當頁面還沒載入完的時候呼叫下面語句，會取不到「add」 這個對象，提示為空或不是對象
複製代碼代碼如下:
document.getElementById("add").onclick = function(){
alert("hello");
}
當使用便可取的對象
複製代碼代碼如下:
window.onload = function(){
document.getElementById("add").onclick = function(){
alert("hello");
}
}
複製代碼代碼如下:
<script type="text/javascript">
//選取的從左邊移到右邊
function toright() {
var firstElement = document.getElementById("first");
var secondElement = document.getElementById("second");
var firstoptionElement = firstElement.getElementsByTagName("option");
var len = firstoptionElement.length;
for(var i=0;i<len;i++){
if(firstElement.selectedIndex != -1){ //selectedIndex 是select 的屬性
secondElement.appendChild(firstoptionElement[firstElement.selectedIndex]);
}
}
}
//全部移到右邊
function allright(){
var firstElement = document.getElementById("first");
var secondElement = document.getElementById("second");
var firstoptionElement = firstElement.getElementsByTagName("option");
var len = firstoptionElement.length;
for(var i=0;i<len;i++){
secondElement.appendChild(firstoptionElement[0]);//option選項選取時候索引為0
}
}
//雙擊移到右邊
function db(){
/* //方法一
var firstElement = document.getElementById("first");
var secondElement = document.getElementById("second");
var firstoptionElement = firstElement.getElementsByTagName("option");
var len = firstoptionElement.length;
for(var i=0;i<len;i++){
if(firstElement.selectedIndex != -1){ //selectedIndex 是select 的屬性
secondElement.appendChild(firstoptionElement[firstElement.selectedIndex]);
}
} */
//方法二
var firstElement = document.getElementById("first");
var secondElement = document.getElementById("second");
secondElement.appendChild(firstElement[firstElement.selectedIndex]);
}
</script>
<style type="text/css">
</style>
</head>
<body>
<table align="left">
<tr>
<td>
<select name="first" size="10" multiple="multiple" id="first" ondblclick="db()">
<option value="1">選項1</option>
<option value="2">選項2</option>
<option value="3">選項3</option>
<option value="4">選項4</option>
<option value="5">選項5</option>
<option value="6">選項6</option>
</select>
</td>
<td valign="middle">
<input id="add" name="add" type="button" value="---->" onclick="toright()"/>
<input id="add_all" name="add_all" type="button" value="==>" onclick="allright()"/>
</td>
<td align="left">
<select name="second" size="10" multiple="multiple" id="second">
<option value="選項8">選項8</option>
</select>
</td>
</tr>
</table>
</body>