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Creating More Effective Rollovers

To make the illusion of animation work, you need to make sure that the replacement image appears immediately, with no delay while it is fetched from the server. To do that, you use JavaScript to place the images into variables used by your script, which preloads all the images into the browser’s cache (so that they are already on the user’s hard disk when they are needed). Then, when the user moves the mouse over an image, the script swaps out one variable containing an image for a second variable containing the replacement image. Listing 4.3 shows how it is done. The visible result is the same as in gray-circle-a.jpg and gray-circle-b.jpg from the previous exercise, but the apparent animation is smoother.

Listing 4.3. The only JavaScript on this HTML page is the pointer to the external .js file.

<!DOCTYPE html>
     <title>A More Effective Rollover</title>
     <script src="script02.js"></script>
     <link rel="stylesheet" href="script01.css">
     <a href="next1.html"><img src="images/button1_off.gif" alt="button1" id="button1"></a>&nbsp;&nbsp;
     <a href="next2.html"><img src="images/button2_off.gif" alt="button2" id="button2"></a>

To keep your JavaScript more manageable, we’ll extract the JavaScript code from the HTML page and put it in an external .js file, as in Listing 4.4 (see Chapter 2 for more about .js files).

Listing 4.4. This is a better way to do rollovers than in Listing 4.1, because it is much more flexible.

window.onload = rolloverInit;

function rolloverInit() {
     for (var i=0; i<document.images.length; i++) {
         if (document.images[i].parentNode.tagName == "A") {

function setupRollover(thisImage) {
     thisImage.outImage = new Image();
     thisImage.outImage.src = thisImage.src;
     thisImage.onmouseout = function() {
        this.src = this.outImage.src;

     thisImage.overImage = new Image();
     thisImage.overImage.src = "images/" + + "_on.gif";
     thisImage.onmouseover = function() {
        this.src = this.overImage.src;

To create a better rollover:

  1. <script src="script02.js"></script>

    This tag is in Listing 4.3, the HTML page. It uses the src attribute to tell the browser where to find the external .js file, which is where the JavaScript resides.

  2. <a href="next1.html"><img src="images/button1_off.gif" alt="button1" id="button1"></a>&nbsp;&nbsp;
    <a href="next2.html"><img src="images/button2_off.gif" alt="button2" id="button2"></a>

    Still in Listing 4.3, these are two typical link tags for the buttons, with image tags embedded in them. The href attribute describes the destination of the link when the user clicks it. In the img tag, the src attribute provides the path to the image before the user rolls over it. The link tags also define the image’s alt text. Note that each of the two buttons also has an id attribute; as described in Chapter 1, the id must be unique for each object. The script uses the image’s id to make the rollover work.

  3. window.onload = rolloverInit;

    Moving to Listing 4.4, the window.onload event handler is triggered when the page has finished loading. The handler calls the rolloverInit() function.

    This handler is used here to make sure that the script doesn’t execute before the page is done loading. That’s because referring to items on the page before the page has finished loading can cause errors if some of the page’s elements haven’t yet been loaded.

  4. function rolloverInit() {
       for (var i=0; i<document.images.length; i++) {

    The rolloverInit() function scans each image on the page, looking to see if the tag around the image is an <a> tag, indicating that it is a link. The first of these two lines begins the function. The second begins a loop that goes through all of the images. The loop begins by setting the counter variable i to 0. Then, each time the loop goes around, if the value of i is less than the number of images in the document, increment i by 1.

  5. if (document.images[i].parentNode.tagName == "A") {

    This is where we test to see if the tag surrounding the image is an anchor tag. We do it by looking at an object and seeing if the object’s value is A (the anchor tag). Let’s break that object apart a bit. The first part of the object, document.images[i], is the current image. Its parentNode property is the container tag that surrounds it, and tagName then provides the name of that container tag. So in English, you can read the part of the line in the parentheses as “For this particular image, is the tag around it an ‘A’?”

  6. setupRollover(document.images[i]);

    If the result of the test in step 5 is true, then the setupRollover function is called and passed the current image.

  7. function setupRollover(thisImage) {

    Take a minute to look at the whole function before we go through it line by line. Here’s the overview: this function adds two new properties to the image object that’s passed in. The new properties are outImage (the version of the image when you’re not on it) and overImage (the version of the image when you are on it), both of which are image objects themselves. Because they’re image objects, once they’re created, we can add their src property. The src for outImage is the current (off) image src. The src value for overImage is calculated based on the id attribute of the original image.

    This line starts off the function with the image that was passed to it by the rolloverInit() function.

  8. thisImage.outImage = new Image();

    This line takes the image object that was passed in and adds the new outImage property to it. Because you can add a property of any kind to an object, and because properties are just objects themselves, what’s happening here is that we’re adding an image object to an image. The parentheses for the new image object are optional, but it’s good coding practice to include them; if needed, you can set properties of the new image object by passing certain parameters.

  9. thisImage.outImage.src = thisImage.src;

    Now we set the source for the new outImage to be the same as the source of thisImage. The default image on the page is always the version you see when the cursor is off the image.

  10. thisImage.onmouseout = function() {
       this.src = this.outImage.src;

    The first line here starts off what’s called an anonymous function—that is, it’s a function without a name. We could name it (say, rollOut()), but as it’s only one line it’s not so necessary.

    In this section, we’re telling the browser to trigger what should happen when the user moves the mouse away from the image. Whenever that happens, we want to set the image source back to the initial source value, that is, the outImage version of the image.

  11. thisImage.overImage = new Image();
    thisImage.overImage.src = "images/" + + "_on.gif";

    In the first line, we create a new image object that will contain the overImage version of the image. The second line sets the source for overImage. It builds the name of the source file on the fly, concatenating "images/" with the id of the image (remember, in Listing 4.3, we saw that those ids were button1 and button2) and adding "_on.gif".

  12. thisImage.onmouseover = function() {
       this.src = this.overImage.src;

    Here we have another anonymous function. This one tells the browser that when the user moves the cursor over the image, it should reset the current image’s source to that of the overImage version, as seen in red-circle-a.jpg and red-circle-b.jpg.

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