function hideAll() {
    var navList = document.getElementById("nav");
    for (var i=0; i<navList.childNodes.length; i++) {
        var node = navList.childNodes[i];
        if (node.tagName == "LI") {
            node.className = node.className.replace(new RegExp("\s?active", "i"), "");
        }
    }
}

window.onload = function () {
    var navList = document.getElementById("nav");
    for (var i=0; i<navList.childNodes.length; i++) {
        var node = navList.childNodes[i];
        if (node.tagName == "LI") {
            node.onclick = function() {
                hideAll();
                this.className += " active";
            }
        }
    }
}