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

<Tabs>
<TabItem label="TypeScript and TSX" icon="seti:typescript">
## Summary
- Rule available since: `v1.0.0`
- Diagnostic Category: [`lint/style/noInferrableTypes`](/reference/diagnostics#diagnostic-category)
- This rule isn't recommended, so you need to enable it.
- This rule has a [**safe**](/linter/#safe-fixes) fix.
- The default severity of this rule is [**information**](/reference/diagnostics#information).
- Sources: 
  - Same as [`@typescript-eslint/no-inferrable-types`](https://typescript-eslint.io/rules/no-inferrable-types)

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

```
## Description
Disallow type annotations for variables, parameters, and class properties initialized with a literal expression.

TypeScript is able to infer the types of parameters, properties, and variables from their default or initial values.
There is no need to use an explicit `:` type annotation for trivially inferred types (boolean, bigint, number, regex, string).
Doing so adds unnecessary verbosity to code making it harder to read.

In contrast to ESLint's rule, this rule allows to use a wide type for `const` declarations.
Moreover, the rule does not recognize `undefined` values, primitive type constructors (String, Number, ...), and `RegExp` type.
These global variables could be shadowed by local ones.

## Examples

### Invalid

```ts
const variable: 1 = 1;
```

<pre class="language-text"><code class="language-text">code-block.ts:1:15 <a href="https://biomejs.dev/linter/rules/no-inferrable-types">lint/style/noInferrableTypes</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This type annotation is trivially inferred from its initialization.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>const variable: 1 = 1;<br />   <strong>   │ </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;">Safe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Remove the type annotation.</span><br />  <br />    <strong>1</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;">c</span><span style="color: Tomato;">o</span><span style="color: Tomato;">n</span><span style="color: Tomato;">s</span><span style="color: Tomato;">t</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">v</span><span style="color: Tomato;">a</span><span style="color: Tomato;">r</span><span style="color: Tomato;">i</span><span style="color: Tomato;">a</span><span style="color: Tomato;">b</span><span style="color: Tomato;">l</span><span style="color: Tomato;">e</span><span style="color: Tomato;"><strong>:</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>1</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">1</span><span style="color: Tomato;">;</span><br />      <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">c</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">v</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">r</span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">b</span><span style="color: MediumSeaGreen;">l</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">1</span><span style="color: MediumSeaGreen;">;</span><br />    <strong>2</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

```ts
let variable: number = 1;
```

<pre class="language-text"><code class="language-text">code-block.ts:1:13 <a href="https://biomejs.dev/linter/rules/no-inferrable-types">lint/style/noInferrableTypes</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This type annotation is trivially inferred from its initialization.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>let variable: number = 1;<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><br />    <strong>2 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Safe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Remove the type annotation.</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;">v</span><span style="color: Tomato;">a</span><span style="color: Tomato;">r</span><span style="color: Tomato;">i</span><span style="color: Tomato;">a</span><span style="color: Tomato;">b</span><span style="color: Tomato;">l</span><span style="color: Tomato;">e</span><span style="color: Tomato;"><strong>:</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>u</strong></span><span style="color: Tomato;"><strong>m</strong></span><span style="color: Tomato;"><strong>b</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">1</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;">v</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">r</span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">b</span><span style="color: MediumSeaGreen;">l</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">1</span><span style="color: MediumSeaGreen;">;</span><br />    <strong>2</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

```ts
class SomeClass {
  readonly field: 1 = 1;
}
```

<pre class="language-text"><code class="language-text">code-block.ts:2:17 <a href="https://biomejs.dev/linter/rules/no-inferrable-types">lint/style/noInferrableTypes</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This type annotation is trivially inferred from its initialization.</span><br />  <br />    <strong>1 │ </strong>class SomeClass &#123;<br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>  readonly field: 1 = 1;<br />   <strong>   │ </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;">Safe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Remove the type annotation.</span><br />  <br />    <strong>1</strong> <strong>1</strong><strong> │ </strong>  class SomeClass &#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;">r</span><span style="color: Tomato;">e</span><span style="color: Tomato;">a</span><span style="color: Tomato;">d</span><span style="color: Tomato;">o</span><span style="color: Tomato;">n</span><span style="color: Tomato;">l</span><span style="color: Tomato;">y</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">f</span><span style="color: Tomato;">i</span><span style="color: Tomato;">e</span><span style="color: Tomato;">l</span><span style="color: Tomato;">d</span><span style="color: Tomato;"><strong>:</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>1</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">1</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;">r</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">d</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">l</span><span style="color: MediumSeaGreen;">y</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">l</span><span style="color: MediumSeaGreen;">d</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">1</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
class SomeClass {
  field: number = 1;
}
```

<pre class="language-text"><code class="language-text">code-block.ts:2:8 <a href="https://biomejs.dev/linter/rules/no-inferrable-types">lint/style/noInferrableTypes</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This type annotation is trivially inferred from its initialization.</span><br />  <br />    <strong>1 │ </strong>class SomeClass &#123;<br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>  field: number = 1;<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><br />    <strong>3 │ </strong>&#125;<br />    <strong>4 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Safe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Remove the type annotation.</span><br />  <br />    <strong>1</strong> <strong>1</strong><strong> │ </strong>  class SomeClass &#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;">f</span><span style="color: Tomato;">i</span><span style="color: Tomato;">e</span><span style="color: Tomato;">l</span><span style="color: Tomato;">d</span><span style="color: Tomato;"><strong>:</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>u</strong></span><span style="color: Tomato;"><strong>m</strong></span><span style="color: Tomato;"><strong>b</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">1</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;">f</span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">l</span><span style="color: MediumSeaGreen;">d</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">1</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
function f(param: number = 1): void {}
```

<pre class="language-text"><code class="language-text">code-block.ts:1:17 <a href="https://biomejs.dev/linter/rules/no-inferrable-types">lint/style/noInferrableTypes</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This type annotation is trivially inferred from its initialization.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>function f(param: number = 1): 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><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;">Safe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Remove the type annotation.</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;">f</span><span style="color: Tomato;">(</span><span style="color: Tomato;">p</span><span style="color: Tomato;">a</span><span style="color: Tomato;">r</span><span style="color: Tomato;">a</span><span style="color: Tomato;">m</span><span style="color: Tomato;"><strong>:</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>u</strong></span><span style="color: Tomato;"><strong>m</strong></span><span style="color: Tomato;"><strong>b</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">1</span><span style="color: Tomato;">)</span><span style="color: Tomato;">:</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">v</span><span style="color: Tomato;">o</span><span style="color: Tomato;">i</span><span style="color: Tomato;">d</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;">f</span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">p</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">r</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">m</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">1</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">:</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">v</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">d</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>

### Valid

```ts
const variable: number = 1;
```

```ts
let variable: 1 | 2 = 1;
```

```ts
class SomeClass {
  readonly field: number = 1;
}
```

```ts
// `undefined` could be shadowed
const variable: undefined = undefined;
```

```ts
// `RegExp` could be shadowed
const variable: RegExp = /a/;
```

```ts
// `String` could be shadowed
let variable: string = String(5);
```

```ts
class SomeClass {
  field: 1 | 2 = 1;
}
```

```ts
function f(param: 1 | 2 = 1): void {}
```

## 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/no_inferrable_types.rs)
- [Test Cases](https://github.com/biomejs/biome/blob/main/crates/biome_js_analyze/tests/specs/style/noInferrableTypes)

</TabItem>
</Tabs>