此计算器只有加、减、乘、除的功能，代码如下：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>计算器</title>
<script type="text/javascript">
function btnAdd() {
//获得文本框中的值，并将其转换成十进制
var Num1 = parseInt(document.getElementById("txt1").value, 10);
var Num2 = parseInt(document.getElementById("txt2").value, 10);
var Num3;
var selectedcmb;//保存被选中的组合框
var opers = document.getElementsByTagName("option");
for (var i = 0; i < opers.length; i++) {
//若组合框被选中,那么被选中的项的selected 属性就为true，就将当前对象存于selectedcmb中
if (opers[i].selected == true) {
selectedcmb = opers[i];
}
}
switch (selectedcmb.value) {
case "+": Num3 = Num1 + Num2;
break;
case "-": Num3 = Num1 - Num2;
break;
case "*": Num3 = Num1 * Num2;
break;
case "/": Num3 = Num1 / Num2;
break;
}
document.getElementById("txt3").value = Num3.toString();
}
</script>
</head>
<body>
<input type="text" id="txt1" />
<!--input type="button" value="+" disabled="disabled" /-->
<select>
<option id="add" value="+" >+</option>
<option id="sub" value="-">-</option>
<option id="mul" value="*">*</option>
<option id="div" value="/">/</option>
</select>
<input type="text" id="txt2" />
<input type="button" value="=" id="btnAdd" />
<input type="text" id="txt3" />
</body>
</html>
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