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

This is an example of using empty brackets in the last call as a close key (from my last interview):

sum(1)(4)(66)(35)(3)()

function sum(numberOne) {
  var count = numberOne;
  return function by(numberTwo) {
    if (numberTwo === undefined) {
      return count;
    } else {
      count += numberTwo;
      return by;
    }
  }
}
console.log(sum(1)(4)(66)(35)(3)());