setInterval 的各种问题

function startStatus(){
var str_space="";
str_num=str_num+dir;
if(str_num>30 || str_num<0){
dir=-dir;
}
for(var i=0;i<str_num;i++){
str_space+=" ";
}
window.status=str_space+str;
}
function MyStart(){
TimerID=setInterval("startStatus();",100);
}I