Jquery Underline on scroll

Question

im wanting to experiment with a feature ive found on a pretty cool site but i have no clue where to start.

http://adamrudzki.com/

The feature is the underline element that moves across as the page scrolls down. I found a similar SO here Underlining menu item but if someone could help with the functionality im after id greatly appreciate it. Not too familiar with Jquery yet.

Thanks in advance !

Answer 1

On your example site each <a> tag there has a <span> element that serves as the underline. But I'm thinking maybe we can cut off the markup and use border-bottom instead. Basically there are two events that plays here - scroll() and click().

This is basic the markup:

<nav> 
    <a>Home</a>
    <a>About</a>
    <a>Portfolio</a>
    <a>Contact</a>
</nav>
<div id="contents">
    <section>Home</section>
    <section>About</section>
    <section>Portfolio</section>
    <section>Contact</section>
</div>

CSS, just would like to emphasize border:

a {
border:0 solid #FFF;
border-bottom-width:0;
}

jQuery scroll() event:

$(window).scroll(function () {
    //get the window scrollTop on scroll
    var st = $(window).scrollTop();
    /* we use each() to iterate on every section and 
      check if the offset position is in relative condition to the
      scrollTop value  
    */
    $('#contents section').each(function (index) {
        var offsetTop = $(this).offset().top,
            h = $(this).height();
        //this condition satisfies that this section is currently on the viewport
        if (st >= offsetTop && st < offsetTop + h) {
           /*find the nav <a> that has the same index to this section
           currently on the viewport and
           show its border-bottom by setting its width.
           */
            $('nav a').eq(index).css({
                'border-bottom-width': '3px'
            });
        } else {
            //hide the border-bottom
            $('nav a').eq(index).css({
                'border-bottom-width': '0'
            });
        }
    });
}).trigger('scroll');

nav <a> click() event:

$('nav a').click(function () {
    /* click has no index argument compared to each() function 
    so we have to get it with index() */
    var index = $(this).index(),
        $target = $('#contents section').eq(index); // find the target section
    //animate scrolling to the target section
    $('html, body').stop(true, true).animate({
        scrollTop: $target.offset().top
    }, 'slow');
});

Note that we are using index to target the exact <section>/<a>, this solution will work properly if <section> is arranged accordingly to nav <a> arrangement position.

See this sample jsfiddle.