ARIA application role test (Javascript)

const DOM_VK_LEFT = 37;
const DOM_VK_RIGHT = 39;
var fruitItemsCount = 0;

function onBodyLoad() {
    var fruitsElem = document.getElementById("fruits");
    fruitsElem.addEventListener("focus", function(e) { onFruitsFocus(e); }, false);
    fruitsElem.addEventListener("keydown", function(e) { onFruitsKeyDown(e); }, false);
    fruitItemsCount = fruitsElem.getElementsByTagName("li").length;
}

function onFruitsFocus(event) {
    var elem = event.target;
    var activeId = elem.getAttribute("aria-activedescendant");
    var active = document.getElementById(activeId);
    active.focus();
}

function onFruitsKeyDown(event) {
    var elem = event.currentTarget;
    var active = elem.getAttribute("aria-activedescendant");
    var fruitNum = active;
    event.preventDefault();
    if (event.keyCode === DOM_VK_RIGHT) {
        ++fruitNum;
        if (fruitNum > fruitItemsCount) {
            fruitNum = 1;
        }
    } else if (event.keyCode === DOM_VK_LEFT) {
        --fruitNum;
        if (fruitNum < 1) {
            fruitNum = fruitItemsCount;
        }
    }
    if (fruitNum != active) {
        var currItem = document.getElementById(fruitNum);
        document.getElementById(active).className = "item";
        currItem.className = "selectedItem";
        elem.setAttribute("aria-activedescendant", fruitNum);
        currItem.focus();
    }
}