已经定义却说没有定义的

[wln5854498]

新人求指教 照着书敲的 某一处说没有被定义 明明已经声明了 请问什么个情况呢

function addLoadEvent(func){
var oldonload = window.onload;
if (typeof window.onload != 'function'){
window.onload = func;
}else{
window.onload = function(){
oldonload();
func();
}
}
}

function insertAfter(newElement,targetElement){
var parent = targetElement.parentNode;
if(parent.lastChild == targetElement){
parent.appendChild(newElement);
}else{
parent.insertBefore(newElement,targetElement.nextSibling);
}
}

function preparePlaceholder(){
if(!document.createElement) return false;
if(!document.createTextNode) return false;
if(!document.getElementById) return false;
if(!document.getElementById("imagegallery")) return false;
var Placeholder = document.createElement("img");
Placeholder.setAttribute("id","placeholder");
placeholder.setAttribute("src","images/placeholder.png"); //浏览器说这里的placeholder没有被定义？？
placeholder.setAttribute("alt","my image gallery");
var description = document.createElement("p");
description.setAttribute("id","description");
var desctext = description.createTextNode("Choose an photo");
description.appendChild(desctext);
var gallery = document.getElementById("imagegallery");
insertAfter(placeholder,gallery);
insertAfter(description,placeholder) ; }



function prepareGallery(){
if(!document.getElementsByTagName) return false;
if(!document.getElementById) return false;
if(!document.getElementById("imagegallery"))return false;
var gallery = document.getElementById("imagegallery");
var links = gallery.getElementsByTagName("a");
for( var i=0; i<links.length; i++){

links[i].onclick = function(){
return showPic(this);
}
links[i].onkeypress = links[i].onclick;
}
}


function showPic(whichpic){
if(!document.getElementById("placeholder"))return false;
var source = whichpic.getAttribute("href");
var placeholder = document.getElementById("placeholder");
placeholder.setAttribute("src",source);
if(!document.getElementById("description"))return false;
if(whichpic.getAttribute("title")){
var text = whichpic.getAttribute("title");
}else{
var text="";
}
var description = document.getElementById("description");
if (description.firstChild.nodeType == 3){
description.firstChild.nodeValue = text;
}
return false;
}
addLoadEvent(preparePlaceholder);
addLoadEvent(prepareGallery);