Implementation of observer pattern for Node.js

Question

I wrote an implementation of observer pattern as part of my online battle Chess game:

const observers = Symbol();
class Observer{
 constructor(){
 if(new.target === Observer){
 throw new Error('Cannot create new Observer object. Observer is supposed to be inherited only.');
 }
 /**
 * @private
 * @type {Set}
 */
 this[observers] = new Set();
 }
 on(observer, event, callback){
 this.getObservers().add({
 observer,
 event,
 callback
 });
 }
 off(observer, event){
 const observers = this.getObservers();
 const observerEntries = this.getObservers().values();
 for(let entry of observerEntries){
 if(entry.observer === observer && entry.event === event){
 observers.delete(entry);
 }
 }
 }
 notify(event, data = {}){
 const observerEntries = this.getObservers().values();
 for(let entry of observerEntries){
 if(entry.event === event){
 entry.callback.call(entry.observer, data);
 }
 }
 }
 getObservers(){
 return this[observers];
 }
}
module.exports = Observer;

The source code for the full game is in a GitHub repository. This is an early version, without many features implemented and possibly with many bugs. I have a humble request to review my code. I'm aware of several things (backend Node code is very messy). The client-side is written in MVC and Observer patterns. I'm looking for feedback (both positive and negative) and advice/guidelines.

• Furman: Question edited, though in case of rather larger projects I don't see any point in posting single short examples of code.
• 200_success: This is outside the scope of this Code Review question, but your login page code crashes here because input[type=text is malformed.

Answer 1

The name of the Observer class is confusing: an Observer contains observers? I think that the class would make more sense if you renamed it Observable.

For the .on() and .off() methods, I think that the event parameter should come first. That would mimic the natural English expression: "on event tell observer to do action". It's also how it's done in other frameworks like jQuery.

I'm mildly annoyed by the excessive vertical spacing of the code.