如何动态改变网页title

var counts=4; var i=1; var ary_p=0; var flag=1; 
//要显示的标题文字存储在这个数组中
var titletxt = new Array(
"第一个title1",
"第二个title2",
"第三个title3",
"第四个title4"
);
//每隔3秒更换一次title名称
function changetitle()
{
window.document.title=titletxt[ary_p];
if(ary_p==3){
ary_p=0;
}else{
ary_p++;
}
setTimeout("changetitle()",3000);
}
changetitle();

//实现打字特效
function dynamictitle() 
{ 
window.document.title=titletxt[ary_p].substr(0, i)+"_"; 

if (flag==0) i--; if (flag==1) i++;
if (i==-1) {flag=1; i=0; ary_p++; ary_p=ary_p%counts;} 
if (i==titletxt[ary_p].length+20) {flag=0; i=titletxt[ary_p].length;} 

window.document.title=titletxt[ary_p].substr(0, i)+"_"; 
setTimeout("dynamictitle()",200);
} 
dynamictitle();