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noUnsafeNegation

src/content/docs/linter/rules/no-unsafe-negation.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/noUnsafeNegation`](/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-unsafe-negation`](https://eslint.org/docs/latest/rules/no-unsafe-negation)

How to configure

json

{
	"linter": {
		"rules": {
			"suspicious": {
				"noUnsafeNegation": "error"
			}
		}
	}
}

Description

Disallow using unsafe negation.

Examples

Invalid

js

!1 in [1,2];

<pre class="language-text"><code class="language-text">code-block.js:1:1 <a href="https://biomejs.dev/linter/rules/no-unsafe-negation">lint/suspicious/noUnsafeNegation</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">The negation operator is used unsafely on the left side of this binary expression.</span> <strong><span style="color: Tomato;">></span></strong> <strong>1 │ </strong>!1 in [1,2]; <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><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><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Wrap the expression with a parenthesis</span> <strong> 1 │ </strong>!<span style="color: MediumSeaGreen;">(</span>1<span style="opacity: 0.8;">·</span>in<span style="opacity: 0.8;">·</span>[1,2]<span style="color: MediumSeaGreen;">)</span>; <strong> │ </strong> <span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">+</span> </code></pre>

js

/**test*/!/** test*/1 instanceof [1,2];

<pre class="language-text"><code class="language-text">code-block.js:1:10 <a href="https://biomejs.dev/linter/rules/no-unsafe-negation">lint/suspicious/noUnsafeNegation</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">The negation operator is used unsafely on the left side of this binary expression.</span> <strong><span style="color: Tomato;">></span></strong> <strong>1 │ </strong>/**test*/!/** test*/1 instanceof [1,2]; <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><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><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><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><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><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>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Wrap the expression with a parenthesis</span> <strong> 1 │ </strong>/**test*/!/**<span style="opacity: 0.8;">·</span>test*/<span style="color: MediumSeaGreen;">(</span>1<span style="opacity: 0.8;">·</span>instanceof<span style="opacity: 0.8;">·</span>[1,2]<span style="color: MediumSeaGreen;">)</span>; <strong> │ </strong> <span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">+</span> </code></pre>

Valid

js

-1 in [1,2];
~1 in [1,2];
typeof 1 in [1,2];
void 1 in [1,2];
delete 1 in [1,2];
+1 instanceof [1,2];

</TabItem> </Tabs>