log

> echo '{"a":1}' | ../node_query_server/bin/qs2 

net.js:614
    throw new TypeError('invalid data');
          ^
TypeError: invalid data
    at WriteStream.Socket.write (net.js:614:11)
    at null._onTimeout (/Users/jan/Work/qs2/node_query_server/index.js:17:20)
    at Timer.listOnTimeout [as ontimeout] (timers.js:112:15)

stream-thing.js

var ndj = require('ndjson');

process.stdin.setEncoding('utf8');

function getRandomInt(min, max) {
  return Math.floor(Math.random() * (max - min)) + min;
}

// var nd_out = process
//   .stdout
//   .pipe(ndj.serialize());

var send = function(data) {
  // console.error('data: ', data);
  setTimeout(function() {
    // nd_out.write(data);
    process.stdout.write(data);
  }, getRandomInt(1, 100));
};

process
  .stdin
  .pipe(ndj.parse())
  .on('data', send);

The old code has an issue where the part of a chunk after the last \n is not a full JSON object, but rather than coding that logic myself, I assume there is something in the streams universe that does that for me, but it is not obvious where to find it. e.g.https://github.com/substack/stream-handbook#consuming-a-readable-stream shows an example (way down), that looks like it does what I want, but it is not clear, whether this is available as an abstraction anywhere.

var offset = 0;

process.stdin.on('readable', function () {
    var buf = process.stdin.read();
    if (!buf) return;
    for (; offset < buf.length; offset++) {
        if (buf[offset] === 0x0a) {
            console.dir(buf.slice(0, offset).toString());
            buf = buf.slice(offset + 1);
            offset = 0;
            process.stdin.unshift(buf);
            return;
        }
    }
    process.stdin.unshift(buf);
});

Updated the gist and added a new call log.

@janl process.stdout.write(data) fails because data is a object (ndjson.parse() returns an object stream). since process.stdout is a binary stream it only accepts strings or buffers.

simply doing process.stdout.write(JSON.stringify(data)) would fix this 😄

the problem here is you are attempting to write a live javascript object to stdout.
this: stdout.write({a: 1}) you need to stringify it first.
one way would be to pipe it to ndjson.stringify() before piping to stdout.