function declarations that contain unsafe references inside loop statements.

## Rule Details

Writing functions within loops tends to result in errors due to the way the function creates a closure around the loop. For example:

```javascript
for (var i = 10; i; i--) {
    (function() { return i; })();
}
```

Generally speaking, it is safer to keep the closure code outside of the loop, or to use `let` / `const` for loop variables so each iteration produces a fresh binding.

This rule disallows any function within a loop that contains unsafe references (e.g. to modified variables).

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

```javascript
for (var i = 0; i < 10; i++) {
    funcs[i] = function() { return i; };
}

for (var i = 0; i < 10; i++) {
    funcs[i] = () => i;
}

for (var i = 0; i < 10; i++) {
    funcs[i] = function() { return i; };
    funcs[i]();
}

var foo = 100;
for (var i = 0; i < 10; i++) {
    funcs[i] = function() { return foo; };
    foo += 1;
}

var foo = 100;
check(function() { return foo; });
foo = 200;
```

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

```javascript
var a = function() {};

for (var i = 0; i < 10; i++) {
    funcs[i] = a;
}

for (let i = 0; i < 10; i++) {
    funcs[i] = function() { return i; };
}

const foo = 100;
for (var i = 0; i < 10; i++) {
    funcs[i] = function() { return foo; };
}

// IIFEs are fine because they execute immediately.
for (var i = 0; i < 10; i++) {
    funcs[i] = (function() { return i; })();
}
```

## Original Documentation

- [ESLint: no-loop-func](https://eslint.org/docs/latest/rules/no-loop-func)
- [Source code](https://github.com/eslint/eslint/blob/v10.8.1/lib/rules/no-loop-func.js)