Function generators.

gistfile1.js

function addX (x) {
  return function (a) {
    return a + x;
  }
}

var add5 = addX(5);
var add10 = addX(10);

// What is result?
var result = add10(add5(addX(4)(6)));

gistfile2.js

// PART TWO

function addX (x) {
  return function (a) {
    return a + x;
  }
}

var makeAdd5 = addX.bind(null, 5);
var add5 = makeAdd5();

// What is result?
var result = add5.apply(null, 10);

Um 25?

DING DING DING .... <hands merrick giant stuffed animal>

Hmmm I'm guessing 15, but less confident in my answer...