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noCatchAssign

src/content/docs/linter/rules/no-catch-assign.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/noCatchAssign`](/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 [**warning**](/reference/diagnostics#warning). - Sources: - Same as [`no-ex-assign`](https://eslint.org/docs/latest/rules/no-ex-assign)

How to configure

json
{
	"linter": {
		"rules": {
			"suspicious": {
				"noCatchAssign": "error"
			}
		}
	}
}

Description

Disallow reassigning exceptions in catch clauses.

Assignment to a catch parameter can be misleading and confusing. It is often unintended and indicative of a programmer error.

Examples

Invalid

js
try {

} catch (e) {
  e;
  e = 10;
}
<pre class="language-text"><code class="language-text">code-block.js:5:3 <a href="https://biomejs.dev/linter/rules/no-catch-assign">lint/suspicious/noCatchAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Orange;">⚠</span></strong> <span style="color: Orange;">Reassigning a </span><span style="color: Orange;"><strong>catch parameter</strong></span><span style="color: Orange;"> is confusing.</span> <strong>3 │ </strong>&#125; catch (e) &#123; <strong>4 │ </strong> e; <strong><span style="color: Tomato;">&gt;</span></strong> <strong>5 │ </strong> e = 10; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>6 │ </strong>&#125; <strong>7 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">The </span><span style="color: lightgreen;"><strong>catch parameter</strong></span><span style="color: lightgreen;"> is declared here:</span> <strong>1 │ </strong>try &#123; <strong>2 │ </strong> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>3 │ </strong>&#125; catch (e) &#123; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>4 │ </strong> e; <strong>5 │ </strong> e = 10; <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Use a local variable instead.</span> </code></pre>

Valid

js
try {

} catch (e) {
  let e = 10;
  e = 100;
}
</TabItem> </Tabs>