简易秒表

<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
<script>
function addZero(n){
   if(n < 10){
       return '0' + n;
   }else{
       return '' + n;
   }
}
window.onload = function(){
   var oTxt = document.getElementById('txt');
   var oBtn1 = document.getElementById('btn1');
   var oBtn2 = document.getElementById('btn2');
   var count = 0;
   var timer = null;

   oBtn1.onclick = function(){
       clearInterval(timer);
       timer = setInterval(function(){
           count++;
           oTxt.value = addZero(parseInt(count/60)) + ':' + addZero(count%60);
       },100);
   }

   oBtn2.onclick = function(){
       clearInterval(timer);
   }
}
</script>
</head>

<body>
<input type="text" value="" id="txt"/>
<input type="button" value="开始" id="btn1"/>
<input type="button" value="暂停" id="btn2"/>
</body>
</html>