observer.js

function Click() {
  this.handlers = [];  // observers
}

Click.prototype = {
  subscribe: (fn) => {
      this.handlers.push(fn);
  },
  unsubscribe: (fn) => {
      this.handlers = this.handlers.filter(...);
  },
  notify: (o, thisObj) => {...}
}

function run() {
  const clickHandler = (item) => { 
      log.add("fired: " + item); 
  };

  let click = new Click();

  click.subscribe(clickHandler);
  click.notify('event #1');
  click.unsubscribe(clickHandler);
  click.notify('event #2');
  click.subscribe(clickHandler);
  click.notify('event #3');
}