Node.js concat files in directory

concat.js

var fs = require('fs');

var dirname = '/some-dir/';

function onFileContent(filename, content) {
  if (!/\.(js|md|txt)$/.test(filename)) {
      return false;
  }
  var str = filename + '\n\n' + content + '\n\n\n\n\n';
  console.log(str);
}

function onError(error) {
  console.log(error);
}

readFiles(dirname, onFileContent, onError);

function readFiles(dirname, onFileContent, onError) {
  fs.readdir(dirname, function(err, filenames) {
    if (err) {
      onError(err);
      return;
    }
    filenames.forEach(function(filename) {
      fs.readFile(dirname + filename, 'utf-8', function(err, content) {
        if (err) {
          onError(err);
          return;
        }
        onFileContent(filename, content);
      });
    });
  });
}