console
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<input type="button" id="btn1" value="开始计时">
<input type="text" id="ipt">
<input type="button" id="btn2" value="结束计时">
<script>
var btn1 = document.getElementById('btn1');
var btn2 = document.getElementById('btn2');
var ipt = document.getElementById('ipt');
var index=0;
var time;
btn1.addEventListener('click',jishi);
btn2.addEventListener('click',stop);
function jishi() {
clearInterval(time);
time=setInterval(function(){
index++;
ipt.value=index;
},1000);
}
function stop(){
clearInterval(time);
}
</script>
</body>
</html>