ts-result

A zero-dependency Result type for TypeScript

Install

$ npm install @haideralsh/ts-result

Usage

Example adapted from the Elm guide

import { Result, Ok, Err } from '@haideralsh/ts-result'

function isReasonableAge(age: number): Result<number, string> {
    if (age < 0) return Err('Please try again after you are born.')
    if (age > 135) return Err('Are you some kind of a turtle?')

    return Ok(age)
}

const ageResult = isReasonableAge(9000)

if (ageResult.ok === true) {
    const age = ageResult.get()
} else {
    console.error(ageResult.getError()) // Are you some kind of a turtle?
}

What is a Result type and why should I use it?

A Result type is the result of a computation that may fail. The type is used for returning and propagating errors. It is helpful to give additional information when things go wrong.

Example

Let us say that we want to insert a user into our database and return the user entity back:

function addUser(name: string, email: string): User {
    const { uuid } = db.query('insert into user values (?, ?)', name, email)
    return User.getById(uuid)
}

However, an insertion operation can sometimes fail. There is no way for the caller of the function above to know that such failure could happen without diving into the implementation of the function.

We could handle the potential failure ourselves and return nothing when the failure happens:

function addUser(name: string, email: string): User | null {
    try {
        const { uuid } = db.query('insert into user values (?, ?)', name, email)
        return User.getById(uuid)
    } catch (err) {
        return null
    }
}

If TypeScript is configured properly, this could be an improvement since TypeScript will warn the caller that the result may be null. However, it doesn't provide them a way to know what is the error that occurred.

We can change the null to be a string instead:

function addUser(name: string, email: string): User | string {
    try {
        const { uuid } = db.query('insert into user values (?, ?)', name, email)
        return User.getById(uuid)
    } catch (err) {
        return err.message
    }
}

This is not any better because now we lose the TypeScript null warning and we still have to check if the returned type is a string or a User which can be non-trivial.

This problem becomes more pronounced when both the okay and error results share the same type. Let us say we only want to return the user identifier instead of the whole User object:

function addUser(name: string, email: string): string {
    try {
        const { uuid } = db.query('insert into user values (?, ?)', name, email)
        return uuid
    } catch (err) {
        return err.message
    }
}

Now we can not tell if the string returned is the user's UUID or the error message.

This is where the Result type comes in. It provides a more elegant way for handling potentially erroneous results.

We can rewrite the above example into this:

import { Result, Ok, Err } from '@haideralsh/ts-result'

function addUser(name: string, email: string): Result<string, string> {
    try {
        const { uuid } = db.query('insert into user values (?, ?)', name, email)
        return Ok(uuid)
    } catch (err) {
        return Err(err.message)
    }
}

Now the caller of this function can use the ok property to know which type of result is returned

const insertResult = addUser('Micheal', 'micheal@example.com')

if (insertResult.ok === true) {
    const userId = insertResult.get() // The user's UUID if the insertion succeeded
} else {
    const errorMsg = insertResult.getError() // The error message if the insertion failed
}

API

Result<T, E>

A Result type accepts two generics. The first (T) must match the value type passed to Ok, while the second (E) must match the value type passed to Err.

A function with a return type Result must return either an Ok or Err or both of them matching their respective types.

import { Result, Ok, Err } from '@haideralsh/ts-result'

function divide(a: number, b: number): Result<number, string> {
    if (b === 0) {
        return Err('Division by zero!')
    }

    return Ok(a / b)
}

Properties and functions on a result wrapped with Ok or Err:

We can use the ok property in order to know if the result was wrapped with an Ok or an Err

ok: boolean

The property ok will be true if a result was wrapped with an Ok or false if it was wrapped with Err.

Example (Ok)

// See `divide` implementation above
const divideResult = divide(1, 1)

console.log(divideResult.ok) // true

Example (Err)

// See `divide` implementation above
const divideResult = divide(1, 0)

console.log(divideResult.ok) // false

getOr(defaultValue: any): any

The getOr function will return the ok value if the result was wrapped with an Ok or the supplied default value if it was wrapped with Err.

Example (Ok)

// See `divide` implementation above
const divideResult = divide(1, 1)

console.log(divideResult.getOr('foo')) // 1

Example (Err)

// See `divide` implementation above
const divideResult = divide(1, 0)

console.log(divideResult.getOr('foo')) // foo

getOrThrow(err?: Error | string): any

The getOrThrow function will return the ok value if the result was wrapped with an Ok or throw an error using the Err value as a message. You can supply the error or the error message to throw as an optional parameter.

Example (Ok)

// See `divide` implementation above
const divideResult = divide(1, 1)

console.log(divideResult.getOrThrow('foo')) // 1

Example (Err)

// See `divide` implementation above
const divideResult = divide(1, 0)

console.log(divideResult.getOrThrow()) // Uncaught Error: Division by zero!
console.log(divideResult.getOrThrow('foo')) // Uncaught Error: foo
console.log(divideResult.getOrThrow(new MyCustomError('foo'))) // Uncaught MyCustomError: foo

getOrRun: <S>(fn: () => S): any

The getOrRun function will return the ok value if the result was wrapped with an Ok or run a function if the result was wrapped with an Err.

Example (Ok)

// See `divide` implementation above
const divideResult = divide(1, 1)

function greeting() {
    return 'Hello world!'
}

console.log(divideResult.getOrRun(greeting)) // 1

Example (Err)

// See `divide` implementation above
const divideResult = divide(1, 0)

function greeting() {
    return 'Hello world!'
}

console.log(divideResult.getOrRun(greeting)) // Hello world!

mapWithDefault<S, R>(defaultValue: S, fn: (parameter: any) => R): R

The mapWithDefault function will apply a function on the ok value if the result was wrapped with an Ok or the error value if the result was wrapped with an Err. The return type will be the same as the supplied function to map.

Example (Ok)

// See `divide` implementation above
const divideResult = divide(1, 1)

function addOne(num: number) {
    return num + 1
}

console.log(divideResult.mapWithDefault(10, addOne)) // 2

Example (Err)

// See `divide` implementation above
const divideResult = divide(1, 0)

function addOne(num: number) {
    return num + 1
}

console.log(divideResult.mapWithDefault(10, addOne)) // 11

Ok(value: T)

The Ok function accepts a value of type T that must match the T generic type passed to Result<T, E>

Properties and functions unique to a result wrapped with an Ok function:

get(): T

The get function will unwrap the value wrapped with an Ok. You have to check if the ok property is true first before being able to use get.

Example

const okResult = OK('foo')

if (okResult.ok === true) console.log(okResult.get()) // foo

map: <S>(fn: (parameter: T) => S): S

The map function will unwrap and apply the supplied function on the value wrapped with an Ok. The supplied function must accept the same type as the wrapped value type. The return type of the map function will be the same as the one of the supplied function. You have to check if the ok property is true first before being able to use map.

Example

const okResult = OK('foo')

function capitalize(str: string) {
    return str.toUpperCase()
}

if (okResult.ok === true) console.log(okResult.map(capitalize)) // FOO

Err(errorValue: E)

The Err function accepts a value of type E that must match the E generic type passed to Result<T, E>

Properties and functions unique to a result wrapped with an Err function:

getError(): E

The getError function will unwrap the value wrapped with an Err. You have to check if the ok property is false first before being able to use getError.

Example

const errResult = Err('foo')

if (errResult.ok === false) console.log(errResult.getError()) // foo