最佳的addEvent事件绑定是怎样诞生的

window.onload = function() {
  var x = document.getElementsByTagName('H3');
  for (var i = 0; i < x.length; i++) {
    x[i].onclick = openClose;
    x[i].relatedElement = x[i].nextSibling; // simplified situation 
    x[i].relatedElement.relatedElement = x[i];
  }
}

function addEvent(obj, type, fn) {
  if (obj.attachEvent) {
    obj['e' + type + fn] = fn;
    obj[type + fn] = function() {
      obj['e' + type + fn](window.event);
    }
    obj.attachEvent('on' + type, obj[type + fn]);
  } else obj.addEventListener(type, fn, false);
}
function removeEvent(obj, type, fn) {
  if (obj.detachEvent) {
    obj.detachEvent('on' + type, obj[type + fn]);
    obj[type + fn] = null;
  } else obj.removeEventListener(type, fn, false);
}

/* 
Original idea by John Resig 
Tweaked by Scott Andrew LePera, Dean Edwards and Peter-Paul Koch 
Fixed for IE by Tino Zijdel (crisp) 
Note that in IE this will cause memory leaks and still doesn't quite function the same as in browsers that do support the W3C event model: 
- event execution order is not the same (LIFO in IE against FIFO) 
- functions attached to the same event on the same element multiple times will also get executed multiple times in IE 
*/
function addEvent(obj, type, fn) {
  if (obj.addEventListener) obj.addEventListener(type, fn, false);
  else if (obj.attachEvent) {
    obj["e" + type + fn] = fn;
    obj.attachEvent("on" + type,
    function() {
      obj["e" + type + fn]();
    });
  }
}
function removeEvent(obj, type, fn) {
  if (obj.removeEventListener) obj.removeEventListener(type, fn, false);
  else if (obj.detachEvent) {
    obj.detachEvent("on" + type, obj["e" + type + fn]);
    obj["e" + type + fn] = null;
  }
}

// written by Dean Edwards, 2005 
// http:/https://img.qb5200.com/download-x/dean.edwards.name/function ;addEvent(element, type, handler) { 
// assign each event handler a unique ID 
// 为事件处理函数设定一个唯一值 
if (!handler.$$guid) handler.$$guid = addEvent.guid++;
// create a hash table of event types for the element 
if (!element.events) element.events = {};
// create a hash table of event handlers for each element/event pair 
var handlers = element.events[type];
if (!handlers) {
  handlers = element.events[type] = {};
  // store the existing event handler (if there is one) 
  // 如果对象已经注册有事件处理，那么要保留下来,并保存为第一个 
  if (element["on" + type]) {
    handlers[0] = element["on" + type];
  }
}
// store the event handler in the hash table 
handlers[handler.$$guid] = handler;
// assign a global event handler to do all the work 
// 指派一个全局函数做统一的事件处理，同时避免了反复注册 
element["on" + type] = handleEvent;
};
// a counter used to create unique IDs 
addEvent.guid = 1;
function removeEvent(element, type, handler) {
  // delete the event handler from the hash table 
  if (element.events && element.events[type]) {
    delete element.events[type][handler.$$guid];
  }
};
function handleEvent(event) {
  // grab the event object (IE uses a global event object) 
  event = event || window.event;
  // get a reference to the hash table of event handlers 
  // 这里的 this 随 handlerEvent function 被触发的source element 变化而变化 
  var handlers = this.events[event.type];
  // execute each event handler 
  for (var i in handlers) {
    //这样写才能保证注册的事件处理函数中的 this 得到正确的引用，直接handlers[i]()是不行的 
    this.$$handleEvent = handlers[i];
    this.$$handleEvent(event);
  }
};