web/_javascript/commons/back-to-top.js

/**
 * Reference: https://bootsnipp.com/snippets/featured/link-to-top-page
 */
$(function() {
    $(window).on('scroll',() => {
        if ($(this).scrollTop() > 50 &&
            $("#sidebar-trigger").css("display") === "none") {
            $("#back-to-top").fadeIn();
        } else {
            $("#back-to-top").fadeOut();
        }
    });

    $("#back-to-top").on('click',() => {
        $("body,html").animate({
            scrollTop: 0
        }, 800);
        return false;
    });
});