function PubSub() {
    this._event = {};

    this.on = function (eventName, handler) {
        if (this._event[eventName]) {
            this._event[eventName].push(handler);
        } else {
            this._event[eventName] = [handler];
        }
    };

    this.emit = function (eventName) {
        var events = this._event[eventName];
        var otherArgs = Array.prototype.slice.call(arguments).slice(1);
        var self = this;

        if (events) {
            events.forEach(function (event) {
                event.apply(self, otherArgs);
            });
        }
    };

    this.off = function (eventName, handler) {
        var events = this._event[eventName];
        if (events) {
            this._event[eventName] = events.filter(function (event) {
                return event !== handler;
            });
        }
    };

    this.once = function (eventName, handler) {
        var self = this;
        function func() {
            var args = Array.prototype.slice.call(arguments);
            handler.apply(self, args);
            this.off(eventName, func);
        }
        this.on(eventName, func);
    };

    this.removeAll = function removeAll(eventName) {
        delete this._event[eventName];
    };
}

// ES6
class PubSub {
    constructor() {
      this._event = {};
    }
    on(eventName, handler) {
      if (this._event[eventName]) {
        this._event[eventName].push(handler);
      } else {
        this._event[eventName] = [handler];
      }
    }
  
    emit(eventName) {
      const events = this._event[eventName];
      const otherArgs = [...arguments].slice(1);
      const self = this;
      if (events) {
        events.forEach(event => {
          event.apply(self, otherArgs);
        });
      }
    }
    off(eventName, handler) {
      const events = this._event[eventName];
      if (events) {
        this._event[eventName] = events.filter(event => {
          return event !== handler;
        });
      }
    }
    once(eventName, handler) {
      const self = this;
      function func() {
        const args = [...arguments];
        handler.apply(self, args);
        this.off(eventName, func);
      }
      this.on(eventName, func);
    }
    removeAll(eventName){
      delete this._event[eventName];
    }
}