Very simple observer pattern example

observer.js

const createObserverPattern = () => {
  let observers = [];
  let register = (observer) => {
    observers.push(observer);
  }
  
  let deregister = (observer) => {
    observers = observers.filter((registeredObserver) => !Object.is(observer, registeredObserver))
  }
  
  let notify = (payload) => {
    observers.forEach((observer) => observer.visit(payload))
  }
  
  return {register, notify, deregister}
}

const myObserver = {
  visit: console.log
}
const {register, notify, deregister} = createObserverPattern()

register(myObserver);
notify({"message": "observed"})