Слайдер на JavaScript

Попробуйте сделать вот так

var images = document.getElementsByClassName("photos");
var next = document.getElementById("next");
var prev = document.getElementById("prev");
var i = 0;
var length = images.length;

next.addEventListener("click", moveRight);
function moveRight() {
    if(i == length-1){
            images[0].style.display = "block";
            images[i].style.display = "none";
            i = 0;
        } else{
            images[i+1].style.display = "block";
            images[i].style.display = "none";
            i++;
        }
}
prev.addEventListener("click", moveLeft);
function moveLeft() {
    if(i == 0){
        images[length-1].style.display = "block";
        images[i].style.display = "none";
        i = length-1;
    } else{
        images[i-1].style.display = "block";
        images[i].style.display = "none";
        i--;
    }