这个为什么不能输出啊？
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var t1 = document.getElementById("txt1").value;
var t2 = document.getElementById("txt2").value;
var sl1 = document.getElementById("select").value;
var result="";
switch(sl1){
case "+":
result = parseFloat(t1) + parseFloat(t2);
break;
case "-":
result = parseFloat(t1) - parseFloat(t2);
break;
case "*":
result = parseFloat(t1) * parseFloat(t2);
break;
default:
result = parseFloat(t1) / parseFloat(t2);
}
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=' = ' 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>
