TS Result

A simple Result type for Typescript loosely inspired by F# Result

Motivation

Typescript doesn't have a standard approach to handling domain errors. Using a Result type helps model errors as domain concepts.

There are various implementations of this type across the internet. They are generally overly complicated or are part of a larger functional library. This library is just an implementation of the Result type that is easy to use in normal Typescript without requiring any other functional patterns.

Installation

To install the package run npm install @getjo/ts-result

Usage

type CreateUserError = "InvalidEmail" | "UserExists"

type User = { email: string }

type UserResult = Result<User, CreateUserError>

type HttpResponse = {
  statusCode: number
  body?: string
}

const createUser = (dto: { email: string }): UserResult => {
  if (!dto.email.includes("@")) {
    return error("InvalidEmail")
  }

  return ok({ email: "[email protected]" })
}

const httpHandler = (dto: { email: string }): HttpResponse => {
  const createUserResult = createUser(dto)

  if (isOk(createUserResult)) {
    return {
      statusCode: 200,
      body: JSON.stringify(unwrapValue(createUserResult)),
    }
  }

  switch (createUserResult.error) {
    case "InvalidEmail":
      return { statusCode: 400 }
    case "UserExists":
      return { statusCode: 409 }
    default:
      return { statusCode: 500 }
  }
}

Documentation

Type annotations

Result<OkType, ErrorType>

Create a result type with an OkType and an ErrorType.

type CreateUserError = "InvalidEmail" | "UserExists"

type User = { email: string }

type UserResult = Result<User, CreateUserError>

Functions

ok(value: OkType)

Create a result with an Ok value.

type CreateUserError = "InvalidEmail" | "UserExists"

type User = { email: string }

type UserResult = Result<User, CreateUserError>

const userResult: UserResult = ok({ email: "[email protected]" })

error(value: ErrorType)

Create a result with an Error value.

type CreateUserError = "InvalidEmail" | "UserExists"

type User = { email: string }

type UserResult = Result<User, CreateUserError>

const userResult: UserResult = error("InvalidEmail")

isOk(result: Result<OkType, ErrorType>)

Typeguard to determine if the Result contains an Ok value.

const okResult = ok("This is fine")

const good = isOk(okResult) // true

isError(result: Result<OkType, ErrorType>)

Typeguard to determine if the Result contains an Error value.

const errorResult = error("Not good")

const bad = isError(errorResult) // true

unwrapValue(result: Result<OkType, ErrorType>)

Returns the value for a Result with an Ok value.

const okResult: Result<string, string> = ok("This is fine")

const okValue = unwrapValue(okResult) // "This is fine"

unwrapError(result: Result<OkType, ErrorType>)

Returns the value for a Result with an Error value.

const errorResult: Result<string, string> = error("Not good")

const errorValue = unwrapError(errorResult) // "Not good"