[рони]

АртемБМ,

let setIndex = up => i = (i + up + sliderItem.length) % sliderItem.length;

buttonRight.addEventListener("click", function () {
    sliderItem[i].classList.remove("block");
    setIndex(1);
    sliderItem[i].classList.add("block")
})
buttonLeft.addEventListener("click", function () {
    sliderItem[i].classList.remove("block");
    setIndex(-1);
    sliderItem[i].classList.add("block")
})