Event emitter

event_emitter.js

const events = Symbol("events");

class EventEmitter {
    constructor() {
        this[events] = new Map();
    }

    addListener(event, callback, once = false) {
        this[events].has(event) || this[events].set(event, new Map());

        const listeners = this[events].get(event);
        listeners.set(callback, once)
    }

    on(...args) {
        this.addListener(...args);
    }

    once(event, callback) {
        this.on(event, callback, true);
    }

    emit(event, ...args) {
        if (!this[events].has(event)) {
            return;
        }

        const listeners = this[events].get(event);
        const currentListeners = [];

        for (let listener of listeners.entries()) {
            currentListeners.push(listener);
        }

        currentListeners.forEach(([listener, once]) => {
            listener.apply(null, args);

            if (once) {
                this.removeListener(event, listener);
            }
        });
    }

    removeListener(event, listener) {
        if (!this[events].has(event)) {
            return;
        }

        const listeners = this[events].get(event);

        if (!listener) {
            listeners.clear();
            this[events].delete(event);
        } else {
            listeners.delete(listener);
        }
    }

    listeners(event) {
        const listeners = this[events].get(event);
        const result = [];

        for (let [listener] of listeners.entries()) {
            result.push(listener);
        }

        return result;
    }
}

example.js

class Observer {
    constructor(id, subject) {
        this.id = id;
        this.subject = subject;
        this.subject.on('change', data => this.onChange(data));
    }

    onChange(data) {
        console.log(`${this.id} notified of change:`, data);
    }
}

const observable = new EventEmitter();
const observers = new Array(5)
    .fill(null)
    .map((_, index) => new Observer(index, observable));
    
observable.emit('change', { someKey: 'some very important data' });
// =>
//    0 notified of change: { someKey: 'some very important data' }
//    1 notified of change: { someKey: 'some very important data' }
//    2 notified of change: { someKey: 'some very important data' }
//    3 notified of change: { someKey: 'some very important data' }
//    4 notified of change: { someKey: 'some very important data' }