cp-async.js

async function () {
  const files = await fs.readdir('a');
  await Promise.all(files.map(async (filename) => {
    const data = await fs.readFile(filename);
    await fs.writeFile('b/' + filename, data);
    console.log(filename);
  }));
}();

cp-callbacks.js

// Copy every file in directory `a` to directory `b`.
fs.readdir('a', function (error, files) {
  if (error) throw error;
  for (var i = 0; i < files.length; ++i) {
    var filename = files[i];
    fs.readFile(filename, function (error, data) {
      if (error) throw error;
      fs.writeFile('b/' + filename, data, function (error) {
        if (error) throw error;
        console.log(filename);
      });
    });
  }
});

cp-generator.js

async(function*() {
  const files = yield fs.readdir('a');
  yield Promise.all(files.map(async(function* (filename) {
    const data = yield fs.readFile(filename);
    yield fs.writeFile('b/' + filename, data);
    console.log(filename);
  })));
})();

cp-promises.js

fs.readdir('a')
  .then(files => Promise.all(
    files.map(filename =>
      fs.readFile(filename)
        .then(data => fs.writeFile('b/' + filename, data))
        .then(() => console.log(filename))
    )
  ))
  .catch(error => throw error);