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                        /JavaScript進階篇

各位大神請教一下，點=號出不來結(jié)果是什么問題？

<!DOCTYPE html>
<html>
?<head>
? <title> 事件</title>?
? <script type="text/javascript">
?? function count(){
??????
??? //獲取第一個輸入框的值
??? var num1=document.getElementById('txt1').value;
?//獲取第二個輸入框的值
? var num2=document.getElementById('txt2').value;
?//獲取選擇框的值
? var select=document.getElementById('select').value;
?
? var sum;
?//獲取通過下拉框來選擇的值來改變加減乘除的運算法則
?if(select=='+'){
???? sum=parseInt(num1)+parseInt(num2);
?}else if(select=='-'){
???? sum=parseInt(num1)-parseInt(num2);
?}else if(select=='*'){
???? sum=parseInt(num1)*parseInt(num2);
?}else if(select=='/'){
???? sum=parseInt(num1)/parseInt(num20;
?}else{
???? document.write("請選擇一個");
?}
??? //設(shè)置結(jié)果輸入框的值
?? 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()"/> 
?? <input type='text' id='fruit' />??
?</body>
</html>