Forget Callback inversion of control, thunks are the way to go!

thunks-rock.js

// Playing with Thunks
// See: http://jsbin.com/qebuluveje/edit?js,console
// Awesome!
const fakeAjax = (text, cb) =>
  setTimeout(() => {
    cb(text);
  }, 5000);

function getFile(file) {
  var text, fn;
  fakeAjax(file, (res) => {
    if (fn) fn(res);
    else text = res;
  });
  return function(cb) {
    if (text) cb(text);
    else fn = cb;
  }
}

let thunk1 = getFile("File 1: bacon bacon bacon");
let thunk2 = getFile("File 2: brie brie brie");
let thunk3 = getFile("File 3: async async async");

const output = (text, index) =>
  console.log(`File #${index}. Contents: ${text}`);

thunk1((text1) => {
  output(text1, 1);
  thunk2((text2) => {
    output(text2, 2)
    thunk3((text3) => {
      output(text3, 3)
      console.log('Complete!')
    });
  });
});