You can find a list of variables and functions Axure exposes here: V7 Variables List. You can also see all options by clicking Insert Variable of Function here:
It’s listed for v7 but it should still be almost all relevant to even v9.
Since for the most part these functions work just as you would expect in JavaScript (there are some exceptions), you can look them up online if you have any questions about what they do or how exactly they work. MDN is a great resource for this. For example. if you’re wondering what the difference between substr() and substring() is…
Note that not every function Axure provides is necessarily a standard JavaScript function, in case you can’t find it online.