JavaScript事件对象event用法分析

var btn = document.getElementsByTagName("button")[0];
var handler = function(event) {
 switch(event.type) {
  case "click":
   alert("click");
  case "mouseover":
   alert("mouseover");
  case "mouseout":
   alert("mouseout");
 }
}
btn.onclick = handler;
btn.onmouseover = handler;
btn.onmouseout = handler;

var EventUtil = { 
 addHandler: function(element, type, handler) {
  ......
 }, 
 getEvent: function(event) {
  return event ? event : window.event;
 },
 getTarget: function(event) {
  return enent.target || target.srcElement;
 },
 preventDefault: function(event) {
  if (event.preventDefault)
   event.preventDefault();
  else
   event.returnValue = false;
 },
 removeHandler: function(element, type, handler) {
  ......
 }, 
 stopPropagation: function() {
  if (event.stopPropagation)
   event.stopPropagation();
  else
   event.cancelBubble = true;
 }
};
var btn = document.getElementsByTagName("button")[0];
btn.onclick = function(event) {
 event = EventUtil.getEvent(event);
 EventUtil.preventDefault(event);
 EventUtil.stopPropagation(event);
 alert(event.target.type); // 输出：click
}