js 动态绑定鼠标事件

<script>
    
   function getElementsByClassName(n) {
   var classElements = [],allElements = document.getElementsByTagName('*');
   for (var i=0; i< allElements.length; i++ )
   {
       if (allElements[i].className == n ) {
           classElements[classElements.length] = allElements[i];
        }
   }
   return classElements;
}
    
   var btnClassElements = getElementsByClassName('btn');
　　for (var i=0; i<btnClassElements.length; i++) {
   　　 btnClassElements[i].onmouseover = function(){
        　　this.style.backgroundImage="url(img/bluebtn.PNG)";
        
    }
    　　btnClassElements[i].onmouseout = function(){
        　　this.style.backgroundImage="url(img/btn.PNG)";
    }

}
</script>