Create an observable that accepts a list of callbacks(returning promises) to execute keeping only an optional number in flight at one time

Rx_throttledPromiseObserver.js

function throttledPromiseObserver(callbacks, maxPending, checkInterval) {
  var remainingCalls = callbacks.length;
  var pendingCalls = 0;
  var callIdx = 0;
  var _checkInterval = checkInterval || 100;
  var _maxPending = maxPending || 0;

  return Rx.Observable.create(bufferedPromiseCalls);

  function bufferedPromiseCalls(observer) {
    check();

    function check() {
      if (remainingCalls) {
        if (_maxPending === 0 || pendingCalls < _maxPending) {
          pendingCalls++;
          remainingCalls--;
          try {
            callbacks[callIdx++]().then(onDone, onDone); //observer.onError
          }
          catch(e){
            observer.onError(e);
          }
        }
        setTimeout(check, _checkInterval);
      }
    }

    function onDone(result) {
      pendingCalls--;
      observer.onNext(result)
      if (remainingCalls === 0 && pendingCalls === 0) {
        observer.onCompleted();
      }
    }
  }
}