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

> Disallow type assertions that narrow a type

`value as T` makes TypeScript treat `value` as a `T` without a check. If `T` is narrower than the real type, the claim can be false, and the code that follows works with a value that isn't what its type says.

An assertion from or to `any` is reported as well. An assertion to a wider type is always true, so it's allowed, and so is `as const`.

{/* 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-unsafe-type-assertion" = "error" # or "warn"
```

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

## Examples

Reported:

{/* bad */}
```ts
function toPort(value: string | number) {
  return value as number;
}
```

{/* bad {"count": 2} */}
```ts
interface Config {
  port: number;
}

const config = JSON.parse("{}") as Config;
const loose = config as any;
```

Not reported:

{/* good */}
```ts
function toPort(value: string | number) {
  return typeof value === "number" ? value : Number(value);
}

const port = 3000 as string | number;
const modes = ["dev", "prod"] as const;
```

## When not to use it

You can't check some values in a way TypeScript understands, such as the result of `document.querySelector()` when you know the markup. If your code has many of these, the rule reports assertions you can't avoid.