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

```ts
const assertLength: <Value, Length>(
  value: Value,
  expectedLength: Length,
) => asserts value is Value & { length: Length };
```

Defined in: [packages/common/src/Assert.ts:834](https://github.com/evoluhq/evolu/blob/ecbac001e0c18bbc45b44715f49ecc3a768c5ec9/packages/common/src/Assert.ts#L834)

Asserts that a value has the expected length and narrows its length.

### Example

```ts

const values: ReadonlyArray<string> = ["Ada", "Grace"];
assertLength(values, 2);
assertType<typeof values.length, 2>();
```