a koan collection for typescript

koan.ts

// Literal type
type Drinks = 'Coke' | 'Milk' | 'Water' | 'Coffee';

type UnUnite<T, K> = T extends K ? never : T;

type Result = UnUnite<string | number | boolean, string>; // Result: number | boolean
type HealthDrinks = UnUnite<Drinks, 'Coke'>;

// `infer`
// "destructuring type". "Pattern matching"
type Flip<T> = T extends [infer A, infer B] ? [B, A] : T;
type _1 = Flip<[number, boolean]>;
type Args<T> = T extends (...a: infer A) => infer R ? A : never;
type UnionFromTuple<T> = T extends [...a: infer A] ? A : T;
type uft = UnionFromTuple<['hello', 'world']>;
type Xs1 = Args<typeof Math.pow>;
type Xs2 = Args<typeof document.getElementById>;

type Head<T> = T extends [infer H, ...infer _] ? H : never;
type Rest<T> = T extends [infer _, ...infer Rest /* or ...a: infer Rest */]
  ? Rest
  : never;
type Rest2<T> = T extends [infer _, ...a: infer R] ? R : never;
type One = Head<[1, 2, 3]>;
type AfterOne = Rest<[1, 2, 3]>;
type NewAfterOne = Rest2<[1, 2, 3]>;