---
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title: useSingleVarDeclarator
description: Learn more about useSingleVarDeclarator
---
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/style/useSingleVarDeclarator`](/reference/diagnostics#diagnostic-category)
- This rule isn't recommended, so you need to enable it.
- This rule has an [**unsafe**](/linter/#unsafe-fixes) fix.
- The default severity of this rule is [**information**](/reference/diagnostics#information).
- Sources: 
  - Same as [`one-var`](https://eslint.org/docs/latest/rules/one-var)

## How to configure
```json title="biome.json"
{
	"linter": {
		"rules": {
			"style": {
				"useSingleVarDeclarator": "error"
			}
		}
	}
}

```
## Description
Disallow multiple variable declarations in the same variable statement

In JavaScript, multiple variables can be declared within a single `var`, `const` or `let` declaration.
It is often considered a best practice to declare every variable separately.
That is what this rule enforces.

## Examples

### Invalid

```js
let foo = 0, bar, baz;
```

<pre class="language-text"><code class="language-text">code-block.js:1:1 <a href="https://biomejs.dev/linter/rules/use-single-var-declarator">lint/style/useSingleVarDeclarator</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Declare variables separately</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>let foo = 0, bar, baz;<br />   <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><br />    <strong>2 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Break out into multiple declarations</span><br />  <br />    <strong>1</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;">l</span><span style="color: Tomato;">e</span><span style="color: Tomato;">t</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">f</span><span style="color: Tomato;">o</span><span style="color: Tomato;">o</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">0</span><span style="color: Tomato;"><strong>,</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">b</span><span style="color: Tomato;">a</span><span style="color: Tomato;">r</span><span style="color: Tomato;"><strong>,</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">b</span><span style="color: Tomato;">a</span><span style="color: Tomato;">z</span><span style="color: Tomato;">;</span><br />      <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">l</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">0</span><span style="color: MediumSeaGreen;"><strong>;</strong></span><br />      <strong>2</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;"><strong>l</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>t</strong></span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">b</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">r</span><span style="color: MediumSeaGreen;"><strong>;</strong></span><br />      <strong>3</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;"><strong>l</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>t</strong></span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">b</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">z</span><span style="color: MediumSeaGreen;">;</span><br />    <strong>2</strong> <strong>4</strong><strong> │ </strong>  <br />  <br /></code></pre>

### Valid

```js
const foo = 0;
let bar;
let baz;
```

```js
for (let i = 0, x = 1; i < arr.length; i++) {}
```

## Related links

- [Disable a rule](/linter/#disable-a-rule)
- [Configure the code fix](/linter#configure-the-code-fix)
- [Rule options](/linter/#rule-options)
- [Source Code](https://github.com/biomejs/biome/blob/main/crates/biome_js_analyze/src/lint/style/use_single_var_declarator.rs)
- [Test Cases](https://github.com/biomejs/biome/blob/main/crates/biome_js_analyze/tests/specs/style/useSingleVarDeclarator)

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