Promises

example.js

function loadFile(filename) {
  let script = ... create script with file

  let promise = new Promise()

  script.onload = function () {
    promise.resolve()
  }

  return promise
}

Promise.all([
  loadFile('file1'),
  loadFile('file2'),
  loadFile('file3'),
]).then(function () {
  // After all files load
})