> ## 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.

# errAsync

> Create an Err variant of ResultAsync

## Overview

The `errAsync` function constructs a `ResultAsync` containing an `Err` variant with the provided error value.

## Signature

```typescript theme={null}
function errAsync<T = never, E extends string = string>(err: E): ResultAsync<T, E>
function errAsync<T = never, E = unknown>(err: E): ResultAsync<T, E>
function errAsync<T = never, E extends void = void>(err: void): ResultAsync<T, void>
```

<ParamField path="err" type="E" required>
  The error value to wrap in a ResultAsync
</ParamField>

**Returns:** `ResultAsync<T, E>` - A ResultAsync containing the Err value

## Usage

### Basic usage

```typescript theme={null}
import { errAsync } from 'neverthrow'

const myResultAsync = errAsync('Oh nooo')

const myResult = await myResultAsync

myResult.isOk() // false
myResult.isErr() // true
```

### Async error handling

```typescript theme={null}
async function validateAge(age: number) {
  if (age < 0) {
    return errAsync('Age cannot be negative')
  }
  if (age < 18) {
    return errAsync('Must be 18 or older')
  }
  return okAsync(age)
}

const result = await validateAge(15)
// Result<number, string>
```

### Custom error types

```typescript theme={null}
type DatabaseError = 
  | { type: 'ConnectionError', message: string }
  | { type: 'QueryError', query: string }

async function queryDatabase(sql: string) {
  return errAsync<User[], DatabaseError>({
    type: 'QueryError',
    query: sql
  })
}
```

## When to use

* Returning an error from an async operation
* Early returns in validation chains
* Converting error states to ResultAsync for consistency

<Warning>
  Like `okAsync`, this wraps the error in a resolved Promise. The Promise itself does not reject.
</Warning>

## Related

<CardGroup cols={2}>
  <Card title="okAsync" icon="check" href="./ok-async">
    Create an Ok variant of ResultAsync
  </Card>

  <Card title="ResultAsync.fromPromise" icon="arrow-right" href="./from-promise">
    Handle rejecting Promises
  </Card>
</CardGroup>