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# no-unnecessary-type-assertion

[Added in v0.1.4](https://github.com/web-infra-dev/rslint/releases/tag/v0.1.4)
## Configuration

| Preset                              | Configured Value |
| ----------------------------------- | ---------------- |
| ✅ ts.configs.recommendedTypeChecked | `"error"`        |
| ✅ ts.configs.strictTypeChecked      | `"error"`        |


```ts title=rslint.config.ts
import { defineConfig, ts } from '@rslint/core';

export default defineConfig([
  ts.configs.recommended,
  {
    rules: {
      '@typescript-eslint/no-unnecessary-type-assertion': 'error',
    },
  },
]);
```

## Rule Details

Disallow type assertions that do not change the type of an expression.

Type assertions (`as` expressions, angle-bracket syntax, and non-null assertions `!`) that don't actually change the type of an expression are unnecessary and add noise to the code. This includes non-null assertions on values that are already non-nullable.

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

```typescript
const foo = 3;
const bar = foo!; // foo is already non-nullable

const str = 'hello' as string; // already a string

declare const value: number;
const num = value as number; // already a number
```

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

```typescript
const foo: number | undefined = getValue();
const bar = foo!; // non-null assertion is meaningful

const value = someUnknown as string; // type narrowing

const x = 3 as const; // const assertions are allowed
```

## Original Documentation

- [typescript-eslint: no-unnecessary-type-assertion](https://typescript-eslint.io/rules/no-unnecessary-type-assertion)
- [Source code](https://github.com/typescript-eslint/typescript-eslint/blob/v8.68.0/packages/eslint-plugin/src/rules/no-unnecessary-type-assertion.ts)