Interface Emitter<KeyMap>

interface Emitter<KeyMap extends EventMap> {
    emit<Key extends string>(eventName: Key, params: KeyMap[Key]): void;
    off<Key extends string>(
        eventName: Key,
        fn: EventReceiver<KeyMap[Key]>,
    ): void;
    on<Key extends string>(
        eventName: Key,
        fn: EventReceiver<KeyMap[Key]>,
    ): void;
}