callback vs async vs promise

cap.js

var fs = require("fs"),
	path = require("path"),
	async = require("async"),
	Q = require("q");

//first version
// var dirName = "foo";
// fs.readdir(dirName, function(err, fileNames) {
// 	if (err) {
// 		throw err;
// 	}

// 	var readCount = 0;
// 	fileNames.forEach(function(fileName) {
// 		var fullPath = path.resolve(dirName, fileName);
// 		fs.readFile(fullPath, function(err, data) {
// 			if (err) {
// 				throw err;
// 			}

// 			console.log(data.length);

// 			if (++readCount == fileNames.length) {
// 				console.log("DONE");
// 			}
// 		});
// 	});
// });

//second version
// var dirName = "foo";
// fs.readdir(dirName, function(err, fileNames) {
// 	if (err) {
// 		throw err;
// 	}

// 	var fullPaths = fileNames.map(function(fileName) {
// 		return path.resolve(dirName, fileName);
// 	});

// 	async.map(fullPaths, fs.readFile, function(err, datas) {
// 		if (err) {
// 			throw err
// 		}

// 		datas.forEach(function(data) {
// 			console.log(data.length);
// 		});

// 		console.log("DONE");
// 	});
// })


//third version
var readdirPromise = function(dirName) {
	var deferred = Q.defer();

	fs.readdir(dirName, function(err, fileNames) {
		if (err) {
			deferred.reject(err);
		} else {
			deferred.resolve(fileNames);
		}
	});

	return deferred.promise;
};

var readFilePromise = function(filePath) {
	var deferred = Q.defer();

	fs.readFile(filePath, function(err, data) {
		if (err) {
			deferred.reject(err);
		} else {
			deferred.resolve(data);
		}
	});

	return deferred.promise;
};
// below two lines are AWESOME！
// var readdirPromise = Q.denodeify(fs.readdir);
// var readFilePromise = Q.denodeify(fs.readFile);

var dirName = "foo";

readdirPromise(dirName).then(function(fileNames) {

	var readFilePromises = fileNames.map(function(fileName) {
		var fullPath = path.resolve(dirName, fileName);
		return readFilePromise(fullPath);
	});

	return Q.all(readFilePromises);
}).then(function(datas) {

	datas.forEach(function(data) {
		console.log(data.length);
	});

	console.log("DONE");
});