javascript监听事件

document.onclick=function(){
      var child=null;
        if(window.event){
            e=window.event;
            child=document.activeElement
        }else{
            child=arguments[0].explicitOriginalTarget;
        }
            //这里写if(id==used) 就做你要做的，else return false
           // alert(child.childNodes[0].childNodes[0].innerHTML);
                                
           }
    }