THIS IS A FORK - ALL CREDIT GOES TO THE AUTHOR NAWORDAR - I WILL EVENTUALLY DO A PR TO SYNC


ts-when

JavaScript functional implementation of switch/case written in TypeScript

Note

This is a fork of a great library [when-switch] which seems no longer maintained.

Usage

You can convert a ts-when use in a functional way, using a single expression:

$3

``js
import when from 'ts-switch'

const getDrinkPrice = drink =>
when(drink)
.is('Coke', 1.5)
.is('Pepsi', 1.8)
.else(2.0)
`

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You can return a value when given assertion is correct

`ts
const x = when
.true(someVariable === true, "some variable is true")
.else("some variable is false")

`

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You can use match method with any object exposing a test method.

#### Regular Expressions

`js
const getCaseStyle = text =>
when(text)
.match(/^([A-Z][a-z]*)+$/, 'UpperCamelCase')
.match(/^([a-z]+[A-Z][a-z]*)+$/, 'LowerCamelCase')
.match(/^([a-z]+_[a-z]+)+$/, 'SnakeCase')
.else('Unknown')
`

#### Custom Type Guard Matcher

`ts
type SpaceObject = { x: number; y: number; z: number }
type Cube = SpaceObject & { width: number }
type Sphere = SpaceObject & { radius: number }

const SpaceObjectSchema = {
test: (_: any): _ is SpaceObject =>
typeof _.x === 'number' &&
typeof _.y === 'number' &&
typeof _.z === 'number'
}

const CubeSchema = {
test: (_: any): _ is Cube =>
typeof _.width === 'number' && SpaceObjectSchema.test(_)
}

const SphereSchema = {
test: (_: any): _ is Sphere =>
typeof _.radius === 'number' && SpaceObjectSchema.test(_)
}

const getObjectVolume = (object: SpaceObject) =>
// Each match handler will receive correct static type
when(object)
.match(CubeSchema, cube => cube.width ** 3)
.match(SphereSchema, sphere => Math.PI 3 / 4 sphere.radius ** 3)
.else(_ => null)
`

> match and is can both be used in the same when expression.

TypeScript

when is fully compatible with TypeScript, and will check the types you return in each is expression:

`ts
const getDrinkPrice = (drink: 'Pepsi' | 'Coke' | 'Orangina'): number =>
when(drink)
.is('Coke', 1.5)
.is('Pepsi', 1.8)
.else(2.0)
`

Here the return type of the when expression will be number

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For each is or else expression added to the current when expression, the type is added as an union to the previous type.

`ts
const getDrinkPrice = (drink: 'Pepsi' | 'Coke' | 'Orangina') =>
when(drink)
.is('Coke', 1.5)
.is('Pepsi', true)
.else('Free')
`

Here the return type of getDrinkPrice expression will be number | string | boolean`

[when-switch]: https://github.com/kube/when-switch