Type narrowing là gì? Các cách narrow type trong TypeScript?

function process(val: string | number | Date) {
  if (typeof val === 'string') return val.toUpperCase(); // typeof
  if (val instanceof Date) return val.toISOString();    // instanceof
  return val.toFixed(2);                                 // number
}

// User-defined type guard
function isUser(obj: unknown): obj is User {
  return typeof obj === 'object' && obj !== null && 'id' in obj;
}