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useFlatMap

src/content/docs/linter/rules/use-flat-map.mdx

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import { Tabs, TabItem } from '@astrojs/starlight/components';

<Tabs> <TabItem label="JavaScript (and super languages)" icon="seti:javascript"> ## Summary - Rule available since: `v1.0.0` - Diagnostic Category: [`lint/complexity/useFlatMap`](/reference/diagnostics#diagnostic-category) - This rule is **recommended**, meaning it is enabled by default. - This rule has a [**safe**](/linter/#safe-fixes) fix. - The default severity of this rule is [**information**](/reference/diagnostics#information). - Sources: - Same as [`unicorn/prefer-array-flat-map`](https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-array-flat-map.md) - Same as [`map_flatten`](https://rust-lang.github.io/rust-clippy/master/#map_flatten)

How to configure

json
{
	"linter": {
		"rules": {
			"complexity": {
				"useFlatMap": "error"
			}
		}
	}
}

Description

Promotes the use of .flatMap() when map().flat() are used together.

Examples

Invalid

js
const array = ["split", "the text", "into words"];
array.map(sentence => sentence.split(' ')).flat();
<pre class="language-text"><code class="language-text">code-block.js:2:1 <a href="https://biomejs.dev/linter/rules/use-flat-map">lint/complexity/useFlatMap</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">The call chain </span><span style="color: lightgreen;"><strong>.map().flat()</strong></span><span style="color: lightgreen;"> can be replaced with a single </span><span style="color: lightgreen;"><strong>.flatMap()</strong></span><span style="color: lightgreen;"> call.</span> <strong>1 │ </strong>const array = [&quot;split&quot;, &quot;the text&quot;, &quot;into words&quot;]; <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>array.map(sentence =&gt; sentence.split(' ')).flat(); <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><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>3 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Safe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Replace the chain with </span><span style="color: lightgreen;"><strong>.flatMap()</strong></span><span style="color: lightgreen;">.</span> <strong>1</strong> <strong>1</strong><strong> │ </strong> const array = [&quot;split&quot;, &quot;the text&quot;, &quot;into words&quot;]; <strong>2</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>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;">s</span><span style="color: Tomato;">e</span><span style="color: Tomato;">n</span><span style="color: Tomato;">t</span><span style="color: Tomato;">e</span><span style="color: Tomato;">n</span><span style="color: Tomato;">c</span><span style="color: Tomato;">e</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">=</span><span style="color: Tomato;">&gt;</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">s</span><span style="color: Tomato;">e</span><span style="color: Tomato;">n</span><span style="color: Tomato;">t</span><span style="color: Tomato;">e</span><span style="color: Tomato;">n</span><span style="color: Tomato;">c</span><span style="color: Tomato;">e</span><span style="color: Tomato;">.</span><span style="color: Tomato;">s</span><span style="color: Tomato;">p</span><span style="color: Tomato;">l</span><span style="color: Tomato;">i</span><span style="color: Tomato;">t</span><span style="color: Tomato;">(</span><span style="color: Tomato;">'</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">'</span><span style="color: Tomato;"><strong>)</strong></span><span style="color: Tomato;">)</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>(</strong></span><span style="color: Tomato;">)</span><span style="color: Tomato;">;</span> <strong>2</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;"><strong>M</strong></span><span style="color: MediumSeaGreen;"><strong>a</strong></span><span style="color: MediumSeaGreen;"><strong>p</strong></span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">c</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;">&gt;</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">c</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">.</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">p</span><span style="color: MediumSeaGreen;">l</span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">;</span> <strong>3</strong> <strong>3</strong><strong> │ </strong> </code></pre>
js
const array = ["split", "the text", "into words"];
array.map(sentence => sentence.split(' ')).flat(1);
<pre class="language-text"><code class="language-text">code-block.js:2:1 <a href="https://biomejs.dev/linter/rules/use-flat-map">lint/complexity/useFlatMap</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">The call chain </span><span style="color: lightgreen;"><strong>.map().flat()</strong></span><span style="color: lightgreen;"> can be replaced with a single </span><span style="color: lightgreen;"><strong>.flatMap()</strong></span><span style="color: lightgreen;"> call.</span> <strong>1 │ </strong>const array = [&quot;split&quot;, &quot;the text&quot;, &quot;into words&quot;]; <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>array.map(sentence =&gt; sentence.split(' ')).flat(1); <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><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>3 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Safe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Replace the chain with </span><span style="color: lightgreen;"><strong>.flatMap()</strong></span><span style="color: lightgreen;">.</span> <strong>1</strong> <strong>1</strong><strong> │ </strong> const array = [&quot;split&quot;, &quot;the text&quot;, &quot;into words&quot;]; <strong>2</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>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;">s</span><span style="color: Tomato;">e</span><span style="color: Tomato;">n</span><span style="color: Tomato;">t</span><span style="color: Tomato;">e</span><span style="color: Tomato;">n</span><span style="color: Tomato;">c</span><span style="color: Tomato;">e</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">=</span><span style="color: Tomato;">&gt;</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">s</span><span style="color: Tomato;">e</span><span style="color: Tomato;">n</span><span style="color: Tomato;">t</span><span style="color: Tomato;">e</span><span style="color: Tomato;">n</span><span style="color: Tomato;">c</span><span style="color: Tomato;">e</span><span style="color: Tomato;">.</span><span style="color: Tomato;">s</span><span style="color: Tomato;">p</span><span style="color: Tomato;">l</span><span style="color: Tomato;">i</span><span style="color: Tomato;">t</span><span style="color: Tomato;">(</span><span style="color: Tomato;">'</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">'</span><span style="color: Tomato;">)</span><span style="color: Tomato;"><strong>)</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>(</strong></span><span style="color: Tomato;"><strong>1</strong></span><span style="color: Tomato;">)</span><span style="color: Tomato;">;</span> <strong>2</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;"><strong>M</strong></span><span style="color: MediumSeaGreen;"><strong>a</strong></span><span style="color: MediumSeaGreen;"><strong>p</strong></span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">c</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;">&gt;</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">c</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">.</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">p</span><span style="color: MediumSeaGreen;">l</span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">;</span> <strong>3</strong> <strong>3</strong><strong> │ </strong> </code></pre>

Valid

js
const array = ["split", "the text", "into words"];
array.map(sentence => sentence.split(' ')).flat(2);
</TabItem> </Tabs>