Publish subscribe factory function

publish_subscribe.js

function publishSubscribe() {
  const subscribers = {};
  function subscribe(eventName, callback) {
    if (!Array.isArray(subscribers[eventName])) {
      subscribers[eventName] = [];
    }
    subscribers[eventName].push(callback);
  }
  function publish(eventName, data) {
    if (!Array.isArray(subscribers[eventName])) {
      return;
    }
    subscribers[eventName].forEach(callback => {
      if (typeof callback === 'function') {
        callback(data);
      }
    })
  }
  return {
    publish,
    subscribe
  };
}