carousel.js

const track = document.getElementById("image-track");

const handleOnDown = e => track.dataset.mouseDownAt = e.clientX;

const handleOnUp = () => {
  track.dataset.mouseDownAt = "0";
  track.dataset.prevPercentage = track.dataset.percentage;

  if (track.dataset.prevPercentage > (-100/6)) {
    track.dataset.percentage = 0;
    track.dataset.prevPercentage = track.dataset.percentage;
  }

  if ((track.dataset.percentage > (-300/6)) && (track.dataset.percentage < (-100/6))) {
    track.dataset.percentage = (-100/3);
    track.dataset.prevPercentage = track.dataset.percentage;
  }

  if ((track.dataset.percentage < (-300/6))) {
    track.dataset.percentage = (-200/3);
    track.dataset.prevPercentage = track.dataset.percentage;
  }

  track.animate({
    transform: `translate(${track.dataset.prevPercentage}%, 0%)`
  }, { duration: 1000, fill: "forwards", easing: 'ease'});
}

const handleOnMove = e => {
  if(track.dataset.mouseDownAt === "0") return;

  const mouseDelta = parseFloat(track.dataset.mouseDownAt) - e.clientX,
        maxDelta = window.innerWidth / 2;

  const percentage = ((mouseDelta / maxDelta) * -100),
        nextPercentage = parseFloat(track.dataset.prevPercentage) + percentage;

  if (nextPercentage > 0) {
    nextPercentage = 0;
  }

  if (nextPercentage < (-200/3)) {
    nextPercentage = -200/3;
  }
  
  track.dataset.percentage = nextPercentage;

  track.animate({
    transform: `translate(${nextPercentage}%, 0%)`
  }, { duration: 1000, fill: "forwards" });

  for(const image of track.getElementsByClassName("project")) {
    image.animate({
      backgroundPosition: `${100 + (nextPercentage)}% center`
    }, { duration: 1000, fill: "forwards" });
  }
}

/* -- Had to add extra lines for touch events -- */

track.onmousedown = e => handleOnDown(e);

track.ontouchstart = e => handleOnDown(e.touches[0]);

track.onmouseup = e => handleOnUp(e);

track.ontouchend = e => handleOnUp(e.touches[0]);

track.onmousemove = e => handleOnMove(e);

track.ontouchmove = e => handleOnMove(e.touches[0]);