.value問(wèn)題，求詳細(xì)解答
<!DOCTYPE?html>
<html>
?<head>
??<title>?事件</title>??
??<script?type="text/javascript">
???function?count(){
???????var?aValue?=?parseInt(document.getElementById("txt1").value);
???????var?bValue?=?parseInt(document.getElementById("txt2").value);
???????var?aSelect?=?document.getElementById("select").value;
???????var?cValue?=?document.getElementById("fruit");??//???????var?cValue?=?document.getElementById("fruit").value;
???????if(aSelect?==?"+")??????????????????????????????//???????if(aSelect?==?"+")??????????????????????????????
???????{???????????????????????????????????????????????//???????{
??????? cValue.value?=?aValue?+?bValue;????????????//????????????cValue?=?aValue?+?bValue;????????????????
???????}???????????????????????????????????????????????//???????}
???????else?if?(aSelect?==?"-")?
???????{
??????? cValue.value?=?aValue?-?bValue;
???????}
???????else?if(aSelect?==?"/")
???????{
??????? cValue.value?=?aValue?/?bValue;
???????}
???????else
???????{
??????? cValue.value?=?aValue?*?bValue;
???????}???????
???}
??</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(guò)?=?按鈕來(lái)調(diào)用創(chuàng)建的函數(shù)，得到結(jié)果-->?
???<input?type='text'?id='fruit'?/>???
?</body>
</html>為什么按照注釋的寫(xiě)法運(yùn)行沒(méi)有效果？.value有什么時(shí)候不能加的？
2017-08-12
“?var?cValue?=?document.getElementById("fruit").value
此時(shí)是將document.getElementById("fruit").value的值賦值給?var?cValue;
而此時(shí)document.getElementById("fruit").value沒(méi)有值，
要在?function?count()函數(shù)最后寫(xiě)document.getElementById("fruit").value=var?cValue;此時(shí)var?cValue是有值的。
將var?cValue的值賦值給document.getElementById("fruit").value。
2017-08-10
你這fruit是輸出框的id，“?var?cValue?=?document.getElementById("fruit").value”你是這么寫(xiě)，但是此時(shí)他并沒(méi)有值