addEventListener
page.addEventListener("change",function(e){
var pageElement = document.querySelector("." + e.target.value)
pageElement.style.zIndex = ++index;
},false) 这段代码改成下面的那样会有什么问题page.onchange=function(){
var pageElement = document.querySelector("." + e.target.value)
pageElement.style.zIndex = ++index;
}