addlatency.js

var addLatency = function() {
  var pending = 0
    , stream
    , ended

  return stream = through(write, end)

  function write(buf) {
    ++pending
    setTimeout(function() {
      --pending
      stream.queue(buf)
      check()
    }, 1000 * Math.random())
  }

  function end() {
    ended = true
    check()
  }

  function check() {
    if(!pending && ended) {
      return stream.queue(null)
    }
  }
}