noRedundantUseStrict

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/noRedundantUseStrict`](/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 [**warning**](/reference/diagnostics#warning). ## How to configure ```json title="biome.json" { "linter": { "rules": { "suspicious": { "noRedundantUseStrict": "error" } } } }

## Description
Prevents from having redundant `"use strict"`.

The directive `"use strict"` **isn't** needed in `.mjs` files, or in `.js` files inside projects where the `package.json` defines library as module:

```json
{
   "type": "module"
}

Instead, .cjs files are considered "scripts" and the directive "use strict" is accepted and advised.

Examples

Invalid

cjs

"use strict";
function foo() {
 	"use strict";
}

<pre class="language-text"><code class="language-text">code-block.cjs:3:3 <a href="https://biomejs.dev/linter/rules/no-redundant-use-strict">lint/suspicious/noRedundantUseStrict</a> FIXABLE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Redundant use strict directive. 1 │ "use strict"; 2 │ function foo() { > 3 │ "use strict"; │ ^^^^^^^^^^^^^ 4 │ } 5 │ ℹ This outer use strict directive already enables strict mode. > 1 │ "use strict"; │ ^^^^^^^^^^^^^ 2 │ function foo() { 3 │ "use strict"; ℹ Safe fix: Remove the redundant use strict directive. 1 1 │ "use strict"; 2 2 │ function foo() { 3 │ - ·→ "use·strict"; 3 │ + ·→ 4 4 │ } 5 5 │ </code></pre>

cjs

"use strict";
"use strict";

function foo() {

}

<pre class="language-text"><code class="language-text">code-block.cjs:2:1 <a href="https://biomejs.dev/linter/rules/no-redundant-use-strict">lint/suspicious/noRedundantUseStrict</a> FIXABLE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Redundant use strict directive. 1 │ "use strict"; > 2 │ "use strict"; │ ^^^^^^^^^^^^^ 3 │ 4 │ function foo() { ℹ This outer use strict directive already enables strict mode. > 1 │ "use strict"; │ ^^^^^^^^^^^^^ 2 │ "use strict"; 3 │ ℹ Safe fix: Remove the redundant use strict directive. 1 1 │ "use strict"; 2 │ - "use·strict"; 3 2 │ 3 │ + 4 4 │ function foo() { 5 5 │ </code></pre>

cjs

function foo() {
"use strict";
"use strict";
}

<pre class="language-text"><code class="language-text">code-block.cjs:3:1 <a href="https://biomejs.dev/linter/rules/no-redundant-use-strict">lint/suspicious/noRedundantUseStrict</a> FIXABLE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Redundant use strict directive. 1 │ function foo() { 2 │ "use strict"; > 3 │ "use strict"; │ ^^^^^^^^^^^^^ 4 │ } 5 │ ℹ This outer use strict directive already enables strict mode. 1 │ function foo() { > 2 │ "use strict"; │ ^^^^^^^^^^^^^ 3 │ "use strict"; 4 │ } ℹ Safe fix: Remove the redundant use strict directive. 1 1 │ function foo() { 2 2 │ "use strict"; 3 │ - "use·strict"; 3 │ + 4 4 │ } 5 5 │ </code></pre>

cjs

class C1 {
	test() {
		"use strict";
	}
}

<pre class="language-text"><code class="language-text">code-block.cjs:3:3 <a href="https://biomejs.dev/linter/rules/no-redundant-use-strict">lint/suspicious/noRedundantUseStrict</a> FIXABLE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Redundant use strict directive. 1 │ class C1 { 2 │ test() { > 3 │ "use strict"; │ ^^^^^^^^^^^^^ 4 │ } 5 │ } ℹ All parts of a class's body are already in strict mode. > 1 │ class C1 { │ ^^^^^^^^^^ > 2 │ test() { > 3 │ "use strict"; > 4 │ } > 5 │ } │ ^ 6 │ ℹ Safe fix: Remove the redundant use strict directive. 1 1 │ class C1 { 2 2 │ test() { 3 │ - → → "use·strict"; 4 │ - → } 3 │ + → → 4 │ + → } 5 5 │ } 6 6 │ </code></pre>

cjs

const C2 = class {
	test() {
		"use strict";
	}
};

<pre class="language-text"><code class="language-text">code-block.cjs:3:3 <a href="https://biomejs.dev/linter/rules/no-redundant-use-strict">lint/suspicious/noRedundantUseStrict</a> FIXABLE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Redundant use strict directive. 1 │ const C2 = class { 2 │ test() { > 3 │ "use strict"; │ ^^^^^^^^^^^^^ 4 │ } 5 │ }; ℹ All parts of a class's body are already in strict mode. > 1 │ const C2 = class { │ ^^^^^^^ > 2 │ test() { > 3 │ "use strict"; > 4 │ } > 5 │ }; │ ^ 6 │ ℹ Safe fix: Remove the redundant use strict directive. 1 1 │ const C2 = class { 2 2 │ test() { 3 │ - → → "use·strict"; 4 │ - → } 3 │ + → → 4 │ + → } 5 5 │ }; 6 6 │ </code></pre>

Valid

cjs

function foo() {

}

cjs

 function foo() {
    "use strict";
}
function bar() {
    "use strict";
}

</TabItem> </Tabs>

Examples

Invalid

Valid

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