add class with JavaScript

Question

I am writing some vanilla JavaScript to create a nice navigation menu. I am stuck on adding an active class.

I am getting elements by class name NOT by id. The belo

Answer 1

Here is a method adapted from Jquery 2.1.1 that take a dom element instead of a jquery object (so jquery is not needed). Includes type checks and regex expressions:

function addClass(element, value) {
    // Regex terms
    var rclass = /[\t\r\n\f]/g,
        rnotwhite = (/\S+/g);

    var classes,
        cur,
        curClass,
        finalValue,
        proceed = typeof value === "string" && value;

    if (!proceed) return element;

    classes = (value || "").match(rnotwhite) || [];

    cur = element.nodeType === 1
        && (element.className
                ? (" " + element.className + " ").replace(rclass, " ")
                : " "
        );

    if (!cur) return element;

    var j = 0;

    while ((curClass = classes[j++])) {

        if (cur.indexOf(" " + curClass + " ") < 0) {

            cur += curClass + " ";

        }

    }

    // only assign if different to avoid unneeded rendering.
    finalValue = cur.trim();

    if (element.className !== finalValue) {

        element.className = finalValue;

    }

    return element;
};