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

You can make use of the below function

function add(num){
   add.sum || (add.sum = 0) // make sure add.sum exists if not assign it to 0
   add.sum += num; // increment it
   return add.toString = add.valueOf = function(){ 
      var rtn = add.sum; // we save the value
      return add.sum = 0, rtn // return it before we reset add.sum to 0
   }, add; // return the function
}

Since functions are objects, we can add properties to it, which we are resetting when it's been accessed.