Affix is not working in Bootstrap 4 (alpha)

Question

According to Bootstrap 3 docs I have added following attributes to a navbar:

Answer 1

To build on Anwar Hussain's answer. I found success with this:

$(window).on('scroll', function (event) {
    var scrollValue = $(window).scrollTop();
    if (scrollValue > 120) {
        $('.navbar').addClass('affix');
    } else{
        $('.navbar').removeClass('affix');
    }
});

This will apply the class to the navbar when scrolling down, but will also remove the class when scrolling back up. Previously, when scrolling back up, the applied class would stay applied to the navbar.