Return types
We support a number of different return types:

Numbers (floats and integers are both supported)
= 1

Strings
= 'Hello world!'

Dates
= new Date (2014,1,28) 
Boolean
= true

Array (only arrays of primitive types are allowed, and all elements of the array must be of the same primitive type)
= [1,1,2,3,5,8]

Object (the values of the object can only be primitive types, and all values of the object must be of the same primitive type)
= {'first': 10, 'second': 20}

Basic Math
= 1+1
= 5*25
= Math.pow(10, 10)
Unsupported functions
Streak's formula functionality is built on Rhino, which is lightly limited in its functionality. You'll find that most common ES5 functions will work, but there is a limit there, too: we do not support XHR or any HTTP requests from formula.
Because of the way formulas are computed, we also advise against using a function like Date.now(), which will not update daily.