void, never, dan Tanda Tangan Fungsi

function logError(msg: string): void {
  console.error(msg);
  // implicit return undefined
}

const fn: () => void = () => 42; // Allowed! Caller ignores return
const fn2: () => undefined = () => 42; // Error

function run(cb: () => void): void { cb(); }
run(() => 'hello'); // OK — return value ignored

function fail(msg: string): never {
  throw new Error(msg);
}
function loop(): never {
  while (true) {}
}

type Shape = 'circle' | 'square';
function area(s: Shape): number {
  switch (s) {
    case 'circle': return 3.14;
    case 'square': return 1;
    default:
      const _: never = s; // Error if a new shape is added
      return 0;
  }
}

type Handler = (event: MouseEvent) => void;
type Transform = (input: string) => string;

interface Formatter {
  (value: string): string;
  locale: string;
}

interface Constructor {
  new (name: string): Animal;
}

function orThrow(val: string | null): string {
  if (val === null) fail('null!'); // never
  return val; // TypeScript knows this is reachable
}