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

Here's a more generic solution that would work for non-unary params as well:

const sum = function (...args) {
  let total = args.reduce((acc, arg) => acc+arg, 0)
  function add (...args2) {
    if (args2.length) {
      total = args2.reduce((acc, arg) => acc+arg, total)
      return add
    }
    return total
  }

  return add
}

document.write( sum(1)(2)() , '
') // with unary params
document.write( sum(1,2)() , '
') // with binary params
document.write( sum(1)(2)(3)() , '
') // with unary params
document.write( sum(1)(2,3)() , '
') // with binary params
document.write( sum(1)(2)(3)(4)() , '
') // with unary params
document.write( sum(1)(2,3,4)() , '
') // with ternary params