# @typescript-eslint/no-unnecessary-type-assertion

> Disallow `as`, `<T>` and `!` assertions that do not change the type

An assertion tells the compiler "trust me". If the expression already has that type, or the place that receives it accepts the original type, the assertion does nothing today. But it stays in the code and silences the compiler when the types change later.

`--fix` removes the assertion.

`bun lint --type-aware` turns this rule on in TypeScript files.

{/* Generated by src/codegen/lint-rule-docs.ts. Do not edit this section. */}

## Configure

Off by default. This rule needs types, so turning it on makes `bun lint` type check your code first.

```toml bunfig.toml
[lint.rules]
"@typescript-eslint/no-unnecessary-type-assertion" = "error" # or "warn"
```

Every option, set to its default:

```toml bunfig.toml
[lint.rules]
"@typescript-eslint/no-unnecessary-type-assertion" = ["error", {
  checkLiteralConstAssertions = false,
  typesToIgnore = [], # list of strings
}]
```

{/* End of the generated section. */}

## Examples

Reported:

{/* bad {"count": 3} */}
```ts
interface User {
  name: string;
  email?: string;
}

function show(user: User) {
  const name = user.name as string;
  const length = user.name!.length;
  const email: string | undefined = user.email!;
  return [name, length, email];
}
```

Not reported:

{/* good */}
```ts
interface User {
  name: string;
  email?: string;
}

function show(user: User) {
  const name = user.name;
  const length = user.name.length;
  const email: string = user.email!;
  return [name, length, email];
}
```

An assertion that narrows or widens the type isn't reported:

{/* good */}
```ts
function parse(text: string) {
  const data = JSON.parse(text) as { id: number };
  let mode = "dark" as "dark" | "light";
  return [data, mode];
}
```

## Options

| Option | Default | What it does |
| --- | --- | --- |
| `checkLiteralConstAssertions` | `false` | Also report `as const` on a literal in a `const` declaration, where the type is already the literal type. |
| `typesToIgnore` | `[]` | Type names, as written after `as`. Assertions to these types are never reported. |

Reported only with `checkLiteralConstAssertions`:

{/* bad {"options": [{"checkLiteralConstAssertions": true}]} */}
```ts
const retries = 3 as const;
```

Not reported with `"typesToIgnore": ["UserId"]`:

{/* good {"options": [{"typesToIgnore": ["UserId"]}]} */}
```ts
type UserId = string;

function key(id: UserId) {
  return id as UserId;
}
```