scrollToAnchor.js

/*! ScrollToAnchor.js v1.1,0 | Paul Browne | 2015 | GNU 2.0  */
(function() {
    var bod = document.getElementsByTagName("body")[0];
    var FIREFOX = /Firefox/i.test(navigator.userAgent);
    if (FIREFOX) {
        bod = document.getElementsByTagName("html")[0];
    }

    function animate(opts) {
        var start = new Date(),
            id = setInterval(function() {
                var timePassed = new Date() - start,
                    progress = timePassed / opts.duration,
                    delta = opts.delta(progress);
                if (progress > 1) {
                    clearInterval(id);
                }
                window.addEventListener('click', function() {
                    clearInterval(id);
                });
                opts.step(delta);
            }, 1);
    }

    function move(duration, too) {
        animate({
            duration: duration || 1200,
            delta: quad,
            step: function(quad) {
                bod.scrollTop = bod.scrollTop + (too - bod.scrollTop) * quad;
            }
        });
    }

    function quad(progress) {
        return Math.pow(progress, 2);
    }

    function boo(evt) {
        var thehref = this.getAttribute('href').slice(1),
            idofhref = document.getElementById(thehref),
            disttotop = idofhref.offsetTop,
            length = Math.abs(disttotop - bod.scrollTop),
            timing;
        if (Math.abs(length) < 500) {
            timing = 1000;
        }
        if (Math.abs(length) > 2500) {
            timing = 2500;
        } else {
            timing = length;
        }
        move(timing, disttotop);
        evt.preventDefault();
    }

    var elem = document.getElementsByTagName("a");
    for (var i = 0; i < elem.length; ++i) {
        var href = elem[i].getAttribute('href');
        if (href.indexOf("#") === 0) {
            elem[i].addEventListener('click', boo);
        }
    }
})();