js实现打字机效果

<span style="font-family: Arial, Helvetica, sans-serif;">function MyText()</span>

{
this[0]=MyText.arguments[0]; //将MyText输入的值传给对象
}

var myText = new MyText("我要输出文字啊啊啊"); //创建对象
var length = myText[0].length;
var num=0;

function typer()
{
document.getElementById("typefield").value = myText[0].substring(0,num); //显示myText[0]从第1个到第num个字符
num++;
if(num<length){
setTimeout("typer()",300);
}}