Typescript Pattern Matching

Pattern matching is a feature built in into many programming languages. Sadly it's not included in JavaScript nor TypeScript. It's like a switch statement on steroids, that lets you write a more declarative code.

This library aims to implement a pattern matching system that works well with other aspects of TS. It uses Builder Pattern to get all the cases and evaluate them. The default case is mandatory.

Installation

Install it using npm (or yarn).

npm install @safe-ts/pattern

It can be imported then in your *.ts files.

import match, { AnyNumber, AnyString } from '@safe-ts/pattern'

// (...)

match(x)
  .case(AnyNumber, () => '...')
  .case(AnyString, () => '...')
  .default(() => '...')

Comparison with other languages

Scala

import scala.util.Random

val x: Int = Random.nextInt(10)

x match {
  case 0 => "zero"
  case 1 => "one"
  case 2 => "two"
  case _ => "other"
}

Elm

patternMatching : Int -> String
patternMatching x =
  case x of
    0 -> "zero"
    1 -> "one"
    2 -> "two"
    _ -> "other"

Typescript with this library

const x = Math.floor(Math.random() * 10);

match(x)
  .case(0, () => 'zero')
  .case(1, () => 'one')
  .case(2, () => 'two')
  .default(() => 'other')

Examples

Simple value based pattern matching

const result = match(5)
  .case(1, () => 'one')
  .case(3, () => 'three')
  .case(5, () => 'five')
  .default(x => `The value is ${x}`)
  
result // 'five'

Partial pattern matching on objects

const result = match(anObject)
  .case({ loading: true }, () => 'Loading...')
  .case({ data: true }, () => 'Data received')
  .default(() => 'Default state')

Partial pattern matching with Any matchers

const result = match(anObject)
  .case({ loading: true }, () => 'Loading...')
  .case({ data: AnyObject }, () => 'Data received')
  .case({ error: AnyObject }, () => 'Error!')
  .default(() => 'Default state')

Typesafe JSON parsing

When parsing JSON strings we do not know if the result will be correctly typed.

interface User {
  name: string
  age: number
}

const jsonString = '{ "definetly": "not an user" }'
const user = JSON.parse(jsonString) as User

user.name // runtime boom

interface User {
  name: string
  age: number
}

const jsonString = '{ "definetly": "not an user" }'

const user = match<User | null>(JSON.parse(jsonString))
  .case({ name: AnyString, age: AnyNumber }, (user: unknown) => user as User)
  .default(() => null)

user && user.name // user will be null if the structure is not right, otherwise it's always guaranteed to be of User type.

kkalamarski/ts-pattern-matching