# unicorn/prefer-array-flat

> Require `.flat()` instead of flattening an array with `concat`, `reduce` or `flatMap`

`concat`, `reduce` and `flatMap(x => x)` can all flatten an array of arrays, but you have to work out that that's what they're doing. `.flat()` says so.

`--fix` rewrites the `flatMap` and `reduce` forms to `.flat()`. `--fix-dangerously` also rewrites `[].concat.apply([], array)`. `[].concat(...array)` is reported, but you have to rewrite it yourself.

{/* Generated by src/codegen/lint-rule-docs.ts. Do not edit this section. */}

## Configure

Off by default. To turn it on:

```toml bunfig.toml
[lint.rules]
"unicorn/prefer-array-flat" = "error" # or "warn"
```

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## Examples

Reported:

{/* bad {"count": 5} */}
```js
const a = pages.flatMap(items => items);
const b = pages.reduce((all, items) => all.concat(items), []);
const c = pages.reduce((all, items) => [...all, ...items], []);
const d = [].concat(...pages);
const e = [].concat.apply([], pages);
```

Not reported:

{/* good */}
```js
const items = pages.flat();
```

A `flatMap` that changes the elements isn't reported:

{/* good */}
```js
const tags = posts.flatMap(post => post.tags);
```