Lesson 12 — Narrowing with in

Tell shapes apart with in

When a union is made of different object shapes, typeof says "object" for all of them. The in operator checks whether a property exists: "r" in s. TypeScript uses that check to narrow — inside the if, s is known to be the shape that has r.

type Cat = { meow: () => void };
type Dog = { bark: () => void };
function speak(p: Cat | Dog) {
  if ("bark" in p) p.bark(); else p.meow();
}

Why it matters: API responses often come in several shapes, and in is the clean way to handle each one.