TypeScript: template literal types — пример кода

// Generate a fully-typed event emitter surface from an event map
type Events = { login: { userId: string }; logout: void };

type Listeners<E> = {
  [K in keyof E & string as `on${Capitalize<K>}`]: (payload: E[K]) => void;
};
type L = Listeners<Events>;
// { onLogin: (p: { userId: string }) => void; onLogout: (p: void) => void }

// Type-safe CSS length values — reject unit-less numbers at compile time
type Unit = 'px' | 'rem' | '%' | 'vh';
type Length = `${number}${Unit}`;

const ok:  Length = '16px';   // ✅
const bad: Length = '16';     // ❌ Type '"16"' is not assignable to type Length

// Parse a query string key path — template inference as a mini-parser
type Split<S extends string, D extends string> =
  S extends `${infer Head}${D}${infer Tail}` ? [Head, ...Split<Tail, D>] : [S];
type Keys = Split<'a.b.c', '.'>; // ['a', 'b', 'c']