js/smooth-scroll.js

/* =============================================================

    Smooth Scroll 2.3
    Animate scrolling to anchor links, by Chris Ferdinandi.
    http://gomakethings.com

    Free to use under the MIT License.
    http://gomakethings.com/mit/
    
 * ============================================================= */

 (function() {

     'use strict';

    // Feature Test
    if ( 'querySelector' in document && 'addEventListener' in window && Array.prototype.forEach ) {

        // Function to animate the scroll
        var smoothScroll = function (anchor, duration) {

            // Calculate how far and how fast to scroll
            var startLocation = window.pageYOffset;
            var endLocation = anchor.offsetTop;
            var distance = endLocation - startLocation;
            var increments = distance/(duration/16);
            var stopAnimation;

            // Scroll the page by an increment, and check if it's time to stop
            var animateScroll = function () {
                window.scrollBy(0, increments);
                stopAnimation();
            };

            // If scrolling down
            if ( increments >= 0 ) {
                // Stop animation when you reach the anchor OR the bottom of the page
                stopAnimation = function () {
                    var travelled = window.pageYOffset;
                    if ( (travelled >= (endLocation - increments)) || ((window.innerHeight + travelled) >= document.body.offsetHeight) ) {
                        clearInterval(runAnimation);
                    }
                };
            }
            // If scrolling up
            else {
                // Stop animation when you reach the anchor OR the top of the page
                stopAnimation = function () {
                    var travelled = window.pageYOffset;
                    if ( travelled <= (endLocation || 0) ) {
                        clearInterval(runAnimation);
                    }
                };
            }

            // Loop the animation function
            var runAnimation = setInterval(animateScroll, 16);
       
        };

        // Define smooth scroll links
        var scrollToggle = document.querySelectorAll('.scroll');

        // For each smooth scroll link
        [].forEach.call(scrollToggle, function (toggle) {

            // When the smooth scroll link is clicked
            toggle.addEventListener('click', function(e) {

                // Prevent the default link behavior
                e.preventDefault();

                // Get anchor link and calculate distance from the top
                var dataID = toggle.getAttribute('href');
                var dataTarget = document.querySelector(dataID);
                var dataSpeed = toggle.getAttribute('data-speed');

                // If the anchor exists
                if (dataTarget) {
                    // Scroll to the anchor
                    smoothScroll(dataTarget, dataSpeed || 500);
                }

            }, false);

        });

    }

 })();