> For the complete documentation index, see [llms.txt](https://musicq.gitbook.io/unwrapit/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://musicq.gitbook.io/unwrapit/apis/maperr.md).

# mapErr

If the unwrapped value is Err, will map the error by calling the given `errMapFn` function.

```ts
const wrapper = await wrap<string, number>(Promise.reject('error'))
wrapper.unwrapOrElse((e: string) => 1) // 1
```