[API reference](https://evolu.dev/docs/api-reference) › [@evolu/common](https://evolu.dev/docs/api-reference/common) › [Result](https://evolu.dev/docs/api-reference/common/Result) › isErr

```ts
function isErr<T, E>(result: Result<T, E>): result is Err<E>;
```

Defined in: [packages/common/src/Result.ts:408](https://github.com/evoluhq/evolu/blob/037c390af081e9944d616ff298a729515c5c5ab7/packages/common/src/Result.ts#L408)

Type guard for [Err](https://evolu.dev/docs/api-reference/common/Result/interfaces/Err) results.

### Example

```ts
import {
  assertErr,
  assertType,
  err,
  isErr,
  type Err,
  type Result,
  type Typed,
} from "@evolu/common";

const getCount = (): Result<number, CountUnavailableError> =>
  err({ type: "CountUnavailable" });

interface CountUnavailableError extends Typed<"CountUnavailable"> {}

const count = getCount();
if (isErr(count)) {
  assertType<Err<CountUnavailableError>, typeof count>();
  assertErr(count, { type: "CountUnavailable" });
}
```