Variadic curried sum function

Question

I need a js sum function to work like this:

sum(1)(2) = 3
sum(1)(2)(3) = 6
sum(1)(2)(3)(4) = 10 
etc.

I heard it can\'t be done. But heard

Answer 1

To make sum(1) callable as sum(1)(2), it must return a function.

The function can be either called or converted to a number with valueOf.

function sum(a) {

   var sum = a;
   function f(b) {
       sum += b;
       return f;
    }
   f.toString = function() { return sum }
   return f
}