useStrictMode

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

<Tabs> <TabItem label="JavaScript (and super languages)" icon="seti:javascript"> ## Summary - Rule available since: `v1.8.0` - Diagnostic Category: [`lint/suspicious/useStrictMode`](/reference/diagnostics#diagnostic-category) - This rule isn't recommended, so you need to enable it. - 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": { "useStrictMode": "error" } } } }

## Description
Enforce the use of the directive `"use strict"` in script files.

The JavaScript [strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode) prohibits some obsolete JavaScript syntaxes and makes some slight semantic changes to allow more optimizations by JavaScript engines.
EcmaScript modules are always in strict mode, while JavaScript scripts are by default in non-strict mode, also known as _sloppy mode_.
A developer can add the `"use strict"` directive at the start of a script file to enable the strict mode in that file.

Biome considers a CommonJS (`.cjs`) file as a script file.
By default, Biome recognizes a JavaScript file (`.js`) as a module file, except if `"type": "commonjs"` is specified in `package.json`.

## Examples

### Invalid

```cjs
var a = 1;

<pre class="language-text"><code class="language-text">code-block.cjs:1:1 <a href="https://biomejs.dev/linter/rules/use-strict-mode">lint/suspicious/useStrictMode</a> FIXABLE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Unexpected absence of the directive "use strict". > 1 │ var a = 1; │ ^^^^^^^^^^ 2 │ ℹ Strict mode allows to opt-in some optimisations of the runtime engines, and it eliminates some JavaScript silent errors by changing them to throw errors. ℹ Check the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode">MDN web docs</a> for more information regarding strict mode. ℹ Safe fix: Insert a top level "use strict". 1 │ + "use·strict"; 1 2 │ var a = 1; 2 3 │ </code></pre>

Valid

cjs

"use strict";

var a = 1;

</TabItem> </Tabs>

Valid

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