Medium fade on scroll effect

gistfile1.txt

/*
    dom nodes
*/
var $btn;

$(document).ready(function () {
    $btn = $('.js-btn');

    fadeFromView({
        element: $btn,
        fadeStart: -200,
        fadeEnd: -100
    });

});

function fadeFromView(options) {
    /*
        fadeStart is relative to the top of the element, e.g:-
            0 = the top of the element
            -100 = 100px above the top of the element
            100 = 100px below the top of the element

        fadeEnd is relative to the bottom of the element, e.g:-
            0 = the bottom of the element
            -100 = 100px above the bottom of the element
            100 = 100px below the bottom of the element
    */

    var defaults = {
            element: null,
            padTop: 0,
            padBottom: 0,
            fadeStart: 0,
            fadeEnd: 0
        };

    var opt = $.extend({}, defaults, options);

    var elHeight = opt.element.outerHeight(),
        elTop = opt.element.offset().top,
        trackStart = elTop + opt.fadeStart,
        trackEnd = elTop + elHeight + opt.fadeEnd,
        trackLength = trackEnd - trackStart;

    $(document).scrollspy({
        onTick: function(element, position, inside, enters, leaves) {
            var opacity,
                trackPos;

            if (position.top < trackStart) {
                /*
                    scroll position is above the start of the track, element should be fully visible
                */
                opt.element.css('opacity', 100);
            } else if (position.top > trackEnd) {
                /*
                    scroll position is below the end of the track, element should be fully hidden
                */
                opt.element.css('opacity', 0);
            } else {
                /*
                    scroll position is within the track, fade the element based on it's position within the track
                */
                trackPos = position.top - trackStart;
                opacity = 1 - (trackPos / trackLength);
                opt.element.css('opacity', opacity);
            }

        }
    });
}