Smooth scroll anchor links

smoothScroll.JS

// Standard Smooth scroll JS - I know kinda verbose D.S.
(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);
        });
    }
})();
// <a class="scroll" data-speed="600" href="#register">Register</a>