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Overview

The fromAsyncThrowable function wraps an async function that might throw exceptions and converts it into a function that returns a ResultAsync. This is the async equivalent of fromThrowable and is safer than using fromPromise with a function call. fromAsyncThrowable is a top-level export that references ResultAsync.fromThrowable.

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Type Signature

function fromAsyncThrowable<A extends readonly any[], R, E>(
  fn: (...args: A) => Promise<R>,
  errorFn?: (err: unknown) => E
): (...args: A) => ResultAsync<R, E>

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Parameters

  • fn: The async function to wrap (returns a Promise that might reject)
  • errorFn: Optional function to map caught errors to a known type E
    • If not provided, the raw error is used
    • Maps both thrown errors (synchronous) and rejected promises (asynchronous)

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Return Value

Returns a new function with the same parameters as the original, but returns a ResultAsync instead of a Promise that might reject.

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Why Use fromAsyncThrowable?

Not all functions that return a Promise are truly async. Some can throw synchronously before returning the promise: 
// DANGEROUS: Can throw synchronously!
function insertUser(user: User): Promise<User> {
  if (!user.id) {
    throw new TypeError('missing user id') // Synchronous throw!
  }
  return db.insert('users', user) // Async operation
}

// This will throw, NOT return a ResultAsync
const result = ResultAsync.fromPromise(
  insertUser(myUser), // Throws before fromPromise can catch
  () => new Error('Database error')
)
With fromAsyncThrowable: 
import { fromAsyncThrowable } from 'neverthrow'

const safeInsertUser = fromAsyncThrowable(
  insertUser, // Function reference, not call
  () => new Error('Database error')
)

// Now it safely catches both sync throws and async rejections
const result = safeInsertUser(myUser)
// result is ResultAsync<User, Error>

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Basic Usage

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Wrapping Database Operations

import { fromAsyncThrowable } from 'neverthrow'
import { db } from './database'

type DbError = { type: 'DatabaseError'; message: string }

const safeInsert = fromAsyncThrowable(
  db.insert,
  (error): DbError => ({
    type: 'DatabaseError',
    message: error instanceof Error ? error.message : 'Unknown error'
  })
)

const result = await safeInsert('users', { name: 'John', email: '[email protected]' })

if (result.isOk()) {
  console.log('Inserted:', result.value)
} else {
  console.log('Error:', result.error.message)
}

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Wrapping Fetch Requests

import { fromAsyncThrowable, ResultAsync } from 'neverthrow'

type FetchError = 
  | { type: 'NetworkError'; message: string }
  | { type: 'InvalidResponse' }

const safeFetch = fromAsyncThrowable(
  fetch,
  (error): FetchError => ({
    type: 'NetworkError',
    message: error instanceof Error ? error.message : 'Request failed'
  })
)

async function getUser(id: string): ResultAsync<User, FetchError> {
  return safeFetch(`https://api.example.com/users/${id}`)
    .andThen(response => {
      if (!response.ok) {
        return err({ type: 'InvalidResponse' })
      }
      return ResultAsync.fromPromise(
        response.json(),
        (): FetchError => ({ type: 'InvalidResponse' })
      )
    })
}

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Real-World Examples

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Email Service

import { fromAsyncThrowable } from 'neverthrow'
import nodemailer from 'nodemailer'

type EmailError = 
  | { type: 'InvalidConfig' }
  | { type: 'SendFailed'; reason: string }

const transporter = nodemailer.createTransport(config)

const safeSendEmail = fromAsyncThrowable(
  transporter.sendMail.bind(transporter),
  (error): EmailError => ({
    type: 'SendFailed',
    reason: error instanceof Error ? error.message : 'Unknown error'
  })
)

async function sendWelcomeEmail(user: User): ResultAsync<void, EmailError> {
  return safeSendEmail({
    from: '[email protected]',
    to: user.email,
    subject: 'Welcome!',
    text: `Welcome ${user.name}!`
  }).map(() => undefined)
}

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File Upload

import { fromAsyncThrowable } from 'neverthrow'
import { S3Client, PutObjectCommand } from '@aws-sdk/client-s3'

type UploadError = { type: 'UploadFailed'; message: string }

const s3Client = new S3Client({ region: 'us-east-1' })

const safeUpload = fromAsyncThrowable(
  async (key: string, body: Buffer) => {
    const command = new PutObjectCommand({
      Bucket: 'my-bucket',
      Key: key,
      Body: body
    })
    return await s3Client.send(command)
  },
  (error): UploadError => ({
    type: 'UploadFailed',
    message: error instanceof Error ? error.message : 'Upload failed'
  })
)

const result = await safeUpload('uploads/image.png', imageBuffer)

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External API with Retries

import { fromAsyncThrowable, ResultAsync, errAsync } from 'neverthrow'

type ApiError = 'NetworkError' | 'Timeout' | 'RateLimited'

const callApi = fromAsyncThrowable(
  async (endpoint: string) => {
    const controller = new AbortController()
    const timeout = setTimeout(() => controller.abort(), 5000)
    
    try {
      const response = await fetch(endpoint, { signal: controller.signal })
      return await response.json()
    } finally {
      clearTimeout(timeout)
    }
  },
  (error): ApiError => {
    if (error instanceof Error) {
      if (error.name === 'AbortError') return 'Timeout'
      if (error.message.includes('rate limit')) return 'RateLimited'
    }
    return 'NetworkError'
  }
)

async function callWithRetry(
  endpoint: string,
  maxRetries: number = 3
): ResultAsync<any, ApiError> {
  let lastError: ApiError = 'NetworkError'
  
  for (let i = 0; i < maxRetries; i++) {
    const result = await callApi(endpoint)
    if (result.isOk()) {
      return result
    }
    lastError = result.error
    
    // Don't retry on rate limit
    if (result.error === 'RateLimited') {
      break
    }
    
    // Wait before retry
    await new Promise(resolve => setTimeout(resolve, 1000 * (i + 1)))
  }
  
  return errAsync(lastError)
}

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Chaining Operations

import { fromAsyncThrowable, safeTry } from 'neverthrow'

const safeDbQuery = fromAsyncThrowable(
  db.query,
  () => 'QueryError' as const
)

const safeDbInsert = fromAsyncThrowable(
  db.insert,
  () => 'InsertError' as const
)

function processUser(userId: string) {
  return safeTry(async function*() {
    const user = yield* safeDbQuery('SELECT * FROM users WHERE id = ?', [userId])
    const validated = yield* validateUser(user)
    yield* safeDbInsert('audit_log', { action: 'user_processed', userId })
    
    return ok(validated)
  })
}

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Comparison with fromPromise

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Using fromPromise (Not Safe for Sync Throws)

import { ResultAsync } from 'neverthrow'

// NOT SAFE if fn can throw synchronously
function wrapper(user: User) {
  return ResultAsync.fromPromise(
    insertUser(user), // Might throw before fromPromise catches
    () => 'Error'
  )
}

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Using fromAsyncThrowable (Safe)

import { fromAsyncThrowable } from 'neverthrow'

// SAFE: Catches both sync throws and async rejections
const safeInsertUser = fromAsyncThrowable(
  insertUser,
  () => 'Error'
)

function wrapper(user: User) {
  return safeInsertUser(user) // Safe!
}

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Key Points

  • Wraps async functions that might throw
  • Catches both synchronous throws and asynchronous rejections
  • Safer than fromPromise when wrapping function calls
  • Returns a function that returns ResultAsync
  • Error mapper is optional but recommended for type safety
  • Use this instead of fromPromise when working with function references

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