Lesson 25 — Ultimate: typed event emitter

Boss level: a fully typed event bus

An event emitter (or bus) lets parts of an app talk without knowing each other: some code calls on(name, handler) to listen, other code calls emit(name, data). An event map type lists every event name with its data type, and generics with K extends keyof E make sure emit("login", …) gets exactly the login data and each handler receives the right type.

type Events = { saved: { id: number } };
bus.on("saved", (d) => d.id);   // d is { id: number }
// bus.emit("saved", { id: "x" }); ← error

Why it matters: this is production-grade type safety for the event systems in UI libraries and Node.