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noSelfAssign

src/content/docs/linter/rules/no-self-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/correctness/noSelfAssign`](/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-self-assign`](https://eslint.org/docs/latest/rules/no-self-assign) - Same as [`self_assignment`](https://rust-lang.github.io/rust-clippy/master/#self_assignment)

How to configure

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

Description

Disallow assignments where both sides are exactly the same.

Self assignments have no effect, so probably those are an error due to incomplete refactoring.

Examples

Invalid

js
a = a;
<pre class="language-text"><code class="language-text">code-block.js:1:5 <a href="https://biomejs.dev/linter/rules/no-self-assign">lint/correctness/noSelfAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">a is assigned to itself.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>a = a; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This is where is assigned.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>a = a; <strong> │ </strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Self assignments have no effect and can be removed.</span> </code></pre>
js
[a] = [a];
<pre class="language-text"><code class="language-text">code-block.js:1:8 <a href="https://biomejs.dev/linter/rules/no-self-assign">lint/correctness/noSelfAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">a is assigned to itself.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>[a] = [a]; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This is where is assigned.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>[a] = [a]; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Self assignments have no effect and can be removed.</span> </code></pre>
js
({a: b} = {a: b});
<pre class="language-text"><code class="language-text">code-block.js:1:15 <a href="https://biomejs.dev/linter/rules/no-self-assign">lint/correctness/noSelfAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">b is assigned to itself.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>(&#123;a: b&#125; = &#123;a: b&#125;); <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This is where is assigned.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>(&#123;a: b&#125; = &#123;a: b&#125;); <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Self assignments have no effect and can be removed.</span> </code></pre>
js
a.b = a.b;
<pre class="language-text"><code class="language-text">code-block.js:1:9 <a href="https://biomejs.dev/linter/rules/no-self-assign">lint/correctness/noSelfAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">b is assigned to itself.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>a.b = a.b; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This is where is assigned.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>a.b = a.b; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Self assignments have no effect and can be removed.</span> </code></pre>
js
a[b] = a[b];
<pre class="language-text"><code class="language-text">code-block.js:1:10 <a href="https://biomejs.dev/linter/rules/no-self-assign">lint/correctness/noSelfAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">b is assigned to itself.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>a[b] = a[b]; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This is where is assigned.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>a[b] = a[b]; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Self assignments have no effect and can be removed.</span> </code></pre>
js
a[b].foo = a[b].foo;
<pre class="language-text"><code class="language-text">code-block.js:1:17 <a href="https://biomejs.dev/linter/rules/no-self-assign">lint/correctness/noSelfAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">foo is assigned to itself.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>a[b].foo = a[b].foo; <strong> │ </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;">This is where is assigned.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>a[b].foo = a[b].foo; <strong> │ </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;">Self assignments have no effect and can be removed.</span> </code></pre>
js
a['b'].foo = a['b'].foo;
<pre class="language-text"><code class="language-text">code-block.js:1:21 <a href="https://biomejs.dev/linter/rules/no-self-assign">lint/correctness/noSelfAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">foo is assigned to itself.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>a['b'].foo = a['b'].foo; <strong> │ </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;">This is where is assigned.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>a['b'].foo = a['b'].foo; <strong> │ </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;">Self assignments have no effect and can be removed.</span> </code></pre>

Valid

js
a &= a;
var a = a;
let a = a;
const a = a;
[a, b] = [b, a];
</TabItem> </Tabs>