启用计时器和关闭计时器，关闭计时器时无效
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>计时器</title>
</head>
<script type="text/javascript">
/*
var num=0;
var i;
function startCount(){
document.getElementById('count').value=num;
num=num+1;
i=setTimeout("startCount()",1000);
}
function stopCount(){
clearTimeout(i);
}
*/
var num=0;
function startCount(){
document.getElementById('count').value=num;
num+=1;
var i=setTimeout("startCount()",1000);
}
function stopCount(){
clearTimeout(i);
}
</script>
</head>
<body>
<form>
<input type="text" id="count" />
<input type="button" value="Start" onclick="startCount()" />
<input type="button" value="Stop" onclick="stopCount()" />
</form>
</body>
</html>
//上面未注释代码哪里错了，怎么无效呢？
