Having trouble streaming response to client using expressjs

You can do this by setting the appropriate headers and then just writing to the response object. Example:

res.writeHead(200, {
  'Content-Type': 'text/plain',
  'Content-Disposition': contentDisposition('foo.data')
});
var c = 0;
var interval = setInterval(function() {
  res.write(JSON.stringify({ foo: Math.random() * 100, count: ++c }) + '\n');
  if (c === 10) {
    clearInterval(interval);
    res.end();
  }
}, 1000);


// extracted from Express, used by `res.download()`
function contentDisposition(filename) {
  var ret = 'attachment';
  if (filename) {
    filename = basename(filename);
    // if filename contains non-ascii characters, add a utf-8 version ala RFC 5987
    ret = /[^\040-\176]/.test(filename)
      ? 'attachment; filename="' + encodeURI(filename) + '"; filename*=UTF-8\'\'' + encodeURI(filename)
      : 'attachment; filename="' + filename + '"';
  }

  return ret;
}

Also, Express/node does not buffer data written to a socket unless the socket is paused (either explicitly or implicitly due to backpressure). Data buffered by node when in this paused state may or may not be combined with other data chunks that already buffered. You can check the return value of res.write() to determine if you should continue writing to the socket. If it returns false, then listen for the 'drain' event and then continue writing again.

The response object is already a writable stream. Express handles sending chunked data automatically, so you won't need to do anything extra but:

response.send(data)

You may also want to check out the built-in pipe method, http://nodejs.org/api/stream.html#stream_event_pipe.