Developing a Type-Safe Event Bus Architecture

type EventMap = Record<string, unknown>;
type EventCallback<T> = (data: T) => void;
 
class TypedEventBus<Events extends EventMap> {
  private listeners: { [K in keyof Events]?: EventCallback<Events[K]>[] } = {};
 
  on<K extends keyof Events>(event: K, callback: EventCallback<Events[K]>): void {
    if (!this.listeners[event]) {
      this.listeners[event] = [];
    }
    this.listeners[event]!.push(callback);
  }
 
  emit<K extends keyof Events>(event: K, data: Events[K]): void {
    const callbacks = this.listeners[event];
    if (callbacks) {
      callbacks.forEach((cb) => cb(data));
    }
  }
}
 
interface AppEvents {
  userLoggedIn: { userId: string; timestamp: number };
  systemError: { code: number; message: string };
}
 
const bus = new TypedEventBus<AppEvents>();
bus.on("userLoggedIn", (data) => console.log(`User ${data.userId} logged in`));
bus.emit("userLoggedIn", { userId: "usr_101", timestamp: Date.now() });