How to have private instance variables in an ES2015 Class

EventEmitter.js

function on(key, listener){
	addEventListener(this.events, key, listener);
}
function off(key, listener){
	if(listener) removeEventListener(this.events, key, listener);
  	else removeEventListeners(this.events, key);
}
function emit(key, eventArgs){
  	for(let listener of this.events[key]){
    	listener(eventArgs);
    }
}
function addEventListener(events, key, listener){
	//if a listenersArray is stored for specific key, push the new listener on array, else create new array containing the listener for specific key
	events[key] ? events[key].push(listener) : events[key] = [listener];
}
function removeEventListener(events, key, listener){
	let index = events[key].indexOf(listener);
  	if(index > -1) events.splice(index, 1);
}
function removeEventListeners(events, key){
	events[key] = [];
}
class EventEmitter{
	constructor(){
	    //privateMembers is not accessible for the client using EventEmitter class
		let privateMembers = {events: {}};
	    this.on = on.bind(privateMembers);
	    this.off = off.bind(privateMembers);
	    this.emit = emit.bind(privateMembers);
	}
}


let myEventEmitter = new EventEmitter();
myEventEmitter.on("add", (eventArgs) => console.log(eventArgs));
myEventEmitter.emit("add", "hello added");
// output is "hello added"
myEventEmitter.off("add");
myEventEmitter.emit("add", "hello added");
// eventListener is not triggered because it is removed
console.log(myEventEmitter);

let myOtherEventEmitter = new EventEmitter();
myEventEmitter.on("add", (eventArgs) => console.log(eventArgs));
myEventEmitter.emit("add", "hello added 2");
// output is "hello added 2"
myEventEmitter.off("add");
myEventEmitter.emit("add", "hello added 2");
// eventListener is not triggered because it is removed
console.log(myOtherEventEmitter);