重写alert方法

window.alert = function(txt)

{
var shield = document.createElement("DIV");
shield.id = "shield";
shield.style.position = "absolute";
shield.style.left = "0px";
shield.style.top = "0px";
shield.style.width = "100%";
shield.style.height = document.body.scrollHeight+"px";
shield.style.background = "#333";
shield.style.textAlign = "center";
shield.style.zIndex = "10000";
shield.style.filter = "alpha(opacity=0)";

  shield.style.opacity = "0.8";
var alertFram = document.createElement("DIV");
alertFram.id="alertFram";
alertFram.style.position = "absolute";
alertFram.style.left = "50%";
alertFram.style.top = "50%";
alertFram.style.marginLeft = "-225px";
alertFram.style.marginTop = "-75px";
alertFram.style.width = "450px";
alertFram.style.height = "150px";
alertFram.style.background = "#ccc";
alertFram.style.textAlign = "center";
alertFram.style.lineHeight = "150px";
alertFram.style.zIndex = "10001";
strHtml = "<ul style=\"list-style:none;margin:0px;padding:0px;width:100%\">\n";
strHtml += " <li style=\"background:#DD828D;text-align:center;font-size:14px;font-weight:bold;height:25px;line-height:25px;border:1px solid #F9CADE;\">提示</li>\n";
strHtml += " <li style=\"background:#fff;text-align:center;font-size:12px;height:120px;line-height:120px;border-left:1px solid #F9CADE;border-right:1px solid #F9CADE;\">"+txt+"</li>\n";

  strHtml += " <li style=\"background:#FDEEF4;text-align:center;font-weight:bold;height:35px;line-height:25px; border:1px solid #F9CADE;\"><input style=\"width:50px;\" type=\"button\" value=\"确 定\" onclick=\"doOk()\" /></li>\n";
strHtml += "</ul>\n";
alertFram.innerHTML = strHtml;
document.body.appendChild(alertFram);
document.body.appendChild(shield);
var c = 0;
this.doAlpha = function(){
if (c++ > 20){clearInterval(ad);return 0;}
shield.style.filter = "alpha(opacity="+c+");";
}
var ad = setInterval("doAlpha()",5);
this.doOk = function(){
alertFram.style.display = "none";
shield.style.display = "none";
}
alertFram.focus();
document.body.onselectstart = function(){return false;};

}

alert('测试alert')