build/dom4.max.js
function createEventListener(type, callback, options) {
    function eventListener(e) {
      if (eventListener.once) {
        e.currentTarget.removeEventListener(
          e.type,
          callback,
          eventListener
        );
        eventListener.removed = true;
      }
      if (eventListener.passive) {
        e.preventDefault = createEventListener.preventDefault;
      }
      if (typeof eventListener.callback === 'function') {
        /* jshint validthis: true */
        eventListener.callback.call(this, e);
      } else if (eventListener.callback) {
        eventListener.callback.handleEvent(e);
      }
      if (eventListener.passive) {
        delete e.preventDefault;
      }
    }
    eventListener.type = type;
    eventListener.callback = callback;
    eventListener.capture = !!options.capture;
    eventListener.passive = !!options.passive;
    eventListener.once = !!options.once;
    // currently pointless but specs say to use it, so ...
    eventListener.removed = false;
    return eventListener;
  }
function createEventListener(type, callback, options) {
    function eventListener(e) {
      if (eventListener.once) {
        e.currentTarget.removeEventListener(
          e.type,
          callback,
          eventListener
        );
        eventListener.removed = true;
      }
      if (eventListener.passive) {
        e.preventDefault = createEventListener.preventDefault;
      }
      if (typeof eventListener.callback === 'function') {
        /* jshint validthis: true */
        eventListener.callback.call(this, e);
      } else if (eventListener.callback) {
        eventListener.callback.handleEvent(e);
      }
      if (eventListener.passive) {
        delete e.preventDefault;
      }
    }
    eventListener.type = type;
    eventListener.callback = callback;
    eventListener.capture = !!options.capture;
    eventListener.passive = !!options.passive;
    eventListener.once = !!options.once;
    // currently pointless but specs say to use it, so ...
    eventListener.removed = false;
    return eventListener;
  }
https://dom.spec.whatwg.org/#interface-eventtarget