---
url: /docs/guide/usage/linter/rules/typescript/no-unsafe-type-assertion.md
---

### What it does

Disallows unsafe type assertions that narrow a type.

### Why is this bad?

Type assertions that narrow a type bypass TypeScript's type-checking and can lead to
runtime errors. Type assertions that broaden a type are safe because TypeScript
essentially knows *less* about a type. Instead of using type assertions to narrow a
type, it's better to rely on type guards, which help avoid potential runtime errors
caused by unsafe type assertions.

### Examples

Examples of **incorrect** code for this rule:

```ts
function f() {
  return Math.random() < 0.5 ? 42 : "oops";
}
const z = f() as number;

const items = [1, "2", 3, "4"];
const number = items[0] as number;
```

Examples of **correct** code for this rule:

```ts
function f() {
  return Math.random() < 0.5 ? 42 : "oops";
}
const z = f() as number | string | boolean;

const items = [1, "2", 3, "4"];
const number = items[0] as number | string | undefined;
```

## How to use

## Version

This rule was added in v1.12.0.

## References