errAsync

Create an Err variant of ResultAsync

Overview

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

Signature

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>

err · E · required

The error value to wrap in a ResultAsync

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

Usage

Basic usage

import { errAsync } from 'neverthrow'

const myResultAsync = errAsync('Oh nooo')

const myResult = await myResultAsync

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

Async error handling

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

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

Like okAsync, this wraps the error in a resolved Promise. The Promise itself does not reject.

Related

okAsync

Create an Ok variant of ResultAsync

ResultAsync.fromPromise

Handle rejecting Promises