> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/supermacro/neverthrow/llms.txt
> Use this file to discover all available pages before exploring further.

# ResultAsync.mapErr

> Transform the Err value of a ResultAsync

## Overview

Maps a `ResultAsync<T, E>` to `ResultAsync<T, F>` by applying a function to the contained `Err` value. The `Ok` value is left untouched.

## Signature

```typescript theme={null}
class ResultAsync<T, E> {
  mapErr<U>(f: (e: E) => U | Promise<U>): ResultAsync<T, U>
}
```

<ParamField path="f" type="(e: E) => U | Promise<U>" required>
  Function to transform the Err value. Can return either a value or a Promise.
</ParamField>

**Returns:** `ResultAsync<T, U>` - A new ResultAsync with the transformed error

## Usage

### Convert error types

```typescript theme={null}
type ApiError = { code: number, message: string }

const result = await fetchData()
  .mapErr((error: Error) => ({
    code: 500,
    message: error.message
  }))

// Result<Data, ApiError>
```

### Async error mapping

```typescript theme={null}
const result = await queryDatabase(sql)
  .mapErr(async (dbError) => {
    // Log error asynchronously
    await logError(dbError)
    return 'Database query failed'
  })
```

### Add context to errors

```typescript theme={null}
const processFile = (filename: string) => {
  return readFile(filename)
    .mapErr(err => `Failed to read ${filename}: ${err}`)
    .andThen(parseJSON)
    .mapErr(err => `Failed to parse ${filename}: ${err}`)
}
```

## Key characteristics

* **Short-circuits on Ok**: If the ResultAsync is Ok, mapErr is not executed
* **Type transformation**: Allows changing the error type in the Result
* **Async support**: Can handle both sync and async error transformation functions

<Tip>
  Use `mapErr` to normalize error types across different operations, making error handling consistent.
</Tip>

## Related

<CardGroup cols={2}>
  <Card title="map" icon="arrow-right" href="./map">
    Transform the Ok value
  </Card>

  <Card title="orElse" icon="rotate-left" href="./or-else">
    Recover from errors
  </Card>
</CardGroup>