Revenge/src/lib/emitter.ts

import { Emitter, EmitterEvent, EmitterListener, EmitterListenerData, EmitterListeners } from "@types";

export const Events = Object.freeze({
    GET: "GET",
    SET: "SET",
    DEL: "DEL",
});

export default function createEmitter(): Emitter {
    return {
        listeners: Object.values(Events).reduce<EmitterListeners>((acc, val: string) => ((acc[val] = new Set<EmitterListener>()), acc), {}) as EmitterListeners,

        on(event: EmitterEvent, listener: EmitterListener) {
            if (!this.listeners[event].has(listener)) this.listeners[event].add(listener);
        },

        off(event: EmitterEvent, listener: EmitterListener) {
            this.listeners[event].delete(listener);
        },

        once(event: EmitterEvent, listener: EmitterListener) {
            const once = (event: EmitterEvent, data: EmitterListenerData) => {
                this.off(event, once);
                listener(event, data);
            };
            this.on(event, once);
        },

        emit(event: EmitterEvent, data: EmitterListenerData) {
            for (const listener of this.listeners[event]) listener(event, data);
        },
    };
}