currying.js

import objectassign from 'object-assign'
Object.assign = objectassign

function add(...xs) {
  const x = xs.reduce((a,b) => a+b)
  return Object.assign((...ys) => add(x, ...ys), { valueOf: () => x })
}

function test(a, b) {
  console.log(a == b ? 'OK' : a + ' != ' + b)
}

test(add(1, 2), 3);
test(add(3)(4)(), 7);
test(add(3)(4)(5), 12);

const three = add(3);
const four = add(4);
test(three, 3);
test(four, 4);
test(three(5), 8);
test(three(6), 9);
test(three(four), 7);
test(three(four)(three(four)), 14);