Promiseパターンを使ったファイル処理のテスト。 引数の受け渡しと取り扱いが見えにくいのはPromiseパターンの欠点かな。

join.js

var fs = require('fs');
var path = require('path');

var promise = require('node-promise');
var fsp = require('node-promise/fs-promise');

const TEXT_DIR_PATH = path.resolve(__dirname, 'texts');
const OUTPUT_FILE_PATH = path.resolve(__dirname, 'output.txt');

var texts = [];

var textDirPaths = [
  path.resolve(TEXT_DIR_PATH, '1'),
  path.resolve(TEXT_DIR_PATH, '2'),
  path.resolve(TEXT_DIR_PATH, '3'),
  path.resolve(TEXT_DIR_PATH, '4')
];

var filePaths = [];

promise.all(textDirPaths.map(function(textDirPath) {
  var alltxtp = new promise.Promise();

  return fsp.readdir(textDirPath).then(function(fileNames) {
    var txtpArr = [];
    fileNames.forEach(function(fileName) {
      if (fileName.match(/\.txt$/)) {
        var filePath = path.join(textDirPath, fileName);
        var txtp = fsp.readFile(filePath);
        txtpArr.push(txtp);
      }
    });
    return promise.all(txtpArr);
  }).then(function(txts) {
    return txts.join('');
  });
})).then(function(txts) {
  fs.writeFile(OUTPUT_FILE_PATH, txts.join('\n\n\n'));
});