when-switch
JavaScript functional implementation of switch/case, inspired by Ruby case/when.
Usage
You can convert a switch-case use in a functional way, using a single expression:
Strict Equality
import when from 'when-switch'
const getDrinkPrice = drink =>
when(drink)
.is('Coke', 1.5)
.is('Pepsi', 1.8)
.else(2.0)Structural Matching
You can use match method with any object exposing a test method.
Regular Expressions
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
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)
matchandiscan both be used in the samewhenexpression.
TypeScript
when is fully compatible with TypeScript, and will check the types you return in each is expression:
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
Union types
For each is or else expression added to the current when expression, the type is added as an union to the previous type.
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