noConstAssign

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/correctness/noConstAssign`](/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 [**error**](/reference/diagnostics#error). - Sources: - Same as [`no-const-assign`](https://eslint.org/docs/latest/rules/no-const-assign)

How to configure

json

{
	"linter": {
		"rules": {
			"correctness": {
				"noConstAssign": "error"
			}
		}
	}
}

Description

Prevents from having const variables being re-assigned.

Trying to assign a value to a const will cause an TypeError when the code is executed.

Examples

Invalid

const a = 1;
a = 4;

<pre class="language-text"><code class="language-text">code-block.js:2:1 <a href="https://biomejs.dev/linter/rules/no-const-assign">lint/correctness/noConstAssign</a> FIXABLE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ✖ Can't assign a because it's a constant. 1 │ const a = 1; > 2 │ a = 4; │ ^ 3 │ ℹ This is where the variable is defined as constant. > 1 │ const a = 1; │ ^ 2 │ a = 4; 3 │ ℹ Unsafe fix: Replace const with let if you assign it to a new value. 1 │ - const·a·=·1; 1 │ + let·a·=·1; 2 2 │ a = 4; 3 3 │ </code></pre>

const a = 2;
a += 1;

<pre class="language-text"><code class="language-text">code-block.js:2:1 <a href="https://biomejs.dev/linter/rules/no-const-assign">lint/correctness/noConstAssign</a> FIXABLE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ✖ Can't assign a because it's a constant. 1 │ const a = 2; > 2 │ a += 1; │ ^ 3 │ ℹ This is where the variable is defined as constant. > 1 │ const a = 2; │ ^ 2 │ a += 1; 3 │ ℹ Unsafe fix: Replace const with let if you assign it to a new value. 1 │ - const·a·=·2; 1 │ + let·a·=·2; 2 2 │ a += 1; 3 3 │ </code></pre>

const a = 1;
++a;

<pre class="language-text"><code class="language-text">code-block.js:2:3 <a href="https://biomejs.dev/linter/rules/no-const-assign">lint/correctness/noConstAssign</a> FIXABLE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ✖ Can't assign a because it's a constant. 1 │ const a = 1; > 2 │ ++a; │ ^ 3 │ ℹ This is where the variable is defined as constant. > 1 │ const a = 1; │ ^ 2 │ ++a; 3 │ ℹ Unsafe fix: Replace const with let if you assign it to a new value. 1 │ - const·a·=·1; 1 │ + let·a·=·1; 2 2 │ ++a; 3 3 │ </code></pre>

const a = 1, b = 2;

a = 2;

<pre class="language-text"><code class="language-text">code-block.js:3:1 <a href="https://biomejs.dev/linter/rules/no-const-assign">lint/correctness/noConstAssign</a> FIXABLE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ✖ Can't assign a because it's a constant. 1 │ const a = 1, b = 2; 2 │ > 3 │ a = 2; │ ^ 4 │ ℹ This is where the variable is defined as constant. > 1 │ const a = 1, b = 2; │ ^ 2 │ 3 │ a = 2; ℹ Unsafe fix: Replace const with let if you assign it to a new value. 1 │ - const·a·=·1,·b·=·2; 1 │ + let·a·=·1,·b·=·2; 2 2 │ 3 3 │ a = 2; </code></pre>

Valid

const a = 10;
let b = 10;
b = 20;

</TabItem> </Tabs>

How to configure

Description

Examples

Invalid

Valid

Related links