為什么if判斷沒有用？
<!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 s=document.getElementById("select").value;
??? var result="";
??? if(txt1!=NaN && txt2!=NaN)
??? {
??????? switch(s)
??????? {
??????????? 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;
??? }
??? else
??? {
??????? alert("請輸入正確數(shù)字！");
??? }
????
???
?? }
? </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>
2016-03-27
因為
NaN != NaN
建議參考如下內(nèi)容，講的比較詳細
http://www.cnblogs.com/onepixel/p/5281796.html