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

## Call Signature

```ts
function mapSet<T, U>(
  set: NonEmptyReadonlySet<T>,
  mapper: (item: T) => U,
): NonEmptyReadonlySet<U>;
```

Defined in: [packages/common/src/Set.ts:231](https://github.com/evoluhq/evolu/blob/037c390af081e9944d616ff298a729515c5c5ab7/packages/common/src/Set.ts#L231)

Maps a set using a mapper function, returning a new readonly set.

Preserves non-empty type.

Note: If the mapper produces duplicate values, the resulting set will have
fewer elements.

### Mapping non-empty sets

```ts
import {
  assertEqual,
  assertType,
  createSet,
  mapSet,
  type NonEmptyReadonlySet,
} from "@evolu/common";

const original = createSet([1, 2, 3]);
const doubled = mapSet(original, (x) => x * 2);
const parity = mapSet(original, (x) => x % 2);

assertType<NonEmptyReadonlySet<number>, typeof doubled>();
assertEqual(doubled, new Set([2, 4, 6]));
assertEqual(parity, new Set([1, 0]));
```

## Call Signature

```ts
function mapSet<T, U>(
  set: ReadonlySet<T>,
  mapper: (item: T) => U,
): ReadonlySet<U>;
```

Defined in: [packages/common/src/Set.ts:236](https://github.com/evoluhq/evolu/blob/037c390af081e9944d616ff298a729515c5c5ab7/packages/common/src/Set.ts#L236)

Possibly empty set.