---
# Don't modify this file manually. This file is auto generated from source, and you will lose your changes next time the website is built.
# Head to the `biomejs/biome` repository, and modify the source code in there.

title: noFlatMapIdentity
description: Learn more about noFlatMapIdentity
---
import { Tabs, TabItem } from '@astrojs/starlight/components';

<Tabs>
<TabItem label="JavaScript (and super languages)" icon="seti:javascript">
## Summary
- Rule available since: `v1.7.0`
- Diagnostic Category: [`lint/complexity/noFlatMapIdentity`](/reference/diagnostics#diagnostic-category)
- This rule is **recommended**, meaning it is enabled by default.
- This rule has an [**unsafe**](/linter/#unsafe-fixes) fix.
- The default severity of this rule is [**information**](/reference/diagnostics#information).
- Sources: 
  - Same as [`flat_map_identity`](https://rust-lang.github.io/rust-clippy/master/#flat_map_identity)

## How to configure
```json title="biome.json"
{
	"linter": {
		"rules": {
			"complexity": {
				"noFlatMapIdentity": "error"
			}
		}
	}
}

```
## Description
Disallow to use unnecessary callback on `flatMap`.

To achieve the same result (flattening an array) more concisely and efficiently, you should use `flat` instead.

## Examples

### Invalid

```js
array.flatMap((arr) => arr);
```

<pre class="language-text"><code class="language-text">code-block.js:1:1 <a href="https://biomejs.dev/linter/rules/no-flat-map-identity">lint/complexity/noFlatMapIdentity</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Avoid unnecessary callback in </span><span style="color: lightgreen;"><strong>flatMap</strong></span><span style="color: lightgreen;"> call.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>array.flatMap((arr) =&gt; arr);<br />   <strong>   │ </strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>2 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">You can just use </span><span style="color: lightgreen;"><strong>flat</strong></span><span style="color: lightgreen;"> to flatten the array.</span><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Replace unnecessary </span><span style="color: lightgreen;"><strong>flatMap</strong></span><span style="color: lightgreen;"> call to </span><span style="color: lightgreen;"><strong>flat</strong></span><span style="color: lightgreen;"> instead.</span><br />  <br />    <strong>1</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>y</strong></span><span style="color: Tomato;"><strong>.</strong></span><span style="color: Tomato;"><strong>f</strong></span><span style="color: Tomato;"><strong>l</strong></span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>t</strong></span><span style="color: Tomato;"><strong>M</strong></span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>p</strong></span><span style="color: Tomato;"><strong>(</strong></span><span style="color: Tomato;">(</span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>)</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>&gt;</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;">)</span><span style="color: Tomato;">;</span><br />      <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;"><strong>a</strong></span><span style="color: MediumSeaGreen;"><strong>r</strong></span><span style="color: MediumSeaGreen;"><strong>r</strong></span><span style="color: MediumSeaGreen;"><strong>a</strong></span><span style="color: MediumSeaGreen;"><strong>y</strong></span><span style="color: MediumSeaGreen;"><strong>.</strong></span><span style="color: MediumSeaGreen;"><strong>f</strong></span><span style="color: MediumSeaGreen;"><strong>l</strong></span><span style="color: MediumSeaGreen;"><strong>a</strong></span><span style="color: MediumSeaGreen;"><strong>t</strong></span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">;</span><br />    <strong>2</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

```js
array.flatMap((arr) => {return arr});
```

<pre class="language-text"><code class="language-text">code-block.js:1:1 <a href="https://biomejs.dev/linter/rules/no-flat-map-identity">lint/complexity/noFlatMapIdentity</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Avoid unnecessary callback in </span><span style="color: lightgreen;"><strong>flatMap</strong></span><span style="color: lightgreen;"> call.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>array.flatMap((arr) =&gt; &#123;return arr&#125;);<br />   <strong>   │ </strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>2 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">You can just use </span><span style="color: lightgreen;"><strong>flat</strong></span><span style="color: lightgreen;"> to flatten the array.</span><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Replace unnecessary </span><span style="color: lightgreen;"><strong>flatMap</strong></span><span style="color: lightgreen;"> call to </span><span style="color: lightgreen;"><strong>flat</strong></span><span style="color: lightgreen;"> instead.</span><br />  <br />    <strong>1</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>y</strong></span><span style="color: Tomato;"><strong>.</strong></span><span style="color: Tomato;"><strong>f</strong></span><span style="color: Tomato;"><strong>l</strong></span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>t</strong></span><span style="color: Tomato;"><strong>M</strong></span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>p</strong></span><span style="color: Tomato;">(</span><span style="color: Tomato;"><strong>(</strong></span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>)</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>&gt;</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>&#123;</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>t</strong></span><span style="color: Tomato;"><strong>u</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>&#125;</strong></span><span style="color: Tomato;">)</span><span style="color: Tomato;">;</span><br />      <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;"><strong>a</strong></span><span style="color: MediumSeaGreen;"><strong>r</strong></span><span style="color: MediumSeaGreen;"><strong>r</strong></span><span style="color: MediumSeaGreen;"><strong>a</strong></span><span style="color: MediumSeaGreen;"><strong>y</strong></span><span style="color: MediumSeaGreen;"><strong>.</strong></span><span style="color: MediumSeaGreen;"><strong>f</strong></span><span style="color: MediumSeaGreen;"><strong>l</strong></span><span style="color: MediumSeaGreen;"><strong>a</strong></span><span style="color: MediumSeaGreen;"><strong>t</strong></span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">;</span><br />    <strong>2</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

### Valid

```js
array.flatMap((arr) => arr * 2);
```

## Related links

- [Disable a rule](/linter/#disable-a-rule)
- [Configure the code fix](/linter#configure-the-code-fix)
- [Rule options](/linter/#rule-options)
- [Source Code](https://github.com/biomejs/biome/blob/main/crates/biome_js_analyze/src/lint/complexity/no_flat_map_identity.rs)
- [Test Cases](https://github.com/biomejs/biome/blob/main/crates/biome_js_analyze/tests/specs/complexity/noFlatMapIdentity)

</TabItem>
</Tabs>