fromPromise()

Convert a Promise into a ResultAsync with error handling

Overview

The fromPromise function transforms a Promise<T> that might reject into a ResultAsync<T, E>. It allows you to handle Promise rejections in a type-safe way by mapping the rejection to a known error type.

fromPromise is a top-level export that references ResultAsync.fromPromise.

Type Signature

function fromPromise<T, E>(
  promise: PromiseLike<T>,
  errorHandler: (unknownError: unknown) => E
): ResultAsync<T, E>

Parameters

• promise: A PromiseLike<T> that might reject
• errorHandler: Required function that maps the rejection value to a known error type E
  • Receives unknown as the error (the rejection value)
  • Must return a value of type E

Return Value

Returns a ResultAsync<T, E> that:

• Resolves to Ok<T> if the promise resolves successfully
• Resolves to Err<E> if the promise rejects

Basic Usage

Converting a Simple Promise

import { fromPromise } from 'neverthrow'

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

const userPromise = fetch('https://api.example.com/user/1')
  .then(res => res.json())

const userResult = fromPromise(
  userPromise,
  (error): FetchError => ({
    type: 'NetworkError',
    message: error instanceof Error ? error.message : 'Unknown error'
  })
)

// userResult type: ResultAsync<User, FetchError>

const user = await userResult

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

Database Query

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

type DbError = 'ConnectionFailed' | 'QueryFailed'

function getUser(id: string) {
  return fromPromise(
    db.query('SELECT * FROM users WHERE id = ?', [id]),
    (): DbError => 'QueryFailed'
  )
}

const result = await getUser('123')

Real-World Examples

HTTP Request with Detailed Error Handling

import { fromPromise, ResultAsync } from 'neverthrow'

type HttpError = 
  | { type: 'NetworkError'; message: string }
  | { type: 'Timeout' }
  | { type: 'InvalidResponse'; status: number }

async function fetchUser(id: string): ResultAsync<User, HttpError> {
  const controller = new AbortController()
  const timeout = setTimeout(() => controller.abort(), 5000)
  
  return fromPromise(
    fetch(`https://api.example.com/users/${id}`, {
      signal: controller.signal
    }),
    (error): HttpError => {
      clearTimeout(timeout)
      if (error instanceof Error && error.name === 'AbortError') {
        return { type: 'Timeout' }
      }
      return {
        type: 'NetworkError',
        message: error instanceof Error ? error.message : 'Unknown error'
      }
    }
  ).andThen(response => {
    clearTimeout(timeout)
    if (!response.ok) {
      return err({ type: 'InvalidResponse', status: response.status })
    }
    return fromPromise(
      response.json(),
      (): HttpError => ({ type: 'NetworkError', message: 'Invalid JSON' })
    )
  })
}

File System Operations

import { fromPromise } from 'neverthrow'
import { promises as fs } from 'fs'

type FileError = 
  | { type: 'NotFound'; path: string }
  | { type: 'PermissionDenied'; path: string }
  | { type: 'Unknown'; message: string }

function readFile(path: string) {
  return fromPromise(
    fs.readFile(path, 'utf-8'),
    (error): FileError => {
      if (error instanceof Error) {
        if (error.message.includes('ENOENT')) {
          return { type: 'NotFound', path }
        }
        if (error.message.includes('EACCES')) {
          return { type: 'PermissionDenied', path }
        }
        return { type: 'Unknown', message: error.message }
      }
      return { type: 'Unknown', message: 'Unknown error' }
    }
  )
}

const result = await readFile('config.json')

result.match(
  (content) => console.log('File content:', content),
  (error) => {
    switch (error.type) {
      case 'NotFound':
        console.log('File not found:', error.path)
        break
      case 'PermissionDenied':
        console.log('Permission denied:', error.path)
        break
      case 'Unknown':
        console.log('Error:', error.message)
        break
    }
  }
)

Multiple Async Operations

import { fromPromise, safeTry } from 'neverthrow'

type AppError = 'DbError' | 'ApiError' | 'ValidationError'

function processOrder(orderId: string) {
  return safeTry(async function*() {
    // Fetch order from database
    const order = yield* fromPromise(
      db.getOrder(orderId),
      (): AppError => 'DbError'
    )
    
    // Validate order
    const validated = yield* validateOrder(order)
      .mapErr((): AppError => 'ValidationError')
    
    // Call payment API
    const payment = yield* fromPromise(
      paymentApi.charge(validated),
      (): AppError => 'ApiError'
    )
    
    // Update order in database
    yield* fromPromise(
      db.updateOrder(orderId, { status: 'paid', paymentId: payment.id }),
      (): AppError => 'DbError'
    )
    
    return ok(payment)
  })
}

Promise.all with Results

import { fromPromise, ResultAsync } from 'neverthrow'

type FetchError = { message: string }

function fetchMultipleUsers(ids: string[]) {
  const promises = ids.map(id =>
    fromPromise(
      fetch(`https://api.example.com/users/${id}`).then(r => r.json()),
      (error): FetchError => ({
        message: error instanceof Error ? error.message : 'Fetch failed'
      })
    )
  )
  
  return ResultAsync.combine(promises)
}

const result = await fetchMultipleUsers(['1', '2', '3'])

if (result.isOk()) {
  console.log('All users:', result.value) // User[]
} else {
  console.log('At least one fetch failed:', result.error)
}

Streaming Response

import { fromPromise } from 'neverthrow'

type StreamError = 'StreamFailed' | 'ReadFailed'

function downloadFile(url: string) {
  return fromPromise(
    fetch(url),
    (): StreamError => 'StreamFailed'
  ).andThen(response => {
    if (!response.ok) {
      return err('StreamFailed' as const)
    }
    return fromPromise(
      response.arrayBuffer(),
      (): StreamError => 'ReadFailed'
    )
  })
}

Chaining with Other Methods

import { fromPromise } from 'neverthrow'

const result = fromPromise(
  fetch('https://api.example.com/data'),
  () => 'FetchError' as const
)
  .andThen(response => {
    if (!response.ok) {
      return err('InvalidResponse' as const)
    }
    return fromPromise(
      response.json(),
      () => 'ParseError' as const
    )
  })
  .map(data => data.items)
  .map(items => items.filter(item => item.active))

// result type: ResultAsync<Item[], 'FetchError' | 'InvalidResponse' | 'ParseError'>

Important Considerations

fromPromise vs fromAsyncThrowable

Use fromPromise when you already have a Promise instance:

const promise = someAsyncFunction()
const result = fromPromise(promise, handleError)

Use fromAsyncThrowable when wrapping a function that might throw synchronously:

const safeFunction = fromAsyncThrowable(someAsyncFunction, handleError)
const result = safeFunction() // Safe from sync throws

Error Handler is Required

Unlike fromThrowable, the error handler parameter is not optional for fromPromise. This is because:

• Promise rejections can be of any type
• TypeScript cannot infer a safe default error type
• You must explicitly handle the unknown rejection value

// This will cause a TypeScript error
const result = fromPromise(somePromise) // Error: Expected 2 arguments

// You must provide an error handler
const result = fromPromise(somePromise, () => 'Error')

Key Points

• Converts Promise<T> to ResultAsync<T, E>
• Error handler parameter is required
• Maps promise rejections to a known error type
• Use for existing Promise instances
• For function wrapping, prefer fromAsyncThrowable
• Works seamlessly with safeTry and other Result methods

Related

• fromAsyncThrowable() - For wrapping async functions
• fromSafePromise() - For Promises that never reject
• ResultAsync - The ResultAsync class
• safeTry() - For combining multiple async operations