RxJS, how to poll an API to continuously check for updated records using a dynamic timestamp

Question

I am new to RxJS and I am trying to write an app that will accomplish the following things:

1. On load, make an AJAX request (faked as fetchItems() f

Answer 1

You seem to mean that modifiedSince is part of the state you carry, so it should appear in the scan. Why don-t you move the action in do into the scan too?. Your seed would then be {modifiedSince: null, itemArray: []}.

Errr, I just thought that this might not work, as you need to feed modifiedSince back to the fetchItem function which is upstream. Don't you have a cycle here? That means you would have to use a subject to break that cycle. Alternatively you can try to keep modifiedSince encapsulated in a closure. Something like

function pollItems (fetchItems, polling_frequency) {
var modifiedSince = null;
var data$ = Rx.Observable.interval(polling_frequency)
  .startWith('run right away')
  .flatMap(function() { 
    // `fetchItems(modifiedSince)` returns an array of items modified after `modifiedSince`
    return fetchItems(modifiedSince);
  })
  .do(function(item) {
    if(item.updatedAt > modifiedSince) {
      modifiedSince = item.updatedAt;
    }
  })
  .scan(function(previous, current) {
    previous.push(current);
    return previous;
  }, []);

return data$;
}

I have to run out to celebrate the new year, if that does not work, I can give another try later (maybe using the expand operator, the other version of scan).