Reverse slider when swiped down

I followed this article for a vertical swipeable cards slider.

This question has two parts.

1. I cant understand on how to reverse the direction of slider when swiped down?

Here is the relevant codepen - https://codepen.io/bmarcelino/pen/vRYPXV

The relevant function to update the cards

function updateUi() {
    requestAnimationFrame(function(){
        elTrans = 0;
        var elZindex = 5;
        var elScale = 1;
        var elOpac = 1;
        var elTransTop = items;
        var elTransInc = elementsMargin;

        for(i = currentPosition; i < (currentPosition + items); i++){
            if(listElNodesObj[i]){
                listElNodesObj[i].classList.add('stackedcards-bottom', 'stackedcards--animatable', 'stackedcards-origin-bottom');

                listElNodesObj[i].style.transform ='scale(' + elScale + ') translateX(0) translateY(' + (elTrans - elTransInc) + 'px) translateZ(0)';
                listElNodesObj[i].style.webkitTransform ='scale(' + elScale + ') translateX(0) translateY(' + (elTrans - elTransInc) + 'px) translateZ(0)';
                listElNodesObj[i].style.opacity = elOpac;
                listElNodesObj[i].style.display = 'block';
                listElNodesObj[i].style.zIndex = elZindex;

                elScale = elScale - 0.04;
                elOpac = elOpac - (1 / items);
                elZindex--;
            }
        }

    });

};

As of now the slider moves in only one direction when swiped - forward. I am looking to understand an implementation of adding the backward movement to the slider.

2. Regarding performance

Also, requestAnimationFrame really helps out in providing a smooth experience while swiping. But is there a limit as to how many cards should be in DOM? I will be calling an API service to get the contents, since it will return media, so will simply setting opacity to 0 help out in any way reducing memory use?

The author argues that removing DOM would force the browser to repaint, which can impact performance substantially? But isn't that virtual list do? What is the performance to cost ratio in such scenarios?

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