当页面还没有加载完的时候调用下面语句，会取不到“add” 这个对象，提示为空或不是对象
document.getElementById("add").onclick = function(){
alert("hello");
}
当使用便可取的对象
window.onload = function(){
document.getElementById("add").onclick = function(){
alert("hello");
}
}
<script type="text/javascript">
//选中的从左边移到右边
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 的属性
secondElement.appendChild(firstoptionElement[firstElement.selectedIndex]);
}
}
}
//全部移动到右边
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选项选中时候索引为0
}
}
//双击移动到右边
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 的属性
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 width="285" height="169" border="0" align="left">
<tr>
<td width="126">
<select name="first" size="10" multiple="multiple" id="first" ondblclick="db()">
<option value="1">选项1</option>
<option value="2">选项2</option>
<option value="3">选项3</option>
<option value="4">选项4</option>
<option value="5">选项5</option>
<option value="6">选项6</option>
</select>
</td>
<td width="69" 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 width="127" align="left">
<select name="second" size="10" multiple="multiple" id="second">
<option value="选项8">选项8</option>
</select>
</td>
</tr>
</table>
</body>