Splicing array to add to an existing array

I am using rss2json to consume an rss feed. There is not a page param to enable pagination. There is a count parameter that I can pass to the request. I am able to load the feed and get results back. I have created a service using ionic to make a request to get the feed:

getRssFeed(rssUrl: string, count: number) {
    return new Promise<any>(resolve => {
        this.http.get(`${ environment.podcast.baseUrl }?rss_url=${ rssUrl }&api_key=${ environment.podcast.apiKey }&count=${ count }`)
            .subscribe(data => {
                resolve(data);
            }, error => {
                console.error('Something really bad happened trying to get rss feed.');
                console.error(error);
            });
    });
}

This works great. I can get the data back - all is well. I am using an infinite scroll component to handle pagination. Again, all is well. I am starting with 10 podcast episodes. I am logging out when I want to load more episodes:

When I scroll, the service makes the correct call, but because the rss2json service does not have a page param, It will return the entire array when I update the count.

So I need to do something like this:

episodes: Array<any>;
count = 10;

...

this.episodes.splice(this.episodes.length, this.count, data.items);

I need to find out how many episodes I already have. The first time I get to the bottom of my list, I'll have 10 (I want to increment +10 each load). So I need to:

• Find out how many episodes I currently have (10, 20, 30 etc.)
• Make the request to get more episodes
• Service returns 20 episodes -- but it will always start at zero.
• Slice the first 10, 20, ?? episodes that are returned, add the remaining 10 to the end of the list.

I am not sure how to achieve this and could use some direction.

Here is how I am requesting more episodes:

 this.myPodcastService.getRssFeed(this.rssUrl, this.count)
     .then(data => {
         if (data) {
             // console.log('data', data.items);
             // data is an object
             // data.items is an array of episodes

             // this.episodes.splice(this.episodes.length, this.count, data.items);
         } else {
             ...
         }
          ...
      });

For example, the first time I get to the end of my episodes, I'll have 10 on the page. I want to go out, get 10 more episodes. So I need to increment my count variable to 20 and pass that in as the count param.

The service will return 20 items. The first 10 I want to delete (They are already on screen). I only need the last 10 episodes...

Now I'll have 20 episodes. The next time I scroll, I'll need to increment my count to 30. The service will return an array of 30 items. I will need to delete (splice) the first 20; leaving only the last 10 -- then add that to the episodes array.

The logging should show something like:

this.episodes[10]
this.episodes[20]
this.episodes[30]

I hope that makes sense. I know what I'm trying to achieve, I'm struggling how to actually do it. Thank you for any suggestions!

EDIT/SOLUTION

Thank you so much for the suggestion! In case someone else comes across this, here is what I came up with that is doing exactly what I need.

// load more episodes using infinite scroll.
loadMoreEpisodes(event) {
    console.log('--> loading more episodes');

    this.count = (this.count + this.count);  // 10, 20, 30...

    this.myPodcastService.getRssFeed(this.rssUrl, this.count)
        .then(data => {
            if (data) {
                // append the new episodes to the existing array
                this.episodes.push(...data.items.splice(-this.episodes.length, this.count));
                event.target.complete();
                console.log('this.episodes', this.episodes);
            } else {
                this.alertCtrl.create({
                    header: 'Error',
                    subHeader: 'Something bad happened',
                    message: 'Something internet related happened & we couldn\'t load the playlist.',
                    buttons: [{ text: 'Ok', role: 'cancel' }]
                }).then(alert => {
                    alert.present();
                });
            }
        });
}