An event emitter, with an event map

emitter.ts

// Usage:

// interface FooEventMap {
//    run: (arg1: string, arg2: number) => void;
// }
//
// class Foo extends Emitter<{ [K in keyof FooEventMap]: FooEventMap[K] }> {
//     run() {
//         this.emit("run", "First argument", 5);
//     }
// }

class Emitter<M extends {
	[K in string]: (...args: any[]) => void
} = { [K in string]: (...args: any[]) => void }> {
	protected _events: { [K in keyof M]?: M[K][] } = {}

	on<K extends keyof M>(event: K, listener: M[K]) {
		this._events[event] = this._events[event] || [];
		this._events[event].push(listener);
	}

	once<K extends keyof M>(event: K, listener: M[K]) {
		this._events[event] = this._events[event] || [];
		const newListener = (...args: any[]) => {
			// @ts-ignore
			this.off(event, newListener);
			listener.apply(this, args);
		}

		// @ts-ignore
		this.on(event, newListener);
	}

	off<K extends keyof M>(event: K, listener: M[K]) {
		this._events[event] = this._events[event] || [];
		const listeners = this._events[event];
		const index = listeners.indexOf(listener);
		if (index !== -1) listeners.splice(index, 1);
	}

	emit<K extends keyof M>(event: K, ...args: Parameters<M[K]>) {
		this._events[event] = this._events[event] || [];
		this._events[event].forEach((listener) => {
			listener.apply(this, args);
		});
	}
}

export default Emitter;