noEvolvingTypes

import { Tabs, TabItem } from '@astrojs/starlight/components';

<Tabs> <TabItem label="TypeScript and TSX" icon="seti:typescript"> ## Summary - Rule available since: `v1.6.3` - Diagnostic Category: [`lint/suspicious/noEvolvingTypes`](/reference/diagnostics#diagnostic-category) - This rule isn't recommended, so you need to enable it. - This rule doesn't have a fix. - The default severity of this rule is [**warning**](/reference/diagnostics#warning). ## How to configure ```json title="biome.json" { "linter": { "rules": { "suspicious": { "noEvolvingTypes": "error" } } } }

## Description
Disallow variables from evolving into `any` type through reassignments.

In TypeScript, variables without explicit type annotations can evolve their types based on subsequent assignments.

When  TypeScript's [noImplicitAny](https://www.typescriptlang.org/tsconfig/#noImplicitAny) is disabled,
variables without explicit type annotations have implicitly the type `any`.
Just like the `any` type, evolved `any` types disable many type-checking rules and should be avoided to maintain strong type safety.
This rule prevents such cases by ensuring variables do not evolve into `any` type, encouraging explicit type annotations and controlled type evolutions.

If you enabled TypeScript's [noImplicitAny](https://www.typescriptlang.org/tsconfig/#noImplicitAny) and want to benefit of evolving types,
then we recommend to disable this rule.

## Examples

### Invalid

```ts
let a;

<pre class="language-text"><code class="language-text">code-block.ts:1:5 <a href="https://biomejs.dev/linter/rules/no-evolving-types">lint/suspicious/noEvolvingTypes</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ The type of this variable may evolve implicitly to any type, including the any type. > 1 │ let a; │ ^ 2 │ ℹ Add an explicit type or initialization to avoid implicit type evolution. </code></pre>

const b = [];

<pre class="language-text"><code class="language-text">code-block.ts:1:7 <a href="https://biomejs.dev/linter/rules/no-evolving-types">lint/suspicious/noEvolvingTypes</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ The type of this variable may evolve implicitly to any type, including the any type. > 1 │ const b = []; │ ^ 2 │ ℹ Add an explicit type or initialization to avoid implicit type evolution. </code></pre>

let c = null;

<pre class="language-text"><code class="language-text">code-block.ts:1:5 <a href="https://biomejs.dev/linter/rules/no-evolving-types">lint/suspicious/noEvolvingTypes</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ The type of this variable may evolve implicitly to any type, including the any type. > 1 │ let c = null; │ ^ 2 │ ℹ Add an explicit type or initialization to avoid implicit type evolution. </code></pre>

Valid

let a: number;
let b = 1;
var c : string;
var d = "abn";
const e: never[] = [];
const f = [null];
const g = ['1'];
const h = [1];
let workspace: Workspace | null = null;

</TabItem> </Tabs>

Valid

Related links