I didn't thought that is a problem. I have different function which returns values; So i created different function to sum both values.
var result = firstFunct() + secondFunct();
This is most probably because your functions are returning string and not a numeric data type. You will need to cast the output of the functions
var result = Number(firstFunct()) + Number(secondFunct());
note, casting to a number could also be done in a shorter way, usin the
var result = (+firstFunct()) + (+secondFunct());