Extended subscribe with unsubscribe return value

extended_subscribe.js

function subscribe(eventName, callback) {
  if (!Array.isArray(subscribers[eventName])) {
    subscribers[eventName] = []
  }
  subscribers[eventName].push(callback);
  const index = subscribers[eventName].length - 1;
  return () => {
    subscribers[eventName].splice(index, 1);
  }
}