Below is my javascript code. It seems to hanging up at the "var data = transport.responseText.evalJSON();" part.

Here is a link to see how my html is made.
http://twisted.visionsminis.com/price/compressors

if (!window.WAUpdater) var WAUpdater = new Object();

WAUpdater.Methods =
{
	options: {
		redirectOnError: true,
		baseUrl: "",
		updateColor:'#666666',
		mode:"add",
		removeRow:null
	},
	_options: new Object,
	setOptions: function(options) {
		Object.extend(this.options, options || {});
	},
	
	showAjaxModal: function()
	{
		Modalbox.show('<div id="MB_loading"><br/>Please wait...</div>'); 
	},
	
	updateMiniCart: function(transport, options)
	{
		this.setOptions(options);
		Modalbox.hide();
		window.scroll(0,0);
	  	var data = transport.responseText.evalJSON();
		//IF THERE'S ANY ERROR
		if(data.error)
		{
			
			if(this.options.redirectOnError)
			{
				if(data.url) window.location = data.url;
				else history.go(-1);
			}
			else
			{
				var error_msg = 'Sorry. There was an error in your operation.';
				if(data.error_msg) error_msg = data.error_msg;
				Modalbox.show('<div id="MB_message">ERROR:'+error_msg+'</div>',{modal:true}); 
			}
			
		}
		else
		{
			var item_count= data.item_count;
			var cart_subtotal = data.cart_subtotal;
			var onepage_checkout = data.onepage_checkout;
			var link_text = data.link_text;
			var cart_subtotal_str = data.cart_subtotal_str;
			var cart_subtotal_tax = data.cart_subtotal_tax;
			var my_cart_str = data.my_cart_str;
			var qty_str = data.qty_str;
			var checkout_str = data.checkout_str;
			var no_cart_str = data.no_cart_str;
			
			//update item count on top menu
			if($$('a.top-link-cart').size()!=0 && link_text)
			{
				$$('a.top-link-cart')[0].update(link_text);
			}
			
			
			if(this.options.removeRow)
			{
				if($$('.mini-cart h5').size()!=0 && item_count==0)$$('.mini-cart h5')[0].remove();
				new Effect.SlideUp(this.options.removeRow);
			}
			
			//update recently added items
			if(data.recent_items)
			{
				if($$('.mini-cart h5').size()!=0) $$('.mini-cart h5')[0].remove();
				if($$('.mini-cart .content').size()!=0) $$('.mini-cart .content')[0].remove();
				if($$('#cart-sidebar').size()!=0) $$('#cart-sidebar')[0].remove();
				// insert recently updated items after checkout button
				if ($$('.mini-cart').size() != 0) {
					$$('.mini-cart')[0].insert(data.recent_items);
					decorateList('cart-sidebar', 'non-recursive');
					new Effect.SlideDown('cart-sidebar');
				}
			}
			
			//total price
			if($$('.mini-cart .head').size()!=0)
			{
				var minicart_itemcount = qty_str;
				
				if(item_count > 0)
				{
					var minicart_string = '<h4>'+my_cart_str+'</h4>'+minicart_itemcount + '<p class="subtotal">'+cart_subtotal_str+'<strong>'+cart_subtotal+'</strong>'+cart_subtotal_tax +'</p>';
					if($$('.mini-cart .actions').size() ==0 && onepage_checkout) 
					{
						$$('.mini-cart .head')[0].insert({
							after: '<div class="actions"><button class="form-button" onclick="setLocation('+this.options.baseUrl+'\'/index.php/checkout/onepage/\')" type="button"><span>'+checkout_str+'</span></button></div>'
						});
					}
				}
				else
				{
					if($$('.mini-cart .actions').size() !=0) $$('.mini-cart .actions')[0].remove();
					var minicart_string = '<h4>'+my_cart_str+'</h4>';
					$$('.mini-cart .head')[0].insert({
						after: '<div class="content"><p>'+no_cart_str+'</p></div>'
					});
				}
				
				$$('.mini-cart .head')[0].update(minicart_string);
				
				new Effect.Highlight($$('.mini-cart .head')[0],{startcolor: this.options.updateColor});
				if($$('.mini-cart .head .head').size()!=0) new Effect.Highlight($$('.mini-cart .head .head')[0],{startcolor: this.options.updateColor});
				if($$('.mini-cart .actions').size()!=0) new Effect.Highlight($$('.mini-cart .actions')[0],{startcolor: this.options.updateColor});
			}
			
			if($$('#shopping-cart-totals-table tfoot').size()!=0)
			{
				$$('#shopping-cart-totals-table tfoot')[0].update(data.totals_footer);
				$$('#shopping-cart-totals-table tbody')[0].update(data.totals_top);
				new Effect.Highlight($$('#shopping-cart-totals-table tfoot')[0],{startcolor: this.options.updateColor});
				new Effect.Highlight($$('#shopping-cart-totals-table tbody')[0],{startcolor: this.options.updateColor});
			}
			
		}
	}
};

Object.extend(WAUpdater, WAUpdater.Methods);
function submit_ajax_cart(url)
{
	new Ajax.Request(url, {
	  method: 'get',
	  onLoad:WAUpdater.showAjaxModal(),
	  onComplete: function(transport)
	  {
	  	bu =  url.substring(0,url.lastIndexOf("/checkout") ) ;
	  	WAUpdater.updateMiniCart(transport, {baseUrl:bu, mode:"ADD"});
	  }
	});
}

function submit_ajax_cart_p(url)
{
	window.opener.submit_ajax_cart(url); 
}

/* from this section on is premium */
function delete_ajax_cart(url,row)
{
	new Ajax.Request(url, {
	  method: 'get',
	  onLoad:WAUpdater.showAjaxModal(),
	  onComplete: function(transport) 
	  {
		bu =  url.substring(0,url.lastIndexOf("/checkout") ) ;
	  	WAUpdater.updateMiniCart(transport, {baseUrl:bu, mode:"DELETE", removeRow:row, redirectOnError: false});
	  }
	});
}

Seems like a debugging issue, have you tried a (JavaScript) debugger? With a debugger you can easily step through the code, and see which variables are filled with what values, and when they change.

See it as a slowmotion running your webpage, while you can inspect every value if it is correct.

I use Netbeans:
www.netbeans.org

But there are more:
Addon for Firefox: https://addons.mozilla.org/nl/firefox/addon/216
Also you may use Firebug: https://addons.mozilla.org/nl/firefox/addon/1843

I hope this helped.

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