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In a past column, I've written about how to display random advertisements when the page loads. To complement that past article, we're going to focus on an oft sought-after feature: automation. This article will focus on rotating the ads, in random order, over a period of time without reloading the page. I also address an issue brought up by the readership of my previous article, that of automatic ad rotating without reloading the page.
To begin, let's have a look at the code that was used in the previous column:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Ad Rotation in JavaScript</title>
<script type="text/javascript"><!--
var returnValue = true;
var links = ["http://yahoo.com/", "http://msn.com/", "http://cnn.com/"];
var imgs = ["yahoo.gif", "msn.gif", "cnn.gif"];
var len = (links.length + imgs.length) / 2;
if((len*2)%2 == 1){returnValue=false;}
function showAd(){
if(returnValue==false)
{document.write('<img
src="blank.gif" alt="">');return false;}
var rand = Math.floor(len*Math.random())
document.write("<a href=\""+links[rand]+"\"
title=\"Go to "+links[rand]+"\"><img src=\""+imgs[rand]+"\"
alt=\"Go to "+links[rand]+"\"></a>");
}
// --></script>
</head><body>
<script type="text/javascript"><!--
showAd();
//--></script>
<noscript> <img src="blank.gif" alt=""> </noscript>
</body></html>
Here, we’ll make a few modifications to the code, which has the effect of optimizing it. Here's the new code:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Ad Rotation in JavaScript</title>
<style type="text/css"><!--
#myAd img {
border: none;
}
--></style>
<script type="text/javascript"><!--
var links = ["http://yahoo.com/", "http://msn.com/", "http://cnn.com/"];
// link addresses
var imgs = ["yahoo.gif", "msn.gif", "cnn.gif"];
// link images
var len = (links.length + imgs.length) / 2; // length
function showAd(){
if(len%2 == 0){
document.write('<img
src="blank.gif" alt="">');
return false;
}
var rand = Math.floor(len*Math.random());
document.write('<a href="'+links[rand]+'"
title="Go to '+links[rand]+'" id="myAd">');
document.write('<img src="'+img[rand].src+'"
alt="Go to '+links[rand]+'" name="myAdImg"></a>');
}
//--></script>
</head><body>
<script type="text/javascript"><!--
showAd();
//--></script>
<noscript><img src="blank.gif" alt=""></noscript>
</body></html>
There aren't a lot of differences, but you'll notice that I've added a STYLE tag below the TITLE tags. This will remove that bothersome 2-pixel, blue border from our image.
Note: For more information, you can read up on the CSS official specification: www.w3.org/TR/CSS2/.
Moving onward, we’ve removed the "returnValue" variable. Instead
of having to add another variable, we did the length calculation inside of our
showAd() function. This if statement simply divides "len" by two,
then checks to see what the value is of the remainder. If it is zero, a blank
image will be displayed to avoid JavaScript errors (remember, it's a fail-safe
device). The last thing we've done is add an ID ("myAd") to the link,
and a NAME ("myAdImg") to the image. This way, we can refer to the
image and its containing link whenever we need to -- which will be set at an
interval of your choice. Let's create the rest of the new code, then examine
it further.
// preload images
for(i=0; i<imgs.length; i++){
img[i] = new Image();
img[i].src = imgs[i];
}
function showAd(){
if(len%2 == 0){
document.write('<img
src="blank.gif" alt="">');
return false;
}
var rand = Math.floor(len*Math.random());
document.write('<a href="'+links[rand]+'" title="Go
to '+links[rand]+'" id="myAd">');
document.write('<img src="'+img[rand].src+'"
alt="Go to '+links[rand]+'" name="myAdImg"></a>');
setInterval("updateAd()", timeBasis*1000);
}
function updateAd(){
var r = Math.floor(len*Math.random());
if(!document.links) return false;
for(i=0; i<document.links.length;
i++){
if(document.links[i].id
== "myAd"){
if(document.links[i].href
!= links[r]){
document.links[i].href
= links[r];
document.images['myAdImg'].src
= img[r].src;
} else {
updateAd();
}
}
}
}
//--></script>
</head><body>
<script type="text/javascript"><!--
showAd();
//--></script>
<noscript><img src="blank.gif" alt=""></noscript>
</body></html>
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Created: June 5, 2003
Revised: November 3, 2004
URL: http://webreference.com/programming/javascript/jf/column6/1
