<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var num1=document.getElementById("txt1" ).value;
var num2=document.getElementById("txt2" ).value;
var sel=document.getElementById("select" ).value;
var req="";
switch(sel)
{
case"+":
req=parseInt(num1)+parseInt(num2);
break;
case"-":
req=parseInt(num1)-parseInt(num2);
break;
case"*":
req=parseInt(num1)*parseInt(num2);
break;
case"/"：
req=parseInt(num1)/parseInt(num2);
}
//获取第一个输入框的值
//获取第二个输入框的值
//获取选择框的值
//获取通过下拉框来选择的值来改变加减乘除的运算法则
//设置结果输入框的值
document.getElementById("fruit").value=req;
}
</script>
</head>
<body>
<input type='text' id='txt1' />
<select id='select'>
<option value='+'>+</option>
<option value="-">-</option>
<option value="*">*</option>
<option value="/">/</option>
</select>
<input type='text' id='txt2' />
<input type='button' value=' = ' onclick="count()" /> <!--通过 = 按钮来调用创建的函数，得到结果-->
<input type='text' id='fruit' />
</body>
</html>
req=parseInt(num1)/parseInt(num2);后面加个break;即可