问仅在向上滚动时使用FIxed导航栏

EN

.nav-up
    top -80px
    background-color #8fd0f3
.nav-down
    top 0
.navbar-wrapper
    position fixed
    // top -7px
    top 37.5px
    // -webkit-transform translateY(37.5px)
    z-index 100
    width 100%
    padding-top 17.5px
    transition top 0.2s ease-in-out
    padding-bottom 17.5px
    .navbar-elements
        width 960px
        margin 0 auto
        position relative
        .logo
            cursor pointer
        .nav-buttons
            text-transform uppercase
            position absolute
            right 0
            top 0
            display inline-block
        .nav-btn    
            display inline-block
            vertical-align middle
            border-radius 4px
            padding 12px 21px 12px 21px
            font-family 'Lato'
            cursor pointer
            // transition opacity 0.3s ease
        .breeder-btn
            background-color rgba(255, 255, 255, 1)
            margin-left 10px
            transition background-color 0.2s ease
            // margin-right 136px
        .breeder-btn:hover
            background-color rgba(255, 255, 255, 0.85)
        .guarantee-btn
            background-color rgba(17, 35, 114, 1)
            color white
            transition background-color 0.2s ease
        .guarantee-btn:hover
            background-color rgba(17,35,114, 0.85)

<div class="navbar-wrapper nav-down">
            <div class="navbar-elements">
                <img src="../img/logo.png" class="logo">
                <div class="nav-buttons">
                    <div class="nav-btn guarantee-btn">
                        our guarantee           
                    </div>
                    <div class="nav-btn breeder-btn">   
                        new breeder sign-up     
                    </div>
                </div>              
            </div>
        </div>

// Hide Header on on scroll 
function HeaderScroll() {
    var delta = 5;
    var navbarHeight = $('.navbar-wrapper').outerHeight();
    console.log(navbarHeight);

    $(window).scroll(function(event){
         hasScrolled()
    });

    setInterval(function() {
        // if (didScroll) {
            hasScrolled();
            // console.log(didScroll);
        // }
    }, 5);
    hasScrolled();
}

 function hasScrolled() {
    var delta = 5;
     var navbarHeight = $('.navbar-wrapper').outerHeight();
    var position = $(window).scrollTop();
    // console.log($(this))
    // console.log(st)
    // Make sure they scroll more than delta
    if(Math.abs(position) <= delta)
        return;

    // If they scrolled down and are past the navbar, add class .nav-up.
    // This is necessary so you never see what is "behind" the navbar.
    if (position > navbarHeight){
        // Scroll Down
        $('.navbar-wrapper').removeClass('nav-down').addClass('nav-up');
    } else {
        // Scroll Up
        if(position + $(window).height() < $(document).height()) {
            // console.log("Scroll up")
            $('.navbar-wrapper').removeClass('nav-up').addClass('nav-down');
        }
    }
}