循环计时器

<input type="button" id="btn" value="开始答题">

var start = 3;
var step = -1;
var k=0;
function count()
{
document.getElementById("btn").value ="倒计时："+ start+"s";
start += step;
if(start <=0){
start = 3;

//倒计时4次停止计时
if(k<4){
　　//在循环时的操作

}else{

　　//清除计时器
　　clearTimeout(timer);
}
}
var timer= setTimeout("count()",1000);

}
window.onload = count;