backpressure.js

function pipe(readable, writable) {
  var unsubscribe = readable.subscribe(next, writable.complete, writable.error);

  function next(value) {
    // the writable's `next` can return a promise; if it does, unsubscribe and wait
    // for the promise to resolve before resubscribing
    var pause = writable.next(value);
    if (pause) {
      unsubscribe();
      pause.then(function() { pipe(readable, writable); })
    }
  }
}

class Writable {
  next(value) {
    // someAsyncProcessing returns a promise, like all well-behaved async things;
    // the pipe will unsubscribe from the upstream until the promise resolves
    return someAsyncProcessing(value);
  }
}