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).

``bash
npm install @safe-ts/pattern
`

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

`typescript

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

// (...)

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

`


Comparison with other languages

$3

`scala
import scala.util.Random

val x: Int = Random.nextInt(10)

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


$3


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

$3

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

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

Examples

$3


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

result // 'five'
`

$3

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

$3

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

$3

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

`typescript
interface User {
name: string
age: number
}

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

user.name // runtime boom
`

`typescript
interface User {
name: string
age: number
}

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

const user = match(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.
``