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# no-array-concat-in-loop

[Added in v0.9.0](https://github.com/web-infra-dev/rslint/releases/tag/v0.9.0)
## Configuration

| Preset                              | Configured Value |
| ----------------------------------- | ---------------- |
| ✅ unicornPlugin.configs.recommended | `"error"`        |


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

export default defineConfig([
  unicornPlugin.configs.recommended,
  {
    rules: {
      'unicorn/no-array-concat-in-loop': 'error',
    },
  },
]);
```

## Rule Details

Disallows accumulating an array with `Array#concat()` inside a loop. Because
`concat()` creates a new array and copies the values accumulated so far, using
it on every iteration can make the loop quadratic in the total number of items.

This rule reports local `let` and `var` variables initialized to an empty
array literal. It does not try to infer arbitrary array-like values or custom
`concat()` methods.

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

```javascript
let result = [];

for (const chunk of chunks) {
  result = result.concat(chunk);
}
```

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

```javascript
const result = [];

for (const chunk of chunks) {
  result.push(...chunk);
}
```

```javascript
const result = chunks.flat();
```

```javascript
let result = [];

for (const chunk of chunks) {
  result = other.concat(chunk);
}
```

The rule does not provide an autofix. Replacing `concat()` with `push()` can
change observable behavior when the previous array is aliased, and the two
methods have different argument-spreading semantics.

## Original Documentation

- [eslint-plugin-unicorn: no-array-concat-in-loop](https://github.com/sindresorhus/eslint-plugin-unicorn/blob/v73.0.0/docs/rules/no-array-concat-in-loop.md)
- [Source code](https://github.com/sindresorhus/eslint-plugin-unicorn/blob/v73.0.0/rules/no-array-concat-in-loop.js)