在一个无穷循环中的计时事件

<!DOCTYPE html>
<html>
<head> 
<meta charset="utf-8"> 
<title>滴答编程(dida100.com)</title> 
<script>
var c=0;
var t;
var timer_is_on=0;
function timedCount(){
	document.getElementById('txt').value=c;
	c=c+1;
	t=setTimeout("timedCount()",1000);
}
function doTimer(){
	if (!timer_is_on)
	{
		timer_is_on=1;
		timedCount();
	}
}
</script> 
</head>

<body>
<form>
<input type="button" value="开始计数!" onClick="doTimer()">
<input type="text" id="txt">
</form>
<p>单击按钮，输入框将从0开始一直计数。</p>
</body>
</html>