Observer

    function EventEmitter () {
        this.events = {};
    }

    EventEmitter.prototype.on = function(name, cb) {
        if(this.events[name]){
            this.events[name].push(cb)
            
        }else{
           this.events[name] = [cb]; 
        }
    };

    EventEmitter.prototype.emit = function(name,...arg) {
        if (this.events[name]) {
            this.events[name].forEach(function (fn) {
                fn.call(this, ...arg);
            });
        }
    };

    EventEmitter.prototype.off = function (name, cb) {
        if(this.events[name]){
            this.events[name] = this.events[name].filter(function (fn) {
                return fn !== cb;
            });

            if (!this.events[name] || this.events[name].length === 0) {
                delete this.events[name];
            }
        }
    }

    EventEmitter.prototype.once = function (name, cb) {
        var _this = this;

        function onlyOnce () {
            cb.call(this, arguments);
            _this.off(name, onlyOnce);
        }

        this.on(name, onlyOnce);
    }

    var events = new EventEmitter();

    events.on('addEvent', function () {
        console.log('on AddEventHanndle');
    });

    events.once('addEvent2', function () {
        console.log('once AddEvent2Hanndle -- 1');
    });

    events.once('addEvent2', function () {
        console.log('once AddEvent2Hanndle -- 2');
    });