<script type="text/javascript">
function btn(a) {
alert(a
); // a相当于 document.getElementById('txt1')
var max= 9;
var min = 0;
if (a.value >= min && a.value <= max) {
a.style.backgroundColor = "red";
}
}
</script>
<body>
<form id="form1" runat="server">
<div>
<input type="text" runat="server" onchange="btn(this)"
id="txt1" />
//this相当于 document.getElementById('txt1')
</div>
</form>
</body>