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.
<div id="toc"></div>Need to see real-life examples of how to leverage this package for error handling? See this repo: https://github.com/parlez-vous/server
eslint-plugin-neverthrow
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)Result.combine
(static class method)Result.combineWithAllErrors
(static class method)Result.safeUnwrap()
ResultAsync
)
okAsync
errAsync
ResultAsync.fromThrowable
(static class method)ResultAsync.fromPromise
(static class method)ResultAsync.fromSafePromise
(static class method)ResultAsync.map
(method)ResultAsync.mapErr
(method)ResultAsync.unwrapOr
(method)ResultAsync.andThen
(method)ResultAsync.orElse
(method)ResultAsync.match
(method)ResultAsync.combine
(static class method)ResultAsync.combineWithAllErrors
(static class method)ResultAsync.safeUnwrap()
> npm install neverthrow
eslint-plugin-neverthrow
As part of neverthrow
s bounty program, user mdbetancourt created eslint-plugin-neverthrow
to ensure that errors are not gone unhandled.
Install by running:
> npm install eslint-plugin-neverthrow
With eslint-plugin-neverthrow
, you are forced to consume the result in one of the following three ways:
.match
.unwrapOr
._unsafeUnwrap
This ensures that you're explicitly handling the error of your Result
.
This plugin is essentially a porting of Rust's must-use
attribute.
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 typeErr
class and typeResult
Type as well as namespace / object from which to call Result.fromThrowable
, Result.combine.ResultAsync
classokAsync
convenience function to create a ResultAsync
containing an Ok
type Result
errAsync
convenience function to create a ResultAsync
containing an Err
type Result
import { ok, Ok, err, Err, Result, okAsync, errAsync, ResultAsync, fromAsyncThrowable, fromThrowable, fromPromise, fromSafePromise, safeTry, } 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!
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
err
Constructs an Err
variant of Result
Signature:
err<T, E>(error: 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:
class Result<T, E> { map<U>(callback: (value: T) => U): Result<U, E> { ... } }
Example:
import { 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:
class Result<T, E> { mapErr<F>(callback: (error: E) => F): Result<T, F> { ... } }
Example:
import { parseHeaders } from '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.unwrapOr
(method)Unwrap the Ok
value, or return the default if there is an Err
Signature:
class Result<T, E> { unwrapOr<T>(value: T): T { ... } }
Example:
const myResult = err('Oh noooo') const multiply = (value: number): number => value * 2 const unwrapped: number = myResult.map(multiply).unwrapOr(10)
Result.andThen
(method)Same idea as map
above. Except you must return a new Result
.
The returned value will be a Result
. As of v4.1.0-beta
, you are able to return distinct error types (see signature below). Prior to v4.1.0-beta
, the error type could not be distinct.
This is useful for when you need to do a subsequent computation using the inner T
value, but that computation might fail.
Additionally, andThen
is really useful as a tool to flatten a Result<Result<A, E2>, E1>
into a Result<A, E2>
(see example below).
Signature:
class Result<T, E> { // Note that the latest version lets you return distinct errors as well. // If the error types (E and F) are the same (like `string | string`) // then they will be merged into one type (`string`) andThen<U, F>( callback: (value: T) => Result<U, F> ): Result<U, E | F> { ... } }
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.asyncAndThen
(method)Same idea as andThen
above, except you must return a new ResultAsync
.
The returned value will be a ResultAsync
.
Signature:
class Result<T, E> { asyncAndThen<U, F>( callback: (value: T) => ResultAsync<U, F> ): ResultAsync<U, E | F> { ... } }
Result.orElse
(method)Takes an Err
value and maps it to a Result<T, SomeNewType>
. This is useful for error recovery.
Signature:
class Result<T, E> { orElse<A>( callback: (error: E) => Result<T, 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) )
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:
class Result<T, E> { match<A, B = A>( okCallback: (value: T) => A, errorCallback: (error: E) => B ): A | B => { ... } }
match
is like chaining map
and mapErr
, with the distinction that with match
both functions must have the same return type.
The differences between match
and chaining map
and mapErr
are that:
match
both functions must have the same return type A
match
unwraps the Result<T, E>
into an A
(the match functions' return type)
Example:
// map/mapErr api // note that you DON'T have to append mapErr // after map which means that you are not required to do // error handling computationThatMightFail().map(console.log).mapErr(console.error) // match api // works exactly the same as above since both callbacks // only perform side effects, // except, now you HAVE to do error handling :) computationThatMightFail().match(console.log, console.error) // Returning values const attempt = computationThatMightFail() .map((str) => str.toUpperCase()) .mapErr((err) => `Error: ${err}`) // `attempt` is of type `Result<string, string>` const answer = computationThatMightFail().match( (str) => str.toUpperCase(), (err) => `Error: ${err}` ) // `answer` is of type `string`
If you don't use the error parameter in your match callback then match
is equivalent to chaining map
with unwrapOr
:
const answer = computationThatMightFail().match( (str) => str.toUpperCase(), () => 'ComputationError' ) // `answer` is of type `string` const answer = computationThatMightFail() .map((str) => str.toUpperCase()) .unwrapOr('ComputationError')
Result.asyncMap
(method)Similar to map
except for two things:
Promise
ResultAsync
You can then chain the result of asyncMap
using the ResultAsync
apis (like map
, mapErr
, andThen
, etc.)
Signature:
class Result<T, E> { asyncMap<U>( callback: (value: T) => Promise<U> ): ResultAsync<U, E> { ... } }
Example:
import { parseHeaders } from '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()
.
Result.fromThrowable
(static class method)Although
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