Typescript Conditionals

conditionals.ts

import { options } from './lib'

/**
 * Conditional Types in typescript are a subtype of generic types
 *
 * They are useful for deriving information from the type system
 */

// Generics aka Parametric Types: Types which take types and return other types
namespace FrontendCoeRules {

    // generic reminder
    type Predicate<T> = (arg: T) => boolean

    let isEven: Predicate<number> = x => x % 2 === -0

    // What if I want to extract number from that type?

    type ReturnType<Func> = Func extends (...args: any[]) => infer R ? R : never

    type IsEvenArg = ReturnType<typeof isEven>

    // ReturnType already exists


    type ArrayKind<T extends any[]> = T extends (infer U)[] ? U : never;

    type Options = ArrayKind<typeof options>

}

// lib.ts

export let options: { label: string, value: string }[] = [{ label: 'cool stuff', value: 'actualEvil' }]