func.js

var add = function(x, y) { return x + y }
var mul = function(x, y) { return x * y }

var make = function(x, arr) {
  if (typeof x == 'number') {
    arr ? arr.push(x) : arr = [x]
    return function(x) { return make(x, arr) }
  } else return arr.reduce(x)
}

var make = function(x, arr) {
  return typeof x == 'number'
    ? (arr ? arr.push(x) : arr = [x], function(x) { return make(x, arr) })
    : arr.reduce(x)
}

var s = make(1)(2)(3)(4)(5)
s(add) // => 15
console.log(s(mul)) // => 120

var x = make(5)(10)(15)
x(add) // => 30
x(mul) // => 750