Example from StrongLoop article -> http://strongloop.com/strongblog/node-js-callback-hell-promises-generators/ implemented with deferred promise library

strongloop-find-largest-file.js

var fs        = require('fs')
  , join      = require('path').join
  , promisify = require('deferred').promisify

  , readdir = promisify(fs.readdir), lstat = promisify(fs.lstat);

module.exports = function (dir) {
  var largest = null, largestSize = -1;
  return readdir(dir).map(function (file) {
    return lstat(join(dir, file))(function (stats) {
      if (!stats.isFile()) return;
      if (stats.size <= largestSize) return;
      largest = file;
      largestSize = stats.size;
    });
  })(function () { return largest; });
};