Have difficulty understanding a logic in JS slider script

JavaScript
1

The source script that I am analyzing is here.

var itemClassName = "carousel__photo";
items = d.getElementsByClassName(itemClassName),
totalItems = items.length,
slide = 0,
moving = true;

// Set classes
function setInitialClasses() {
  // Targets the previous, current, and next items
  // This assumes there are at least three items.
  items[totalItems - 1].classList.add("prev");
  items[0].classList.add("active");
  items[1].classList.add("next");
}

To understand better I initially assume only 3 slides. In that case:

items[totalItems - 1] → This will be items[2].
So why on the third item we are putting calls prev

Currently, only the first one(items[0]) is visible.