You can use Firebug generate logs. This is a great way to finding errors in a page.
Execute the following code in Firefox.
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
var a = "w3";
var b = " ";
//console.log("%s%d", "w3", resource);
The above code is intended to output "w3resource". But it shows "w3" only. So let's uncomment the line console.log("%s%d", "w3", resource); and run Firebug.
So, now we can easily understand that resource is not defined. Check the code back, and yes, var b = "";