NeverThrow 🙅

Description

Encode failure into your program.

This package contains a Result type that represents either success (Ok) or failure (Err).

For asynchronous tasks, neverthrow offers a ResultAsync class which wraps a Promise<Result<T, E>> and gives you the same level of expressivity and control as a regular Result<T, E>.

ResultAsync is thenable meaning it behaves exactly like a native Promise<Result> ... except you have access to the same methods that Result provides without having to await or .then the promise! Check out the wiki for examples and best practices.

Need to see real-life examples of how to leverage this package for error handling? See this repo: https://github.com/parlez-vous/server

Table Of Contents

• Installation
• Top-Level API
• API Documentation
  • Synchronous API (Result)
    • ok
    • err
    • Result.isOk (method)
    • Result.isErr (method)
    • Result.map (method)
    • Result.mapErr (method)
    • Result.unwrapOr (method)
    • Result.andThen (method)
    • Result.asyncAndThen (method)
    • Result.orElse (method)
    • Result.match (method)
    • Result.asyncMap (method)
    • Result.fromThrowable (static class method)
  • Asynchronous API (ResultAsync)
    • okAsync
    • errAsync
    • ResultAsync.fromPromise (static class method)
    • ResultAsync.map (method)
    • ResultAsync.mapErr (method)
    • ResultAsync.unwrapOr (method)
    • ResultAsync.andThen (method)
    • ResultAsync.match (method)
  • Utilities
    • combine
  • Testing
• A note on the Package Name

Installation

> npm install neverthrow

Top-Level API

neverthrow exposes the following:

• ok convenience function to create an Ok variant of Result
• err convenience function to create an Err variant of Result
• Ok class and type
• Err class and type
• Result Type as well as namespace / object from which to call Result.fromThrowable
• ResultAsync class
• okAsync convenience function to create a ResultAsync containing an Ok type Result
• errAsync convenience function to create a ResultAsync containing an Err type Result
• combine utility function that allows you to turn Result<T, E>[] into Result<T[], E>, or a ResultAsync<T, E>[] into ResultAsync<T[], E> (just like Promise.all)

import {
  ok,
  Ok,
  err,
  Err,
  Result,
  okAsync,
  errAsync,
  ResultAsync,
  combine
} from 'neverthrow'

Check out the wiki for help on how to make the most of neverthrow.

If you find this package useful, please consider sponsoring me or simply buying me a coffee!

API Documentation

Synchronous API (Result)

ok

Constructs an Ok variant of Result

Signature:

ok<T, E>(value:  T): Ok<T, E> { ... }

Example:

import { ok } from 'neverthrow'

const myResult = ok({ myData: 'test' }) // instance of `Ok`

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

⬆️ Back to top

err

Constructs an Err variant of Result

Signature:

err<T, E>(err:  E):  Err<T, E> { ... }

Example:

import { err } from 'neverthrow'

const myResult = err('Oh noooo') // instance of `Err`

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

⬆️ Back to top

Result.isOk (method)

Returns true if the result is an Ok variant

Signature:

isOk(): boolean { ... }

⬆️ Back to top

Result.isErr (method)

Returns true if the result is an Err variant

Signature:

isErr(): boolean { ... }

⬆️ Back to top

Result.map (method)

Maps a Result<T, E> to Result<U, E> by applying a function to a contained Ok value, leaving an Err value untouched.

This function can be used to compose the results of two functions.

Signature:

type MapFunc = <T, U>(f: T) => U
map<U>(fn: MapFunc):  Result<U, E> { ... }

Example:

const { getLines } from 'imaginary-parser'
// ^ assume getLines has the following signature:
// getLines(str: string): Result<Array<string>, Error>

// since the formatting is deemed correct by `getLines`
// then it means that `linesResult` is an Ok
// containing an Array of strings for each line of code
const linesResult = getLines('1\n2\n3\n4\n')

// this Result now has a Array<number> inside it
const newResult = linesResult.map(
  (arr: Array<string>) => arr.map(parseInt)
)

newResult.isOk() // true

⬆️ Back to top

Result.mapErr (method)

Maps a Result<T, E> to Result<T, F> by applying a function to a contained Err value, leaving an Ok value untouched.

This function can be used to pass through a successful result while handling an error.

Signature:

type MapFunc = <E>(e: E) => F
mapErr<U>(fn: MapFunc):  Result<T, F> { ... }

Example:

import { parseHeaders } 'imaginary-http-parser'
// imagine that parseHeaders has the following signature:
// parseHeaders(raw: string): Result<SomeKeyValueMap, ParseError>

const rawHeaders = 'nonsensical gibberish and badly formatted stuff'

const parseResult = parseHeaders(rawHeaders)

parseResult.mapErr(parseError => {
  res.status(400).json({
    error: parseError
  })
})

parseResult.isErr() // true

⬆️ Back to top

Result.unwrapOr (method)

Unwrap the Ok value, or return the default if there is an Err

Signature:

unwrapOr<T>(v: T): T { ... }

Example:

const myResult = err('Oh noooo')

const multiply = (val: number): number => val * 2

const unwrapped: number = myResult.map(multiply).unwrapOr(10)

⬆️ Back to top

Result.andThen (method)

Same idea as map above. Except you must return a new Result.

The returned value will be a Result.

This is useful for when you need to do a subsequent computation using the inner T value, but that computation might fail.

andThen is really useful as a tool to flatten a Result<Result<A, E2>, E1> into a Result<A, E2> (see example below).

Signature:

type AndThenFunc = <T, U>(t:  T) => Result<U, E>
andThen<U>(f: AndThenFunc): Result<U, E> { ... }

Example 1: Chaining Results

import { err, ok } from 'neverthrow'

const sq = (n: number): Result<number, number> => ok(n ** 2)

ok(2)
  .andThen(sq)
  .andThen(sq) // Ok(16)

ok(2)
  .andThen(sq)
  .andThen(err) // Err(4)

ok(2)
  .andThen(err)
  .andThen(sq) // Err(2)

err(3)
  .andThen(sq)
  .andThen(sq) // Err(3)

Example 2: Flattening Nested Results

// It's common to have nested Results
const nested = ok(ok(1234))

// notNested is a Ok(1234)
const notNested = nested.andThen(innerResult => innerResult)

⬆️ Back to top

Result.asyncAndThen (method)

Same idea as andThen above. Except you must return a new ResultAsync.

The returned value will be a ResultAsync.

Signature:

type AndThenAsyncFunc = (t:  T) => ResultAsync<U, E>
asyncAndThen<U>(f: AndThenAsyncFunc): ResultAsync<U, E> { ... }

⬆️ Back to top

Result.orElse (method)

Takes an Err value and maps it to a Result<T, SomeNewType>. This is useful for error recovery.

Signature:

type ErrorCallback = <A>(e:  E) => Result<T, A>
orElse<A>(f: ErrorCallback<A>): Result<T, A> { ... }

Example:

enum DatabaseError {
  PoolExhausted = 'PoolExhausted',
  NotFound = 'NotFound',
}

const dbQueryResult: Result<string, DatabaseError> = err(DatabaseError.NotFound)

const updatedQueryResult = dbQueryResult.orElse(dbError =>
  dbError === DatabaseError.NotFound
    ? ok('User does not exist') // error recovery branch: ok() must be called with a value of type string
    //
    //
    // err() can be called with a value of any new type that you want
    // it could also be called with the same error value
    //     
    //     err(dbError)
    : err(500) 
)

⬆️ Back to top

Result.match (method)

Given 2 functions (one for the Ok variant and one for the Err variant) execute the function that matches the Result variant.

Match callbacks do not necessitate to return a Result, however you can return a Result if you want to.

Signature:

match<A>(
  okFn: (t: T) =>  A,
  errFn: (e: E) =>  A
): A => { ... }

match is like chaining map and mapErr, with the distinction that with match both functions must have the same return type.

Example:

const result = computationThatMightFail()

const successCallback = (someNumber: number) => {
  console.log('> number is: ', someNumber)
}

const failureCallback = (someFailureValue: string) => {
  console.log('> boooooo')
}

// method chaining api
// note that you DONT have to append mapErr
// after map which means that you are not required to do
// error handling
result.map(successCallback).mapErr(failureCallback)

// match api
// works exactly the same as above,
// except, now you HAVE to do error handling :)
myval.match(successCallback, failureCallback)

⬆️ Back to top

Result.asyncMap (method)

Similar to map except for two things:

• the mapping function must return a Promise
• asyncMap returns a ResultAsync

You can then chain the result of asyncMap using the ResultAsync apis (like map, mapErr, andThen, etc.)

Signature:

type MappingFunc = (t: T) => Promise<U>
asyncMap<U>(fn: MappingFunc):  ResultAsync<U, E> { ... }

Example:

import { parseHeaders } 'imaginary-http-parser'
// imagine that parseHeaders has the following signature:
// parseHeaders(raw: string): Result<SomeKeyValueMap, ParseError>

const asyncRes = parseHeaders(rawHeader)
  .map(headerKvMap => headerKvMap.Authorization)
  .asyncMap(findUserInDatabase)

Note that in the above example if parseHeaders returns an Err then .map and .asyncMap will not be invoked, and asyncRes variable will resolve to an Err when turned into a Result using await or .then().

⬆️ Back to top

Result.fromThrowable (static class method)

Although Result is not an actual JS class, the way that fromThrowable has been implemented requires that you call fromThrowable as though it were a static method on Result. See examples below.

The JavaScript community has agreed on the convention of throwing exceptions. As such, when interfacing with third party libraries it's imperative that you wrap third-party code in try / catch blocks.

This function will create a new function that returns an Err when the original function throws.

It is not possible to know the types of the errors thrown in the original function, therefore it is recommended to use the second argument errorFn to map what is thrown to a known type.

Example:

import { Result } from 'neverthrow'

type ParseError = { message: string }
const toParseError = (): ParseError => ({message: "Parse Error" })

const safeJsonParse = Result.fromThrowable(JSON.parse, toParseError)

// the function can now be used safely, if the function throws, the result will be an Err
const res = safeJsonParse("{");

⬆️ Back to top

Asynchronous API (ResultAsync)

okAsync

Constructs an Ok variant of ResultAsync

Signature:

okAsync<T, E>(value: T): ResultAsync<T, E>

Example:

import { okAsync } from 'neverthrow'

const myResultAsync = okAsync({ myData: 'test' }) // instance of `ResultAsync`

const myResult = await myResultAsync // instance of `Ok`

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

⬆️ Back to top

errAsync

Constructs an Err variant of ResultAsync

Signature:

errAsync<T, E>(err:  E):  ResultAsync<T, E>

Example:

import { errAsync } from 'neverthrow'

const myResultAsync = errAsync('Oh nooo') // instance of `ResultAsync`

const myResult = await myResultAsync // instance of `Err`

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

⬆️ Back to top

ResultAsync.fromPromise (static class method)

Transforms a Promise<T> into a ResultAsync<T, E>.

The second argument handles the rejection case of the promise. If it is ommited, the code might throw because neverthrow does not know if the promise you are passing to fromPromise has any promise rejection logic associated to it (via a .catch method call or catch (err) {} block).

Signature:

fromPromise<U, E>(p: Promise<U>, f?: (e: unknown) => E):  ResultAsync<U, E> { ... }

Example:

import { ResultAsync } from 'neverthrow'
import { insertIntoDb } from 'imaginary-database'
// insertIntoDb(user: User): Promise<User>

const res = ResultAsync.fromPromise(insertIntoDb(myUser), () => new Error('Database error'))
// res has a type of ResultAsync<User, Error>

⬆️ Back to top

ResultAsync.map (method)

Maps a ResultAsync<T, E> to ResultAsync<U, E> by applying a function to a contained Ok value, leaving an Err value untouched.

The applied function can be synchronous or asynchronous (returning a Promise<U>) with no impact to the return type.

This function can be used to compose the results of two functions.

Signature:

type MapFunc = <T>(f: T | Promise<T>) => U
map<U>(fn: MapFunc):  ResultAsync<U, E> { ... }

Example:

const { findUsersIn } from 'imaginary-database'
// ^ assume findUsersIn has the following signature:
// findUsersIn(country: string): ResultAsync<Array<User>, Error>

const usersInCanada = findUsersIn("Canada")

// Let's assume we only need their names
const namesInCanada = usersInCanada.map((users: Array<User>) => users.map(user => user.name))
// namesInCanada is of type ResultAsync<Array<string>, Error>

// We can extract the Result using .then() or await
namesInCanada.then((namesResult: Result<Array<string>, Error>) => {
  if(namesResult.isErr()){
    console.log("Couldn't get the users from the database", namesResult.error)
  }
  else{
    console.log("Users in Canada are named: " + namesResult.value.join(','))
  }
})

⬆️ Back to top

ResultAsync.mapErr (method)

Maps a ResultAsync<T, E> to ResultAsync<T, F> by applying a function to a contained Err value, leaving an Ok value untouched.

The applied function can be synchronous or asynchronous (returning a Promise<F>) with no impact to the return type.

This function can be used to pass through a successful result while handling an error.

Signature:

type MapFunc = <E>(e: E) => F | Promise<F>
mapErr<U>(fn: MapFunc):  ResultAsync<T, F> { ... }

Example:

const { findUsersIn } from 'imaginary-database'
// ^ assume findUsersIn has the following signature:
// findUsersIn(country: string): ResultAsync<Array<User>, Error>

// Let's say we need to low-level errors from findUsersIn to be more readable
const usersInCanada = findUsersIn("Canada").mapErr((e: Error) => {
  // The only error we want to pass to the user is "Unknown country"
  if(e.message === "Unknown country"){
    return e.message
  }
  // All other errors will be labelled as a system error
  return "System error, please contact an administrator."
})

// usersInCanada is of type ResultAsync<Array<User>, string>

usersInCanada.then((usersResult: Result<Array<User>, string>) => {
  if(usersResult.isErr()){
    res.status(400).json({
      error: usersResult.error
    })
  }
  else{
    res.status(200).json({
      users: usersResult.value
    })
  }
})

⬆️ Back to top

ResultAsync.unwrapOr (method)

Unwrap the Ok value, or return the default if there is an Err.
Works just like Result.unwrapOr but returns a Promise<T> instead of T.

Signature:

unwrapOr<T>(v: T):  Promise<T> { ... }

Example:

const unwrapped: number = await errAsync(0).unwrapOr(10)
// unwrapped = 10

⬆️ Back to top

ResultAsync.andThen (method)

Same idea as map above. Except the applied function must return a Result or ResultAsync.

ResultAsync.andThen always returns a ResultAsync no matter the return type of the applied function.

This is useful for when you need to do a subsequent computation using the inner T value, but that computation might fail.

andThen is really useful as a tool to flatten a ResultAsync<ResultAsync<A, E2>, E1> into a ResultAsync<A, E2> (see example below).

Signature:

type AndThenFunc = (t:  T) => ResultAsync<U, E> | Result<U, E>
andThen<U>(f: AndThenFunc): ResultAsync<U, E> { ... }

Example

const { validateUser } from 'imaginary-validator'
const { insertUser } from 'imaginary-database'
const { sendNotification } from 'imaginary-service'

// ^ assume validateUser, insertUser and sendNotification have the following signatures:
// validateUser(user: User): Result<User, Error>
// insertUser(user): ResultAsync<User, Error>
// sendNotification(user): ResultAsync<void, Error>

const resAsync = validateUser(user)
               .andThen(insertUser)
               .andThen(sendNotification)

// resAsync is a ResultAsync<void, Error>

resAsync.then((res: Result<void, Error>) => {
  if(res.isErr()){
    console.log("Oops, at least one step failed", res.error)
  }
  else{
    console.log("User has been validated, inserted and notified successfully.")
  }
})

⬆️ Back to top

ResultAsync.orElse (method)

Takes an Err value and maps it to a ResultAsync<T, SomeNewType>. This is useful for error recovery.

Signature:

type ErrorCallback = <A>(e:  E) => Result<T, A> | ResultAsync<T, A>
orElse<A>(f: ErrorCallback<A>): ResultAsync<T, A> { ... }

⬆️ Back to top

ResultAsync.match (method)

Given 2 functions (one for the Ok variant and one for the Err variant) execute the function that matches the ResultAsync variant.

The difference with Result.match is that it always returns a Promise because of the asynchronous nature of the ResultAsync.

Signature:

match<A>(
  okFn: (t:  T) =>  A,
  errFn: (e:  E) =>  A
): Promise<A> => { ... }

Example:

const { validateUser } from 'imaginary-validator'
const { insertUser } from 'imaginary-database'

// ^ assume validateUser and insertUser have the following signatures:
// validateUser(user: User): Result<User, Error>
// insertUser(user): ResultAsync<User, Error>

// Handle both cases at the end of the chain using match
const resultMessage = await validateUser(user)
        .andThen(insertUser)
        .match(
            (user: User) => `User ${user.name} has been successfully created`,
            (e: Error) =>  `User could not be created because ${e.message}`
        )

// resultMessage is a string

⬆️ Back to top

Utilities

combine

Combine lists of Results or lists of ResultAsyncs.

If you're familiar with Promise.all, the combine function works conceptually the same.

combine works on both heterogeneous and homogeneous lists. This means that you can have lists that contain different kinds of Results and still be able to combine them. Note that you cannot combine lists that contain both Results and ResultAsyncs.

The combine function takes a list of results and returns a single result. If all the results in the list are Ok, then the return value will be a Ok containing a list of all the individual Ok values.

If just one of the results in the list is an Err then the combine function returns that Err value (it short circuits and returns the first Err that it finds).

Formally speaking:

// homogeneous lists
function combine<T, E>(resultList: Result<T, E>[]): Result<T[], E>

// heterogeneous lists
function combine<T1, T2, E1, E2>(resultList: [ Result<T1, E1>, Result<T2, E2> ]): Result<[ T1, T2 ], E1 | E2>
function combine<T1, T2, T3, E1, E2, E3> => Result<[ T1, T2, T3 ], E1 | E2 | E3>
function combine<T1, T2, T3, T4, E1, E2, E3, E4> => Result<[ T1, T2, T3, T4 ], E1 | E2 | E3 | E4>
// ... etc etc ad infinitum

Additionally, this same function also works for ResultAsync. And thanks to typescript function overloading, the types can be distinguished.

function combine<T, E>(asyncResultList: ResultAsync<T, E>[]): ResultAsync<T[], E>

⬆️ Back to top

Testing

Result instances have two unsafe methods, aptly called _unsafeUnwrap and _unsafeUnwrapErr which should only be used in a test environment.

_unsafeUnwrap takes a Result<T, E> and returns a T when the result is an Ok, otherwise it throws a custom object.

_unsafeUnwrapErr takes a Result<T, E> and returns a E when the result is an Err, otherwise it throws a custom object.

That way you can do something like:

expect(myResult._unsafeUnwrap()).toBe(someExpectation)

However, do note that Result instances are comparable. So you don't necessarily need to unwrap them in order to assert expectations in your tests. So you could also do something like this:

import { ok } from 'neverthrow'

// ...

expect(callSomeFunctionThatReturnsAResult("with", "some", "args")).toEqual(ok(someExpectation));

By default, the thrown value does not contain a stack trace. This is because stack trace generation makes error messages in Jest harder to understand. If you want stack traces to be generated, call _unsafeUnwrap and / or _unsafeUnwrapErr with a config object:

_unsafeUnwrapErr({
  withStackTrace: true,
})

// ^ Now the error object will have a `.stack` property containing the current stack

If you find this package useful, please consider sponsoring me or simply buying me a coffee!

A note on the Package Name

Although the package is called neverthrow, please don't take this literally. I am simply encouraging the developer to think a bit more about the ergonomics and usage of whatever software they are writing.

Throwing and catching is very similar to using goto statements - in other words; it makes reasoning about your programs harder. Secondly, by using throw you make the assumption that the caller of your function is implementing catch. This is a known source of errors. Example: One dev throws and another dev uses the function without prior knowledge that the function will throw. Thus, and edge case has been left unhandled and now you have unhappy users, bosses, cats, etc.

With all that said, there are definitely good use cases for throwing in your program. But much less than you might think.