Piping multiple file streams using Node.js

I want to stream multiple files, one after each other, to the browser. To illustrate, think of having multiple CSS files which shall be delivered concatenated as one. The code I am using is: ``` var directory = path.join(__dirname, 'css'); fs.readdir(directory, function (err, files) { async.eachSeries(files, function (file, callback) { if (!endsWith(file, '.css')) { return callback(); } // (1) var currentFile = path.join(directory, file); fs.stat(currentFile, function (err, stats) { if (stats.isDirectory()) { return callback(); } // (2) var stream = fs.createReadStream(currentFile).on('end', function () { callback(); // (3) }); stream.pipe(res, { end: false }); // (4) }); }, function () { res.end(); // (5) }); }); ``` The idea is that I - filter out all files that do not have the file extension `.css`. - filter out all directories. - proceed with the next file once a file has been read completely. - pipe each file to the response stream without closing it. - end the response stream once all files have been piped. The problem is that only the first `.css` file gets piped, and all remaining files are missing. It's as if (3) would directly jump to (5) after the first (4). The interesting thing is that if I replace line (4) with ``` stream.on('data', function (data) { console.log(data.toString('utf8')); }); ``` everything works as expected: I see multiple files. If I then change this code to ``` stream.on('data', function (data) { res.write(data.toString('utf8')); }); ``` all files expect the first are missing again. What am I doing wrong? PS: The error happens using Node.js 0.8.7 as well as using 0.8.22. UPDATE Okay, it works if you change the code as follows: ``` var directory = path.join(__dirname, 'css'); fs.readdir(directory, function (err, files) { var concatenated = ''; async.eachSeries(files, function (file, callback) { if (!endsWith(file, '.css')) { return callback(); } var currentFile = path.join(directory, file); fs.stat(currentFile, function (err, stats) { if (stats.isDirectory()) { return callback(); } var stream = fs.createReadStream(currentFile).on('end', function () { callback(); }).on('data', function (data) { concatenated += data.toString('utf8'); }); }); }, function () { res.write(concatenated); res.end(); }); }); ``` But: Why? Why can't I call `res.write` multiple times instead of first summing up all the chunks, and then write them all at once?