Ajax :: Returning A Value Directly From A Function?

I'm attempting to simplify my javascript code when it comes to ajax, but afterwords it only prints 'undefined' to the screen rather than what I want it to print.

I want to be able to put something like this on my main page onclick="document.getElementById('output').innerHTML = print_output();" where print_output() is the ajax function. This way I don't have to use a function to assign values directly to innerHTML and I don't have to muck about with a js file whenever I want to change my page layout.

To do this, I created a recursive function:

function print_output(return_value) {

if a value has been passed to the function, simply return it
if(return_value || return_value == 0) {
return return_value;
}

[Code].....

but like I said, it prints 'undefined' out to the screen. Why isn't this printing the contents that it receives from 'index.php' like it's supposed to?