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

I'm posting this revision as its own post since I apparently don't have enough reputation yet to just leave it as a comment. This is a revision of @Rafael 's excellent solution.

function sum (n) {
    var v = x => sum (n + x);
    v.valueOf = () => n; 
    return v;
}

console.log(+sum(1)(2)(3)(4)); //10

I didn't see a reason to keep the v.toString bit, as it didn't seem necessary. If I erred in doing so, please let me know in the comments why v.toString is required (it passed my tests fine without it). Converted the rest of the anonymous functions to arrow functions for ease of reading.