Highlights any menu item which links to the current page and deactivates it. Use it by adding ROB.utils.setCurrentPageActive()

utilities.js

/**
 * @author Rob Davis
 * @licence CC-BY
 */
var ROB = ROB || {};
/**
 * create an IIFE which extends a 'ROB' module if it already exists and creates
 * a submodule called 'utils'.
 */
ROB.utils = (function (oldModule) {
    "use strict";
    const returnObject = oldModule;

    /**
     * Gives the current filename of the url or index.html if no name (this is less
     * than perfect since giving just a path might mean index.html, index.php for instance
     *
     * @method getCurrentPageName
     * @returns the filename of the current url or index.html if there is no filename
     */
    const getCurrentPageName = function () {
        let pagePathName= window.location.pathname;
        return pagePathName.substring(pagePathName.lastIndexOf("/") + 1) || 'index.html';
    };

    /**
     * changes the link of the current page to make it void add adds a class
     * called active to it, this is prevent a menu linking to the current page from
     * being doing anything.
     */
    returnObject.setCurrentPageActive = function (menuLinkParent, activeClassName) {
        menuLinkParent = menuLinkParent || 'nav';
        activeClassName = activeClassName || 'current';
        let pn = getCurrentPageName(),
            currentLink = document.querySelector(`${menuLinkParent} a[href="${pn}"]`);
        if (currentLink) {
            currentLink.setAttribute('href', 'javascript:void(0)');
            currentLink.classList.add(activeClassName);
        }
    };

    return returnObject;

}(ROB.utils || {}));

If you want a simple version, not inside an object wrapper using the extending module pattern, then you can use this:

/**
 * @author Rob Davis
 * @licence CC-BY
 */

/**
 * changes the link of the current page to make it void add adds a class
 * called active to it, this is prevent a menu linking to the current page from
 * being doing anything.
 */
function setCurrentPageActive(menuLinkParent, activeClassName) {
    const getCurrentPageName = function () {
        let pagePathName = window.location.pathname;
        return pagePathName.substring(pagePathName.lastIndexOf("/") + 1) || 'index.html';
    };

    menuLinkParent = menuLinkParent || 'nav';
    activeClassName = activeClassName || 'current';

    let pn = getCurrentPageName(),
        currentLink = document.querySelector(`${menuLinkParent} a[href="${pn}"]`);
    if (currentLink) {
        currentLink.setAttribute('href', 'javascript:void(0)');
        currentLink.classList.add(activeClassName);
    }
}

and then you can call it, perhaps like:

<script type="text/javascript" src="simpleHighlight.js"></script>
<script>
    window.addEventListener('load', function init () {
        setCurrentPageActive();
    })
</script>