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README.md

NeverThrow

Description

Encode failure into your program.

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

Read the blog post which explains why you'd want to use this package.

This package works for both JS and TypeScript. However, the types that this package provides will allow you to get compile-time guarantees around error handling if you are using TypeScript.

neverthrow draws inspiration from Rust, and Elm. It is also a great companion to fp-ts.

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

Installation

> npm install neverthrow

Usage

Create Ok or Err instances with the ok and err functions.

import { ok, err } from 'neverthrow'

// something awesome happend

const yesss = ok(someAesomeValue)

// moments later ...

const mappedYes = yesss.map(doingSuperUsefulStuff)

Result is defined as follows:

type  Result<T, E>
  =  Ok<T, E>
  |  Err<T, E>

Ok<T, E>: contains the success value of type T

Err<T, E>: contains the failure value of type E

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 for you to construct an Ok variant in an OOP way using new
  • Err class for you to construct an Err variant in an OOP way using new
  • Result type - only available in TypeScript
import { ok, Ok, err, Err, Result } from 'neverthrow'

API

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

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

Result.isOk (method)

Returns true if the result is an Ok variant

Signature:

isOk():  boolean { ... }

Result.isErr (method)

Returns true if the result is an Err variant

Signature:

isErr():  boolean { ... }

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>(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

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

Result.andThen (method)

Same idea as map above. Except you must return a new 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 ExtendFunc = (t:  T) => Result<U, E>
extend<U>(f: ExtendFunc): 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)

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 is sort of like combining map and mapErr.

Signature:

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

Example:

const result = computationThatMightFail()

const matched = result.match(
  (innerOkValue) => { return 'Yey' },
  (innerErrValue) => { return 'OhNooo' }
)

Result.asyncMap (method)

Similar to map except for two things:

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

Signature:

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

Example:

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

const promise = 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 promise variable will contain a Err inside of the promise.


Result._unsafeUnwrap (method)

Returns the inner Ok value.

Try to avoid unwrapping Results, as it defeats the purpose of using Results in the first place.

Ironically, this function will throw if the Result is an Err.

import { ok } from 'neverthrow'

const result = ok(12)

const twelve = result._unsafeUnwrap()

Result._unsafeUnwrapErr (method)

Returns the inner Err value.

Try to avoid unwrapping Results, as it defeats the purpose of using Results in the first place.

Ironically, this function will throw if the Result is an Ok.

import { err } from 'neverthrow'

const result = err(12)

const twelve = result._unsafeUnwrapErr()

Wrapping a Dependency that throws

incomplete documenation ... Examples to come soon

  • axios
  • knex

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.

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