javascript next and previous buttons not working

[mle39920]

I am having trouble executing the next and previous buttons on my page. The functions for nextBottonClick and previousButtonC lick specifically. Any help is greatly appreciated.

Thansk

// Variable declarations.
var imageCache = [];  // Array to hold the images from the invisible UL list.
var imageItem = 0;    // Holds the currently displayed image index.
var images = 0;       // Total images we have on the page to display.
// Elements from the page we need to refer to.
var captionNode;
var imageNode;

// Here is the $ function to simplify use of document.getElementById().
var $ = function (id) { 
	return document.getElementById(id); 
}

// Executed upon window load event.
window.onload = function () {
    // Setting up various elements into variables to use later.
    var listNode = $("image_list");  
    var nextButton = $("next");
    var previousButton = $("previous");
    // Filling in the caption and image node variables.  
    captionNode = $("caption");
    imageNode = $("image");

    // The images are placed within <a> tags for convenience.  They aren't used as links ever.
    var links = listNode.getElementsByTagName("a");
    
    // Process image links.  A for loop is used to touch each of the images within the <a> tags
    // and store the image information into an array element.
    var i, linkNode, image;
    for ( i = 0; i < links.length; i++ ) {
        linkNode = links[i];
        // Pre-load image and copy title properties.
        image = new Image();
        image.src = linkNode.getAttribute("href");
        image.title = linkNode.getAttribute("title");
        imageCache.push(image);
    }
    // Now record the total images we have.
    images = imageCache.length;
    
    // Set up the button handlers.
    nextButton.onclick = nextButtonClick;
    previousButton.onclick = previousButtonClick;
}

function nextButtonClick() {
	//TODO: Complete the next button click event handler to display the next image.
	if(imageNode < images){
		imageNode++
	}
	else{
		imageNode = 1
	}
	document.slideImage.src = image[imageNode-1]
	//      Roll the image over when we reach the end of the image set.
	
	// Increase image item index by 1;
	
	// Check to see if we are at the end of the image list.
	// Remember, arrays are indexed starting a zero so if we have a number that is equal
	// to the number of images, we have reached that point.
	// If so, go back to the first image.
	
	
    // Now set the image src and title properties to the appropriate image.
    // To retrieve an image from imageCache with index imageItem, use imageCache[imageItem].
}

function previousButtonClick() {
    // Decrease image counter by 1.
		if(imageNode > 1){
		imageNode--
	}
	else{
		imageNode = images
	}
	document.slideImage.src = image[imageNode-1]
	

    // Check to see if we are less than zero.
    // If so, go to the last image.  Keep in mind that the index of the last
    // image is one less than the number of images.

    // Now set the image src and title properties to the appropriate image.
    
}