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
复制代码 代码如下:
document.getElementById("add").onclick = function(){
alert("hello");
}
当使用便可取的对象
复制代码 代码如下:
window.onload = function(){
document.getElementById("add").onclick = function(){
alert("hello");
}
}
复制代码 代码如下:
<script type="text/javascript"> <br>//选中的从左边移到右边 <br>function toright() { <br>var firstElement = document.getElementById("first"); <br>var secondElement = document.getElementById("second"); <br>var firstoptionElement = firstElement.getElementsByTagName("option"); <br>var len = firstoptionElement.length; <br>for(var i=0;i<len;i++){ <br>if(firstElement.selectedIndex != -1){ //selectedIndex 是select 的属性 <br>secondElement.appendChild(firstoptionElement[firstElement.selectedIndex]); <br>} <br>} <br>} <br>//全部移动到右边 <br>function allright(){ <br>var firstElement = document.getElementById("first"); <br>var secondElement = document.getElementById("second"); <br>var firstoptionElement = firstElement.getElementsByTagName("option"); <br>var len = firstoptionElement.length; <br>for(var i=0;i<len;i++){ <br>secondElement.appendChild(firstoptionElement[0]);//option选项选中时候索引为0 <br>} <br>} <br>//双击移动到右边 <br>function db(){ <br>/* //方法一 <br>var firstElement = document.getElementById("first"); <br>var secondElement = document.getElementById("second"); <br>var firstoptionElement = firstElement.getElementsByTagName("option"); <br>var len = firstoptionElement.length; <br>for(var i=0;i<len;i++){ <br>if(firstElement.selectedIndex != -1){ //selectedIndex 是select 的属性 <br>secondElement.appendChild(firstoptionElement[firstElement.selectedIndex]); <br>} <br>} */ <br>//方法二 <br>var firstElement = document.getElementById("first"); <br>var secondElement = document.getElementById("second"); <br>secondElement.appendChild(firstElement[firstElement.selectedIndex]); <br>} <br><br></script>
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