how to define global variables that are accessible via javascript

[eluser]chubbypama[/eluser]
I have a ci application that includes some ajax calls. After I call ajax methods, i sometimes dynamically build html forms. I would like to be able to "know" the value of base_url() in my javascript. Someone has suggested that I add global js variable declarations to my index.php file that's in the root of my CI application.
How would I do this?