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 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.