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

```ts
function undefinedOr<ValueType>(
  type: ValidateUnionTypeMember<ValueType>,
): UnionType<readonly [ValueType, LiteralType<undefined>]>;
```

Defined in: [packages/common/src/Type.ts:3729](https://github.com/evoluhq/evolu/blob/037c390af081e9944d616ff298a729515c5c5ab7/packages/common/src/Type.ts#L3729)

Union [Type](https://evolu.dev/docs/api-reference/common/Type/interfaces/Type) containing the supplied Type and `undefined`.

This does not make an object property optional. It changes only the values
accepted when the property is present.

### Example

```ts

const StringOrUndefined = undefinedOr(String);

assertOk(StringOrUndefined.fromUnknown(undefined), undefined);
```