JS IE浏览器下的event事件

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 1.IE浏览器下 默认值为null
 2.dom0无法采用传参的方式获取event;
     box.onclick = function (event) {
       console.log(event);//undefined
     }
 3.获取event对象的兼容性写法：
     box.onclick = function (event) {
     event = event || window.event;
        console.log(event);
     }

 4.srcElement 等同于非IE浏览器下的target
     描述：获取当前事件的触发者
     例子：
      box.onclick = function (event) {
        event = event|| window.event;
        //兼容性写法
         var target = event.target|| event.srcElement;
          console.log(event.srcElement);
       };
5.cancelBubble
    描述：阻止事件冒泡
    语法：event.cancelBubble = true
    例子：
         box.onclick = function (event) {
             event = event|| window.event;
    
             console.log('box被点击了');
             event.cancelBubble = true;
         };
         document.body.onclick = function () {
             console.log(' body  被点击了');
         };
 6.returnValue
     描述：取消节点默认行为
     语法：event.returnValue = false;
     例子：
         a.onclick = function (event) {
             event = event|| window.event;
             event.returnValue = false;
         };