readFileAsync('file1',
  (f1) => readFileAsync('file2',
    (f2) => readFileAsync('file3',
      (f3) => {
        /* Do things */
})));

(readFileAsync 'file1 (lambda (f1)
  (readFileAsync 'file2 (lambda (f2)
    (readFileAsync 'file3 (lambda (f3) (operation f1 f2 f3))))))

var promises = ['file1', 'file2', 'file3'].map((name) => {
  return readFileAsync(name);
});

Promise.all(promises)
  .then(console.log)
  .done();