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noReturnAssign

src/content/docs/linter/rules/no-return-assign.mdx

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

<Tabs> <TabItem label="JavaScript (and super languages)" icon="seti:javascript"> :::caution This rule is part of the [nursery](/linter/#nursery) group. This means that it is experimental and the behavior can change at any time. ::: ## Summary - Rule available since: `v2.3.11` - Diagnostic Category: [`lint/nursery/noReturnAssign`](/reference/diagnostics#diagnostic-category) - This rule doesn't have a fix. - The default severity of this rule is [**error**](/reference/diagnostics#error). - Sources: - Same as [`no-return-assign`](https://eslint.org/docs/latest/rules/no-return-assign)

How to configure

json
{
	"linter": {
		"rules": {
			"nursery": {
				"noReturnAssign": "error"
			}
		}
	}
}

Description

Disallow assignments in return statements.

In return statements, it is common to mistype a comparison operator (such as ==) as an assignment operator (such as =). Moreover, the use of assignments in a return statement is confusing. Return statements are often considered side-effect free.

Examples

Invalid

js
function f(a) {
    return a = 1;
}
<pre class="language-text"><code class="language-text">code-block.js:2:12 <a href="https://biomejs.dev/linter/rules/no-return-assign">lint/nursery/noReturnAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;"><strong>Return statements</strong></span><span style="color: Tomato;"> should not contain </span><span style="color: Tomato;"><strong>assignments</strong></span><span style="color: Tomato;">.</span> <strong>1 │ </strong>function f(a) &#123; <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong> return a = 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>3 │ </strong>&#125; <strong>4 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Assignments inside return statements are easy to mistake for comparison operators (&#96;==&#96;), and add unexpected side effects to normally-pure code. </span> <span style="color: lightgreen;">If the assignment is intentional, move it outside of the return statement.</span> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit </span><span style="color: lightgreen;"><a href="https://biomejs.dev/linter/#nursery">https://biomejs.dev/linter/#nursery</a></span><span style="color: lightgreen;"> for more information.</span> </code></pre>

Valid

js
function f(a) {
    a = 1;
    return a;
}
js
function f(a) {
    return a == 1;
}
</TabItem> </Tabs>