为非IE内核游览器添加onmouseenter 和 onmouseleave事件

好像各大类库中jQuery是最后支持这两东东的，关于它们的优势可以参考reglib类库作者的一篇博文《Goodbye mouseover, hello mouseenter》。我们可以在IE观察下它的效果。

注意，我们在红色与蓝色之间移动，并没有触发outer元素的mouseenter事件。无错，它们与mouseover,mouseout最大的区别是它们不会冒泡。

<!doctype html>
<html dir="ltr" lang="zh-CN">
  <head id="head">
    <meta charset="utf-8"/>
    <title>mouseenter and mouseleave by司徒正美</title>
    <style title="text/css">
      div {
        background:red;
      }
      ul {
        background:blue;
      }
    </style>

if (parent != this) {
            handler.call(this, e);
          }
        };
      };
        var $ = function(id){
          return document.getElementById(id);
        };
        window.onload = function(){
          function myFn(e){
            e = e || window.event;
            var target = e.target ? e.target : e.srcElement;
            if(target.nodeType === 3){
              target = target.parentNode;
            }
            $(\'memo\').innerHTML = e.type + \': \' + target.id;
          };
          addEvent($("inner"),"mouseover",myFn,false);
          addEvent($("inner"),"mouseout",myFn,false);
          addEvent($("outer"),"mouseover",myFn,false);
        }
    </script>

</head>
  <body>
    <div id="outer">
      <p>注意：div与ul都绑定了onmouseenter，而只有绑定了onmouseleave</p>
      <ul id = "inner">
        <li>司徒正美</li>
        <li>司徒正美</li>
        <li>司徒正美</li>
        <li>司徒正美</li>
      </ul>
    </div>
    <p id="memo"></p>
  </body>
</html>

下面试图用mouseover与mouseout模拟mouseenter与mouseleave。

      function addEvent(el, type, handler, capture) {
        if (typeof el.addEventListener != \'undefined\') {
          if (type === \'mouseenter\') {
            el.addEventListener(\'mouseover\', withinElement(handler), capture);
          } else if (type === \'mouseleave\') {
            el.addEventListener(\'mouseout\', withinElement(handler), capture);
          } else {
            el.addEventListener(type, handler, capture);
          }
        } else if (typeof el.attachEvent != \'undefined\') {
          el.attachEvent(\'on\' + type, handler);
        } else {
          el[\'on\' + type] = handler;
        }
      }
      function withinElement(handler) {
          return function (e) {
            var parent = e.relatedTarget;
            while ( parent && parent != this ) {
              try { parent = parent.parentNode; }
              catch(e) { break; }
            }
            if ( parent != this )
              handler.call(this, e);
          };
        };
      var $ = function(id){
        return document.getElementById(id);
      }
      window.onload = function(){
        function myFn(e){
          e = e || window.event;
          var target = e.target ? e.target : e.srcElement;
          if(target.nodeType === 3){
            target = target.parentNode;
          }
          $(\'memo\').innerHTML = e.type + \': \' + target.id;
        };
        addEvent($("inner"),"mouseenter",myFn,false);
        addEvent($("inner"),"mouseleave",myFn,false);
        addEvent($("outer"),"mouseenter",myFn,false);
      }

看看jQuery的实现，它用了一个冒牌事件对象，因此我们可以在FF中用e.type能打印出mouseenter，除此与上面没多大改善。