My solution to https://github.com/frantic/currying

nuts.js

function add() {
  var args = [].slice.call(arguments);
  var sum = args.reduce(function(a, b){ return a + b; }, 0);
  
  return numify(sum);

  function numify(num){
    var ret = function(a){
      return a === undefined ? numify(num) : numify(a + num);
    }

    ret.valueOf = function(){ return num; };

    return ret;
    
  }
}

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

var three = add(3);
var 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);

function test(a, b) {
  if (a == b) {
    console.log('OK');
  } else {
    console.log(a + ' != ' + b);
  }
}