emit وon الآمنان نوعيًا

interface Events { login: { userId: string }; logout: void }
function emit<K extends keyof Events>(event: K, payload: Events[K]) {
  console.log("emit", event, JSON.stringify(payload));
}
emit("login", { userId: "u1" });

interface Events { login: { userId: string } }
function emit<K extends keyof Events>(event: K, payload: Events[K]) {}
// emit("login", { id: "u1" }); // Error: id is not userId
emit("login", { userId: "u1" });
console.log("ok");

interface Events { login: { userId: string } }
function emit<K extends keyof Events>(event: K, payload: Events[K]) {}
// emit("signup", { userId: "x" }); // Error: "signup" not a key
emit("login", { userId: "x" });
console.log("only known events");

interface Events { login: { userId: string }; logout: void }
function on<K extends keyof Events>(event: K, handler: (payload: Events[K]) => void) {
  console.log("subscribed to", event);
}
on("login", (p) => console.log(p.userId));

interface Events { message: { text: string } }
function on<K extends keyof Events>(event: K, handler: (p: Events[K]) => void) {}
on("message", (p) => {
  // p is { text: string }
  console.log(p.text.toUpperCase());
});

interface Events { logout: void }
function on<K extends keyof Events>(event: K, handler: (p: Events[K]) => void) {}
on("logout", () => console.log("logged out"));

interface Events { ping: { at: number } }
type Handler<K extends keyof Events> = (p: Events[K]) => void;
const handlers: { ping: Handler<"ping">[] } = { ping: [] };
function on<K extends keyof Events>(e: K, h: Handler<K>) { handlers[e].push(h as Handler<"ping">); }
function emit<K extends keyof Events>(e: K, p: Events[K]) { handlers[e].forEach(h => h(p as Events["ping"])); }
on("ping", p => console.log("at", p.at));
emit("ping", { at: 5 });

interface Events { add: { x: number; y: number } }
function emit<K extends keyof Events>(e: K, p: Events[K]) {
  return e;
}
emit("add", { x: 1, y: 2 });
// emit("add", { x: 1 }); // Error: y missing
console.log("payload checked");

interface Events { open: void; close: void }
function emit<K extends keyof Events>(e: K, p: Events[K]) {}
// Typing emit(" suggests "open" and "close"
emit("open", undefined as void);
console.log("autocompleted names");

interface Events { a: { x: number }; b: { y: string } }
function emit<K extends keyof Events>(e: K, p: Events[K]) { return p; }
console.log(emit("a", { x: 1 }), emit("b", { y: "hi" }));

interface Events { tick: { n: number } }
function emit<K extends keyof Events>(e: K, p: Events[K]) { return e; }
function on<K extends keyof Events>(e: K, h: (p: Events[K]) => void) { return e; }
console.log(emit("tick", { n: 1 }), on("tick", p => p.n));

interface Events { login: { userId: string } }
function emit<K extends keyof Events>(e: K, p: Events[K]) { return e; }
console.log(emit("login", { userId: "u1" }));