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# use-unknown-in-catch-callback-variable

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

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


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

export default defineConfig([
  ts.configs.recommended,
  {
    rules: {
      '@typescript-eslint/use-unknown-in-catch-callback-variable': 'error',
    },
  },
]);
```

## Rule Details

Enforce typing the rejection callback parameter of `.catch()` and `.then()` as `unknown`. Similar to how catch clause variables should be typed as `unknown` (since any value can be thrown), Promise rejection callback parameters should also be typed as `unknown`. This prevents unsafe property accesses and assumptions about the shape of the rejection reason.

The rule checks `.catch(callback)` and `.then(onFulfilled, onRejected)` calls on thenable types and flags the rejection callback parameter when it is not typed as `unknown`.

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

```typescript
promise.catch(err => {});
promise.catch((err: Error) => {});
promise.then(undefined, err => {});
promise.then(undefined, (err: string) => {});
```

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

```typescript
promise.catch((err: unknown) => {});
promise.then(undefined, (err: unknown) => {});
promise.catch((...args: [unknown]) => {});
```

## Original Documentation

- [typescript-eslint: use-unknown-in-catch-callback-variable](https://typescript-eslint.io/rules/use-unknown-in-catch-callback-variable)
- [Source code](https://github.com/typescript-eslint/typescript-eslint/blob/v8.67.0/packages/eslint-plugin/src/rules/use-unknown-in-catch-callback-variable.ts)