测试没问题，出错的同学可以参考哈！
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var x = parseInt(document.getElementById("txt1").value);
var y = parseInt(document.getElementById("txt2").value);
var s = document.getElementById("select").value;
var z = document.getElementById("fruit").value;
switch(s){
case "+":
z= x + y;
break;
case "-":
z= x - y;
break;
case "*":
z= x * y;
break;
case "/":
z= x / y;
break;
}
document.getElementById("fruit").value = z;
}
</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>
