Custom EventEmitter

eventemitter.js

class EventEmitter {

    constructor() {
        this.callbacks = {};
        this.noEventYet = [];
    }

    addEventListener(eventName, callback) {
        const callbacks = this.callbacks[eventName];
        if (!Array.isArray(callbacks)) {
            this.callbacks[eventName] = [];
        }
        callbacks.push(callback);

        // Check if there was an event called before any callback registered
        const indexOfNotYetEvent = this.noEventYet.indexOf(eventName);
        if (indexOfNotYetEvent >= 0) {
            callback();
        }
        this.noEventYet.slice(indexOfNotYetEvent, indexOfNotYetEvent + 1);
    }

    removeEventListener(eventName, callback) {
        const callbacks = this.callbacks[eventName];
        if (Array.isArray(callbacks)) {
            const indexOfcallback = callbacks.findIndex(alreadyRegesriedCallback => {
                return alreadyRegesriedCallback === callback;
            });
            callbacks.slice(indexOfcallback, indexOfcallback + 1);
        } else {
            throw new Error(`Event ${eventName} is not exist`);
        }
    }

    dispatchEvent(eventName) {
        const callbacks = this.callbacks[eventName];
        if (Array.isArray(callbacks)) {
            callbacks.forEach(callback => {
                callback();
            });
        } else {
            // throw new Error(`Event ${eventName} is not exist`);
            this.noEventYet.push(eventName);
        }
    }

}

const eventEmitter = new EventEmitter();
function logClick() {
    log('Log click');
}

eventEmitter.addEventListener('click', logClick);
eventEmitter.removeEventListener('click', logClick);

eventEmitter.addEventListener('load_todo', () => {});

eventEmitter.addEventListener('load_todo', () => {});

eventEmitter.dispatchEvent('load_todo');