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title: noConfusingVoidType
description: Learn more about noConfusingVoidType
---
import { Tabs, TabItem } from '@astrojs/starlight/components';

<Tabs>
<TabItem label="TypeScript and TSX" icon="seti:typescript">
## Summary
- Rule available since: `v1.2.0`
- Diagnostic Category: [`lint/suspicious/noConfusingVoidType`](/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 [**warning**](/reference/diagnostics#warning).
- Sources: 
  - Same as [`@typescript-eslint/no-invalid-void-type`](https://typescript-eslint.io/rules/no-invalid-void-type)

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

```
## Description
Disallow `void` type outside of generic or return types.

`void` in TypeScript refers to a function return that is meant to be ignored.
Attempting to use a void type outside of a return type or a type parameter is often a sign of programmer error.
`void` can also be misleading for other developers even if used correctly.

>The `void` type means cannot be mixed with any other types, other than `never`, which accepts all types.
If you think you need this then you probably want the `undefined` type instead.


The code action suggests using `undefined` instead of `void`.
It is unsafe because a variable with the `void` type cannot be assigned to a variable with the `undefined` type.

## Examples

### Invalid

```ts
let foo: void;
```

<pre class="language-text"><code class="language-text">code-block.ts:1:10 <a href="https://biomejs.dev/linter/rules/no-confusing-void-type">lint/suspicious/noConfusingVoidType</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: Orange;">⚠</span></strong> <span style="color: Orange;"><strong>void</strong></span><span style="color: Orange;"> is confusing outside a return type or a type parameter.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>let foo: void;<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><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;">Use </span><span style="color: lightgreen;"><strong>undefined</strong></span><span style="color: lightgreen;"> instead.</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><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;"><strong>v</strong></span><span style="color: Tomato;"><strong>o</strong></span><span style="color: Tomato;"><strong>i</strong></span><span style="color: Tomato;"><strong>d</strong></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><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;"><strong>u</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>f</strong></span><span style="color: MediumSeaGreen;"><strong>i</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;">;</span><br />    <strong>2</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

```ts
function logSomething(thing: void) {}
```

<pre class="language-text"><code class="language-text">code-block.ts:1:30 <a href="https://biomejs.dev/linter/rules/no-confusing-void-type">lint/suspicious/noConfusingVoidType</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: Orange;">⚠</span></strong> <span style="color: Orange;"><strong>void</strong></span><span style="color: Orange;"> is confusing outside a return type or a type parameter.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>function logSomething(thing: void) &#123;&#125;<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><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;">Use </span><span style="color: lightgreen;"><strong>undefined</strong></span><span style="color: lightgreen;"> instead.</span><br />  <br />    <strong>1</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;">f</span><span style="color: Tomato;">u</span><span style="color: Tomato;">n</span><span style="color: Tomato;">c</span><span style="color: Tomato;">t</span><span style="color: Tomato;">i</span><span style="color: Tomato;">o</span><span style="color: Tomato;">n</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">l</span><span style="color: Tomato;">o</span><span style="color: Tomato;">g</span><span style="color: Tomato;">S</span><span style="color: Tomato;">o</span><span style="color: Tomato;">m</span><span style="color: Tomato;">e</span><span style="color: Tomato;">t</span><span style="color: Tomato;">h</span><span style="color: Tomato;">i</span><span style="color: Tomato;">n</span><span style="color: Tomato;">g</span><span style="color: Tomato;">(</span><span style="color: Tomato;">t</span><span style="color: Tomato;">h</span><span style="color: Tomato;">i</span><span style="color: Tomato;">n</span><span style="color: Tomato;">g</span><span style="color: Tomato;">:</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;"><strong>v</strong></span><span style="color: Tomato;"><strong>o</strong></span><span style="color: Tomato;"><strong>i</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;">)</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">&#123;</span><span style="color: Tomato;">&#125;</span><br />      <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">u</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">c</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">l</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">g</span><span style="color: MediumSeaGreen;">S</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">m</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;">h</span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">g</span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;">h</span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">g</span><span style="color: MediumSeaGreen;">:</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;"><strong>u</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>f</strong></span><span style="color: MediumSeaGreen;"><strong>i</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">&#123;</span><span style="color: MediumSeaGreen;">&#125;</span><br />    <strong>2</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

```ts
interface Interface {
    prop: void;
}
```

<pre class="language-text"><code class="language-text">code-block.ts:2:11 <a href="https://biomejs.dev/linter/rules/no-confusing-void-type">lint/suspicious/noConfusingVoidType</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: Orange;">⚠</span></strong> <span style="color: Orange;"><strong>void</strong></span><span style="color: Orange;"> is confusing outside a return type or a type parameter.</span><br />  <br />    <strong>1 │ </strong>interface Interface &#123;<br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>    prop: void;<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><br />    <strong>3 │ </strong>&#125;<br />    <strong>4 │ </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;">Use </span><span style="color: lightgreen;"><strong>undefined</strong></span><span style="color: lightgreen;"> instead.</span><br />  <br />    <strong>1</strong> <strong>1</strong><strong> │ </strong>  interface Interface &#123;<br />    <strong>2</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">p</span><span style="color: Tomato;">r</span><span style="color: Tomato;">o</span><span style="color: Tomato;">p</span><span style="color: Tomato;">:</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;"><strong>v</strong></span><span style="color: Tomato;"><strong>o</strong></span><span style="color: Tomato;"><strong>i</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;">;</span><br />      <strong>2</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">p</span><span style="color: MediumSeaGreen;">r</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">p</span><span style="color: MediumSeaGreen;">:</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;"><strong>u</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>f</strong></span><span style="color: MediumSeaGreen;"><strong>i</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;">;</span><br />    <strong>3</strong> <strong>3</strong><strong> │ </strong>  &#125;<br />    <strong>4</strong> <strong>4</strong><strong> │ </strong>  <br />  <br /></code></pre>

```ts
type PossibleValues = number | void;
```

<pre class="language-text"><code class="language-text">code-block.ts:1:32 <a href="https://biomejs.dev/linter/rules/no-confusing-void-type">lint/suspicious/noConfusingVoidType</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: Orange;">⚠</span></strong> <span style="color: Orange;"><strong>void</strong></span><span style="color: Orange;"> is confusing inside a union type.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>type PossibleValues = number | void;<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><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;">Use </span><span style="color: lightgreen;"><strong>undefined</strong></span><span style="color: lightgreen;"> instead.</span><br />  <br />    <strong>1</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;">t</span><span style="color: Tomato;">y</span><span style="color: Tomato;">p</span><span style="color: Tomato;">e</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">P</span><span style="color: Tomato;">o</span><span style="color: Tomato;">s</span><span style="color: Tomato;">s</span><span style="color: Tomato;">i</span><span style="color: Tomato;">b</span><span style="color: Tomato;">l</span><span style="color: Tomato;">e</span><span style="color: Tomato;">V</span><span style="color: Tomato;">a</span><span style="color: Tomato;">l</span><span style="color: Tomato;">u</span><span style="color: Tomato;">e</span><span style="color: Tomato;">s</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;">n</span><span style="color: Tomato;">u</span><span style="color: Tomato;">m</span><span style="color: Tomato;">b</span><span style="color: Tomato;">e</span><span style="color: Tomato;">r</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;"><strong>v</strong></span><span style="color: Tomato;"><strong>o</strong></span><span style="color: Tomato;"><strong>i</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;">;</span><br />      <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;">y</span><span style="color: MediumSeaGreen;">p</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">P</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">b</span><span style="color: MediumSeaGreen;">l</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">V</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">l</span><span style="color: MediumSeaGreen;">u</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">s</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;">n</span><span style="color: MediumSeaGreen;">u</span><span style="color: MediumSeaGreen;">m</span><span style="color: MediumSeaGreen;">b</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">r</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;"><strong>u</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>f</strong></span><span style="color: MediumSeaGreen;"><strong>i</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;">;</span><br />    <strong>2</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

### Valid

```ts
function foo(): void {};
```

```ts
function doSomething(this: void) {}
```

```ts
function printArg<T = void>(arg: T) {}
```

## 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/suspicious/no_confusing_void_type.rs)
- [Test Cases](https://github.com/biomejs/biome/blob/main/crates/biome_js_analyze/tests/specs/suspicious/noConfusingVoidType)

</TabItem>
</Tabs>