為什么點擊等號之后沒有效果，求解
<!DOCTYPE html>
<html>
?<head>
? <title> 事件</title>?
? <script type="text/javascript">
?? function count(){
?????? var first=document.getElementById("txt1").value;
?????? var se=document.getElementById("txt2").value;
?????? var sel=document.getElementById("select").value;
?????? var sum=0;
??? switch sel{
??????? case'+':sum=parseInt(first)+parseInt(se);break;
??????? case"-":sum=parseInt(first)-parseInt(se);break;
??????? case'*':sum=parseInt(first)*parseInt(se);break;
??????? case"/":sum=parseInt(first)/parseInt(se);break;
??? }
????? document.getElementById('fruit').value = sum;
?? }
? </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()"/> <!--通過 = 按鈕來調(diào)用創(chuàng)建的函數(shù)，得到結(jié)果-->
?? <input type='text' id='fruit' />??
?</body>
</html>
2020-02-19