<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var txt1 = parseInt( document.getElementById('txt1').value);
var txt2 = parseInt( document.getElementById('txt2').value);
var select = document.getElementById('select').value;
var result = '';
switch (select)
{
case '+':
result = txt1 + txt2;
break;
case '-':
result = txt1 - txt2;
break;
case '*':
result = txt1 * txt2;
break;
case '/':
result = txt1 / txt2;
break;
}
document.getElementById('fruit').value = result;
}
</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=' = ' /> <!--通过 = 按钮来调用创建的函数，得到结果-->
<input type='text' id='fruit' />
</body>
</html>
body里面注释那里你得添加命令运行函数啊 onclick="count()"