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noConstantCondition

src/content/docs/linter/rules/no-constant-condition.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/correctness/noConstantCondition`](/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-constant-condition`](https://eslint.org/docs/latest/rules/no-constant-condition)

How to configure

json
{
	"linter": {
		"rules": {
			"correctness": {
				"noConstantCondition": "error"
			}
		}
	}
}

Description

Disallow constant expressions in conditions

Examples

Invalid

js
if (false) {
    doSomethingUnfinished();
}
<pre class="language-text"><code class="language-text">code-block.js:1:5 <a href="https://biomejs.dev/linter/rules/no-constant-condition">lint/correctness/noConstantCondition</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Unexpected constant condition.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>if (false) &#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><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> doSomethingUnfinished(); <strong>3 │ </strong>&#125; </code></pre>
js
if (Boolean(1)) {
    doSomethingAlways();
}
<pre class="language-text"><code class="language-text">code-block.js:1:5 <a href="https://biomejs.dev/linter/rules/no-constant-condition">lint/correctness/noConstantCondition</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Unexpected constant condition.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>if (Boolean(1)) &#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><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> doSomethingAlways(); <strong>3 │ </strong>&#125; </code></pre>
js
if (undefined) {
    doSomethingUnfinished();
}
<pre class="language-text"><code class="language-text">code-block.js:1:5 <a href="https://biomejs.dev/linter/rules/no-constant-condition">lint/correctness/noConstantCondition</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Unexpected constant condition.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>if (undefined) &#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><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> doSomethingUnfinished(); <strong>3 │ </strong>&#125; </code></pre>
js
for (;-2;) {
    doSomethingForever();
}
<pre class="language-text"><code class="language-text">code-block.js:1:7 <a href="https://biomejs.dev/linter/rules/no-constant-condition">lint/correctness/noConstantCondition</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Unexpected constant condition.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>for (;-2;) &#123; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> doSomethingForever(); <strong>3 │ </strong>&#125; </code></pre>
js
while (typeof x) {
    doSomethingForever();
}
<pre class="language-text"><code class="language-text">code-block.js:1:8 <a href="https://biomejs.dev/linter/rules/no-constant-condition">lint/correctness/noConstantCondition</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Unexpected constant condition.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>while (typeof x) &#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><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> doSomethingForever(); <strong>3 │ </strong>&#125; </code></pre>
js
var result = 0 ? a : b;
<pre class="language-text"><code class="language-text">code-block.js:1:14 <a href="https://biomejs.dev/linter/rules/no-constant-condition">lint/correctness/noConstantCondition</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">Unexpected constant condition.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>var result = 0 ? a : b; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> </code></pre>

Valid

js
if (x === 0) {
    doSomething();
}

for (;;) {
    doSomethingForever();
}

while (typeof x === "undefined") {
    doSomething();
}

do {
    doSomething();
} while (x);

var result = x !== 0 ? a : b;

// Exception
while (true) {
    if (x) { break; }
    x = f();
}
</TabItem> </Tabs>