js代码没有出现错误，但是网页中没有预想的效果

html

Image Gallery

Snapshots

• Ballon

• Tree

• Sea

Choose an image.

js代码

window.onload = prepareGallery;

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);

}

}

}

function showPic(whichpic){

if(!document.getElementById)

{return false;}

var source = whichpic.getAttribute("href");

if(!document.getElementById.getElementById("placeholder"))

{return false;}

var placeholder = document.getElementById("placeholder");

if(placeholder.nodeName != "IMG")

{return false;}

placeholder.setAttribute("src",source);

if(document.getElementById("description"))

{

if(whichpic.getAttribute("title")){

var title = whichpic.getAttribute("title");

}else{

title = "";

}

var description = document.getElementById("description");

if(description.firstChild.nodeType == 3){

description.firstChild.nodeValue = title;

}

}

return true;

}