<head runat="server">
<title>JavaScript实现加法计算器</title>
<script type="text/javascript">
function Sum()
{
var txtbox1 = document.getElementById("txtbox1");
var txtbox2 = document.getElementById("txtbox2");
var sum = document.getElementById("sum");
sum.value = parseInt(txtbox1.value) + parseInt(txtbox2.value);
}
</script>
</head>
<body>
<input type="text" id="txtbox1" />+
<input type="text" id="txtbox2" />
<input type="button" value="=" onclick="Sum()" />
<input type="text" id="sum" />
</body>
原文：http://www.cnblogs.com/Zbuxu/p/5184647.html