重写原生alert,弹出层过一会就消失

 window.alert = function(str) {
 	       if(document.querySelectorAll("div.shieldClass").length!=0){
 	       	return false;
 	       }
            // 	遮罩层
            var shield = document.createElement("DIV");
            shield.className = "shieldClass";
            shield.id = "shield";
            shield.style.position = "absolute";
            shield.style.left = "0px";
            shield.style.top = "0px";
            shield.style.width = "100%";
            shield.style.height = "100%";
            //弹出对话框时的背景颜色
            shield.style.background = "#111";
            shield.style.textAlign = "center";
            shield.style.zIndex = "25000";
            shield.style.opacity = "0.4";
            //背景透明 IE有效
            var alertFram = document.createElement("DIV");
            alertFram.className = "alertFramClass";
            alertFram.id = "alertFram";
            alertFram.style.position = "absolute";
            alertFram.style.left = "50%";
            alertFram.style.top = "40%";
            alertFram.style.background = "rgba(0,0,0,0.7)";
            alertFram.style.textAlign = "center";
            alertFram.style.zIndex = "25001";
            alertFram.style.borderRadius="6px";
            strHtml = "<p style='text-align:center;padding:15px 15px;font-size:12px;font-weight: normal;color:#fff'>" + str +"</p>"
            alertFram.innerHTML = strHtml;
            document.body.appendChild(alertFram);
            document.body.appendChild(shield);
            var o = document.getElementById("alertFram");
            var body=document.getElementsByTagName("body")[0]
            var w = o.offsetWidth; //宽度
                o.style.marginLeft="-"+w/2+"px";
            setTimeout(function(){
             	var shieldDom=document.querySelectorAll("div.shieldClass");
                var alertFramDom=document.querySelectorAll("div.alertFramClass");
           	   for(var i=0;i<shieldDom.length;i++){
           	   	 body.removeChild(shieldDom[i]);
           	   };
          	   for(var j=0;j<alertFramDom.length;j++){
           	   	 body.removeChild(alertFramDom[j]);
           	  };
            },1000);

  }