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noFunctionAssign

src/content/docs/linter/rules/no-function-assign.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/suspicious/noFunctionAssign`](/reference/diagnostics#diagnostic-category) - This rule is **recommended**, meaning it is enabled by default. - This rule doesn't have a fix. - The default severity of this rule is [**error**](/reference/diagnostics#error). - Sources: - Same as [`no-func-assign`](https://eslint.org/docs/latest/rules/no-func-assign)

How to configure

json
{
	"linter": {
		"rules": {
			"suspicious": {
				"noFunctionAssign": "error"
			}
		}
	}
}

Description

Disallow reassigning function declarations.

Examples

Invalid

js
function foo() { };
foo = bar;
<pre class="language-text"><code class="language-text">code-block.js:1:10 <a href="https://biomejs.dev/linter/rules/no-function-assign">lint/suspicious/noFunctionAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Do not reassign a function declaration.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>function foo() &#123; &#125;; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong>foo = bar; <strong>3 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Reassigned here.</span> <strong>1 │ </strong>function foo() &#123; &#125;; <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>foo = bar; <strong> │ </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;">Use a local variable instead.</span> </code></pre>
js
function foo() {
    foo = bar;
 }
<pre class="language-text"><code class="language-text">code-block.js:1:10 <a href="https://biomejs.dev/linter/rules/no-function-assign">lint/suspicious/noFunctionAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Do not reassign a function declaration.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>function foo() &#123; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> foo = bar; <strong>3 │ </strong> &#125; <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Reassigned here.</span> <strong>1 │ </strong>function foo() &#123; <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong> foo = bar; <strong> │ </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;">Use a local variable instead.</span> </code></pre>
js
foo = bar;
function foo() { };
<pre class="language-text"><code class="language-text">code-block.js:2:10 <a href="https://biomejs.dev/linter/rules/no-function-assign">lint/suspicious/noFunctionAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Do not reassign a function declaration.</span> <strong>1 │ </strong>foo = bar; <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>function foo() &#123; &#125;; <strong> │ </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;">Reassigned here.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>foo = bar; <strong> │ </strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong>function foo() &#123; &#125;; <strong>3 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Reassignment happens here because the function declaration is hoisted.</span> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Use a local variable instead.</span> </code></pre>
js
[foo] = bar;
function foo() { };
<pre class="language-text"><code class="language-text">code-block.js:2:10 <a href="https://biomejs.dev/linter/rules/no-function-assign">lint/suspicious/noFunctionAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Do not reassign a function declaration.</span> <strong>1 │ </strong>[foo] = bar; <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>function foo() &#123; &#125;; <strong> │ </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;">Reassigned here.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>[foo] = bar; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong>function foo() &#123; &#125;; <strong>3 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Reassignment happens here because the function declaration is hoisted.</span> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Use a local variable instead.</span> </code></pre>
js
({ x: foo = 0 } = bar);
function foo() { };
<pre class="language-text"><code class="language-text">code-block.js:2:10 <a href="https://biomejs.dev/linter/rules/no-function-assign">lint/suspicious/noFunctionAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Do not reassign a function declaration.</span> <strong>1 │ </strong>(&#123; x: foo = 0 &#125; = bar); <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>function foo() &#123; &#125;; <strong> │ </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;">Reassigned here.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>(&#123; x: foo = 0 &#125; = bar); <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong>function foo() &#123; &#125;; <strong>3 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Reassignment happens here because the function declaration is hoisted.</span> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Use a local variable instead.</span> </code></pre>
js
function foo() {
    [foo] = bar;
 }
<pre class="language-text"><code class="language-text">code-block.js:1:10 <a href="https://biomejs.dev/linter/rules/no-function-assign">lint/suspicious/noFunctionAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Do not reassign a function declaration.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>function foo() &#123; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> [foo] = bar; <strong>3 │ </strong> &#125; <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Reassigned here.</span> <strong>1 │ </strong>function foo() &#123; <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong> [foo] = bar; <strong> │ </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;">Use a local variable instead.</span> </code></pre>
js
(function () {
    ({ x: foo = 0 } = bar);
    function foo() { };
 })();
<pre class="language-text"><code class="language-text">code-block.js:3:14 <a href="https://biomejs.dev/linter/rules/no-function-assign">lint/suspicious/noFunctionAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Do not reassign a function declaration.</span> <strong>1 │ </strong>(function () &#123; <strong>2 │ </strong> (&#123; x: foo = 0 &#125; = bar); <strong><span style="color: Tomato;">&gt;</span></strong> <strong>3 │ </strong> function foo() &#123; &#125;; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>4 │ </strong> &#125;)(); <strong>5 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Reassigned here.</span> <strong>1 │ </strong>(function () &#123; <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong> (&#123; x: foo = 0 &#125; = bar); <strong> │ </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> function foo() &#123; &#125;; <strong>4 │ </strong> &#125;)(); <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Reassignment happens here because the function declaration is hoisted.</span> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Use a local variable instead.</span> </code></pre>

Valid

js
function foo() {
    var foo = bar;
 }
js
function foo(foo) {
    foo = bar;
 }
js
function foo() {
    var foo;
    foo = bar;
 }
js
var foo = () => {};
foo = bar;
js
var foo = function() {};
foo = bar;
js
var foo = function() {
    foo = bar;
 };
js
import bar from 'bar';
function foo() {
    var foo = bar;
}
</TabItem> </Tabs>