# @typescript-eslint/use-unknown-in-catch-callback-variable

> Require the parameter of a promise rejection callback to be typed `unknown`

A promise can reject with anything: an `Error`, a string, `undefined`. TypeScript still types the parameter of a `.catch()` callback as `any`, so `error.message` compiles, and then throws if the value is `undefined`. With `: unknown`, TypeScript makes you check what you got first.

The second argument of `.then()` is checked too. TypeScript's `useUnknownInCatchVariables` does the same for `catch (error)` clauses, but it doesn't reach these callbacks.

{/* 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/use-unknown-in-catch-callback-variable" = "error"
```

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

## Examples

Reported:

{/* bad {"count": 3} */}
```ts
declare function save(): Promise<void>;

save().catch(error => {
  console.error(error.message);
});
save().catch((error: Error) => {
  console.error(error.message);
});
save().then(
  () => console.log("saved"),
  ({ message }) => console.error(message),
);
```

Not reported:

{/* good */}
```ts
declare function save(): Promise<void>;

save().catch((error: unknown) => {
  console.error(error instanceof Error ? error.message : String(error));
});
save().catch(() => {
  console.error("could not save");
});
```