class ClassEventsES6 {
                constructor() {
                    this.listeners = new Map();
                    this.onceListeners = new Map();
                    this.triggerdLabels = new Map();
                }

                // help-function for onReady and onceReady
                // the callbackfunction will execute, 
                // if the label has already been triggerd with the last called parameters
                _fCheckPast(label, callback) {
                    if (this.triggerdLabels.has(label)) {
                        callback(this.triggerdLabels.get(label));
                        return true;
                    } else {
                        return false;
                    }
                }

                // execute the callback everytime the label is trigger
                on(label, callback, checkPast = false) {
                    this.listeners.has(label) || this.listeners.set(label, []);
                    this.listeners.get(label).push(callback);
                    if (checkPast)
                        this._fCheckPast(label, callback);
                }

                // execute the callback everytime the label is trigger
                // check if the label had been already called 
                // and if so excute the callback immediately
                onReady(label, callback) {
                    this.on(label, callback, true);
                }

                // execute the callback onetime the label is trigger
                once(label, callback, checkPast = false) {
                    this.onceListeners.has(label) || this.onceListeners.set(label, []);
                    if (!(checkPast && this._fCheckPast(label, callback))) {
                        // label wurde nocht nicht aufgerufen und 
                        // der callback in _fCheckPast nicht ausgeführt
                        this.onceListeners.get(label).push(callback);
                }
                }
                // execute the callback onetime the label is trigger
                // or execute the callback if the label had been called already
                onceReady(label, callback) {
                    this.once(label, callback, true);
                }

                // remove the callback for a label
                off(label, callback = true) {
                    if (callback === true) {
                        // remove listeners for all callbackfunctions
                        this.listeners.delete(label);
                        this.onceListeners.delete(label);
                    } else {
                        // remove listeners only with match callbackfunctions
                        let _off = (inListener) => {
                            let listeners = inListener.get(label);
                            if (listeners) {
                                inListener.set(label, listeners.filter((value) => !(value === callback)));
                            }
                        };
                        _off(this.listeners);
                        _off(this.onceListeners);
                }
                }

                // trigger the event with the label 
                trigger(label, ...args) {
                    let res = false;
                    this.triggerdLabels.set(label, ...args); // save all triggerd labels for onready and onceready
                    let _trigger = (inListener, label, ...args) => {
                        let listeners = inListener.get(label);
                        if (listeners && listeners.length) {
                            listeners.forEach((listener) => {
                                listener(...args);
                            });
                            res = true;
                        }
                    };
                    _trigger(this.onceListeners, label, ...args);
                    _trigger(this.listeners, label, ...args);
                    this.onceListeners.delete(label); // callback for once executed, so delete it.
                    return res;
                }
            }
            
// +++ here starts the example +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
class TestClassEvents extends ClassEventsES6 {
     constructor() {
        super();
        this.once('sayHallo', this.fStartToTalk);
        this.on('sayHallo', this.fSayHallo);
     }

     fStartToTalk() {
         console.log('I start to talk... ');
     }

     fSayHallo(name = 'Nobody') {
        console.log('Hallo ' + name);
     }
}

let testClassEvents = new TestClassEvents();

testClassEvents.trigger('sayHallo', 'Tony');
testClassEvents.trigger('sayHallo', 'Tim');

testClassEvents.onReady('sayHallo', e => console.log('I already said hello to ' + e));
testClassEvents.trigger('sayHallo', 'Angie');
testClassEvents.off('sayHallo');
testClassEvents.trigger('sayHallo', 'Peter');
console.log('I dont say hallo to Peter, because the event is off!')