[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<T>(
  type: T,
): UnionType<
  [
    Type<
      "Undefined",
      undefined,
      undefined,
      UndefinedError,
      undefined,
      UndefinedError
    >,
    T,
  ]
>;
```

Defined in: [packages/common/src/Type.ts:3886](https://github.com/evoluhq/evolu/blob/e7144e2bbe9069362b62dec1b64a8aa922b8f1b0/packages/common/src/Type.ts#L3886)

`union(undefined, T)` [Type](https://evolu.dev/docs/api-reference/common/Type/interfaces/Type).

### Example

```ts
const UndefinedOrString = undefinedOr(String);
UndefinedOrString.from("world"); // ok("world")
UndefinedOrString.from(undefined); // ok()
UndefinedOrString.from(42); // err(...)
```