Class name problem

className.js

/**
 * Write a function that generates a unique class name every time
 * that it's called. The class name should follow the following
 * pattern:
 * 
 * fn(); // returns 'a'
 * fn(); // returns 'b'
 * ...
 * fn(); // returns 'z'
 * fn(); // returns 'aa'
 * fn(); // returns 'ab'
 * ...
 * fn(); // returns 'zz'
 * fn(); // returns 'aaa'
 * 
 * And so on and so forth.
 */
function className() {}