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> and gives you the same level of expressivity and control as a regular Result.

ResultAsync is thenable meaning it behaves exactly like a native Promise ... 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

``sh
> 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[] into Result, or a ResultAsync[] into ResultAsync (just like Promise.all)


`typescript
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

$3

#### ok

Constructs an Ok variant of Result

Signature:

`typescript
ok(value: T): Ok { ... }
`

Example:

`typescript
import { ok } from 'neverthrow'

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

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

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---

#### err

Constructs an Err variant of Result

Signature:

`typescript
err(err: E): Err { ... }
`

Example:

`typescript
import { err } from 'neverthrow'

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

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

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---

#### Result.isOk (method)

Returns true if the result is an Ok variant

Signature:

`typescript
isOk(): boolean { ... }
`

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---

#### Result.isErr (method)

Returns true if the result is an Err variant

Signature:

`typescript
isErr(): boolean { ... }
`

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---

#### Result.map (method)

Maps a Result to Result 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:

`typescript
type MapFunc = (f: T) => U
map(fn: MapFunc): Result { ... }
`

Example:

`typescript
const { getLines } from 'imaginary-parser'
// ^ assume getLines has the following signature:
// getLines(str: string): Result, 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 inside it
const newResult = linesResult.map(
(arr: Array) => arr.map(parseInt)
)

newResult.isOk() // true
`

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---

#### Result.mapErr (method)

Maps a Result to Result 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:

`typescript
type MapFunc = (e: E) => F
mapErr(fn: MapFunc): Result { ... }
`

Example:

`typescript
import { parseHeaders } 'imaginary-http-parser'
// imagine that parseHeaders has the following signature:
// parseHeaders(raw: string): Result

const rawHeaders = 'nonsensical gibberish and badly formatted stuff'

const parseResult = parseHeaders(rawHeaders)

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

parseResult.isErr() // true
`

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---

#### Result.unwrapOr (method)

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

Signature:

`typescript
unwrapOr(v: T): T { ... }
`

Example:

`typescript
const myResult = err('Oh noooo')

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

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

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---

#### 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, E1> into a Result (see example below).

Signature:

`typescript

type AndThenFunc = (t: T) => Result
andThen(f: AndThenFunc): Result { ... }

`

Example 1: Chaining Results

`typescript
import { err, ok } from 'neverthrow'

const sq = (n: number): Result => 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

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

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

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---

#### Result.asyncAndThen (method)

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

The returned value will be a ResultAsync.

Signature:

`typescript

type AndThenAsyncFunc = (t: T) => ResultAsync
asyncAndThen(f: AndThenAsyncFunc): ResultAsync { ... }

`

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---

#### Result.orElse (method)

Takes an Err value and maps it to a Result. This is useful for error recovery.

Signature:

`typescript
type ErrorCallback = (e: E) => Result
orElse(f: ErrorCallback): Result { ... }
`

Example:

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

const dbQueryResult: Result = 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)
)
`

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---


#### 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:

`typescript
match(
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:

`typescript
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)
`

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---

#### 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:

`typescript
type MappingFunc = (t: T) => Promise
asyncMap(fn: MappingFunc): ResultAsync { ... }
`

Example:

`typescript
import { parseHeaders } 'imaginary-http-parser'
// imagine that parseHeaders has the following signature:
// parseHeaders(raw: string): Result

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().

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---

#### 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:

`typescript
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("{");
`

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---

$3

#### okAsync

Constructs an Ok variant of ResultAsync

Signature:

`typescript
okAsync(value: T): ResultAsync
`

Example:

`typescript
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
`

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---

#### errAsync

Constructs an Err variant of ResultAsync

Signature:

`typescript
errAsync(err: E): ResultAsync
`

Example:

`typescript
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
`

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---

#### ResultAsync.fromPromise (static class method)

Transforms a Promise into a ResultAsync.

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:

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

Example:

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

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

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---

#### ResultAsync.map (method)

Maps a ResultAsync to ResultAsync 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) with no impact to the return type.

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

Signature:

`typescript
type MapFunc = (f: T | Promise) => U
map(fn: MapFunc): ResultAsync { ... }
`

Example:

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

const usersInCanada = findUsersIn("Canada")

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

// We can extract the Result using .then() or await
namesInCanada.then((namesResult: Result, 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(','))
}
})
`

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---

#### ResultAsync.mapErr (method)

Maps a ResultAsync to ResultAsync 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) with no impact to the return type.

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

Signature:

`typescript
type MapFunc = (e: E) => F | Promise
mapErr(fn: MapFunc): ResultAsync { ... }
`

Example:

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

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

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---

#### 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 instead of T.

Signature:

`typescript
unwrapOr(v: T): Promise { ... }
`

Example:

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

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---

#### 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, E1> into a ResultAsync (see example below).

Signature:

`typescript
type AndThenFunc = (t: T) => ResultAsync | Result
andThen(f: AndThenFunc): ResultAsync { ... }
`

Example

`typescript

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
// insertUser(user): ResultAsync
// sendNotification(user): ResultAsync

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

// resAsync is a ResultAsync

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

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---


#### ResultAsync.orElse (method)

Takes an Err value and maps it to a ResultAsync. This is useful for error recovery.

Signature:

`typescript
type ErrorCallback = (e: E) => Result | ResultAsync
orElse(f: ErrorCallback): ResultAsync { ... }
`

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---

#### 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:

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

Example:

`typescript

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

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

// 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
`

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---

$3

#### 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:

`typescript
// homogeneous lists
function combine(resultList: Result[]): Result

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

`typescript
function combine(asyncResultList: ResultAsync[]): ResultAsync
`

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---

$3

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

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

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

That way you can do something like:

`typescript
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:

`typescript
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:

`typescript
_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 throw`s 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.