code: function updateEventListeners(el, prevEvents, events) {
    var eventName;
  
    if (!prevEvents || !events || prevEvents === events) {
      return;
    }
  
    for (eventName in events) {
      // Didn't change.
      if (prevEvents[eventName] === events[eventName]) {
        continue;
      }
  
      // If changed, remove old previous event listeners.
      if (prevEvents[eventName]) {
        removeEventListeners(el, eventName, prevEvents[eventName]);
      }
  
      // Add new event listeners.
      addEventListeners(el, eventName, events[eventName]);
    }
  
    // See if event handlers were removed.
    for (eventName in prevEvents) {
      if (!events[eventName]) {
        removeEventListeners(el, eventName, prevEvents[eventName]);
      }
    }
  }
docs: Handle diffing of previous and current event maps.
  
  @param {Element} el
  @param {Object} prevEvents - Previous event map.
  @param {Object} events - Current event map.