Simple smooth scroll Javascript

smoothscroll.js

// Smoothscroll
function ssc_init() {
    if (!document.body) return;
    var e = document.body;
    var t = document.documentElement;
    var n = window.innerHeight;
    var r = e.scrollHeight;
    ssc_root = document.compatMode.indexOf("CSS") >= 0 ? t : e;
    ssc_activeElement = e;
    ssc_initdone = true;
    if (top != self) {
        ssc_frame = true;
    }
    else if (r > n && (e.offsetHeight <= n || t.offsetHeight <= n)) {
        ssc_root.style.height = "auto";
        if (ssc_root.offsetHeight <= n) {
            var i = document.createElement("div");
            i.style.clear = "both";
            e.appendChild(i);
        }
    }
    if (!ssc_fixedback) {
        e.style.backgroundAttachment = "scroll";
        t.style.backgroundAttachment = "scroll";
    }
    if (ssc_keyboardsupport) {
        ssc_addEvent("keydown", ssc_keydown);
    }
}

function ssc_scrollArray(e, t, n, r) {
    r || (r = 1e3);
    ssc_directionCheck(t, n);
    ssc_que.push({
        x: t,
        y: n,
        lastX: t < 0 ? .99 : -.99,
        lastY: n < 0 ? .99 : -.99,
        start: +(new Date)
    });
    if (ssc_pending) {
        return;
    }
    var i = function () {
        var s = +(new Date);
        var o = 0;
        var u = 0;
        for (var a = 0; a < ssc_que.length; a++) {
            var f = ssc_que[a];
            var l = s - f.start;
            var c = l >= ssc_animtime;
            var h = c ? 1 : l / ssc_animtime;
            if (ssc_pulseAlgorithm) {
                h = ssc_pulse(h);
            }
            var p = f.x * h - f.lastX >> 0;
            var d = f.y * h - f.lastY >> 0;
            o += p;
            u += d;
            f.lastX += p;
            f.lastY += d;
            if (c) {
                ssc_que.splice(a, 1);
                a--;
            }
        }
        if (t) {
            var v = e.scrollLeft;
            e.scrollLeft += o;
            if (o && e.scrollLeft === v) {
                t = 0;
            }
        }
        if (n) {
            var m = e.scrollTop;
            e.scrollTop += u;
            if (u && e.scrollTop === m) {
                n = 0;
            }
        }
        if (!t && !n) {
            ssc_que = [];
        }
        if (ssc_que.length) {
            setTimeout(i, r / ssc_framerate + 1);
        }
        else {
            ssc_pending = false;
        }
    };
    setTimeout(i, 0);
    ssc_pending = true;
}

function ssc_wheel(e) {
    if (!ssc_initdone) {
        init();
    }
    var t = e.target;
    var n = ssc_overflowingAncestor(t);
    if (!n || e.defaultPrevented || ssc_isNodeName(ssc_activeElement, "embed") || ssc_isNodeName(t, "embed") && /\.pdf/i.test(t.src)) {
        return true;
    }
    var r = e.wheelDeltaX || 0;
    var i = e.wheelDeltaY || 0;
    if (!r && !i) {
        i = e.wheelDelta || 0;
    }
    if (Math.abs(r) > 1.2) {
        r *= ssc_stepsize / 120;
    }
    if (Math.abs(i) > 1.2) {
        i *= ssc_stepsize / 120;
    }
    ssc_scrollArray(n, -r, -i);
    e.preventDefault();
}

function ssc_keydown(e) {
    var t = e.target;
    var n = e.ctrlKey || e.altKey || e.metaKey;
    if (/input|textarea|embed/i.test(t.nodeName) || t.isContentEditable || e.defaultPrevented || n) {
        return true;
    }
    if (ssc_isNodeName(t, "button") && e.keyCode === ssc_key.spacebar) {
        return true;
    }
    var r, i = 0,
        s = 0;
    var o = ssc_overflowingAncestor(ssc_activeElement);
    var u = o.clientHeight;
    if (o == document.body) {
        u = window.innerHeight;
    }
    switch (e.keyCode) {
    case ssc_key.up:
        s = -ssc_arrowscroll;
        break;
    case ssc_key.down:
        s = ssc_arrowscroll;
        break;
    case ssc_key.spacebar:
        r = e.shiftKey ? 1 : -1;
        s = -r * u * .9;
        break;
    case ssc_key.pageup:
        s = -u * .9;
        break;
    case ssc_key.pagedown:
        s = u * .9;
        break;
    case ssc_key.home:
        s = -o.scrollTop;
        break;
    case ssc_key.end:
        var a = o.scrollHeight - o.scrollTop - u;
        s = a > 0 ? a + 10 : 0;
        break;
    case ssc_key.left:
        i = -ssc_arrowscroll;
        break;
    case ssc_key.right:
        i = ssc_arrowscroll;
        break;
    default:
        return true;
    }
    ssc_scrollArray(o, i, s);
    e.preventDefault();
}

function ssc_mousedown(e) {
    ssc_activeElement = e.target;
}

function ssc_setCache(e, t) {
    for (var n = e.length; n--;) ssc_cache[ssc_uniqueID(e[n])] = t;
    return t;
}

function ssc_overflowingAncestor(e) {
    var t = [];
    var n = ssc_root.scrollHeight;
    do {
        var r = ssc_cache[ssc_uniqueID(e)];
        if (r) {
            return ssc_setCache(t, r);
        }
        t.push(e);
        if (n === e.scrollHeight) {
            if (!ssc_frame || ssc_root.clientHeight + 10 < n) {
                return ssc_setCache(t, document.body);
            }
        }
        else if (e.clientHeight + 10 < e.scrollHeight) {
            overflow = getComputedStyle(e, "").getPropertyValue("overflow");
            if (overflow === "scroll" || overflow === "auto") {
                return ssc_setCache(t, e);
            }
        }
    } while (e = e.parentNode);
}

function ssc_addEvent(e, t, n) {
    window.addEventListener(e, t, n || false);
}

function ssc_removeEvent(e, t, n) {
    window.removeEventListener(e, t, n || false);
}

function ssc_isNodeName(e, t) {
    return e.nodeName.toLowerCase() === t.toLowerCase();
}

function ssc_directionCheck(e, t) {
    e = e > 0 ? 1 : -1;
    t = t > 0 ? 1 : -1;
    if (ssc_direction.x !== e || ssc_direction.y !== t) {
        ssc_direction.x = e;
        ssc_direction.y = t;
        ssc_que = [];
    }
}

function ssc_pulse_(e) {
    var t, n, r;
    e = e * ssc_pulseScale;
    if (e < 1) {
        t = e - (1 - Math.exp(-e));
    }
    else {
        n = Math.exp(-1);
        e -= 1;
        r = 1 - Math.exp(-e);
        t = n + r * (1 - n);
    }
    return t * ssc_pulseNormalize;
}

function ssc_pulse(e) {
    if (e >= 1) return 1;
    if (e <= 0) return 0;
    if (ssc_pulseNormalize == 1) {
        ssc_pulseNormalize /= ssc_pulse_(1);
    }
    return ssc_pulse_(e);
}
var ssc_framerate = 150;
var ssc_animtime = 500;
var ssc_stepsize = 150;
var ssc_pulseAlgorithm = true;
var ssc_pulseScale = 6;
var ssc_pulseNormalize = 1;
var ssc_keyboardsupport = true;
var ssc_arrowscroll = 50;
var ssc_frame = false;
var ssc_direction = {
    x: 0,
    y: 0
};
var ssc_initdone = false;
var ssc_fixedback = true;
var ssc_root = document.documentElement;
var ssc_activeElement;
var ssc_key = {
    left: 37,
    up: 38,
    right: 39,
    down: 40,
    spacebar: 32,
    pageup: 33,
    pagedown: 34,
    end: 35,
    home: 36
};
var ssc_que = [];
var ssc_pending = false;
var ssc_cache = {};
setInterval(function () {
    ssc_cache = {};
}, 10 * 1e3);
var ssc_uniqueID = function () {
    var e = 0;
    return function (t) {
        return t.ssc_uniqueID || (t.ssc_uniqueID = e++);
    };
}();
function detectIE() {
  var ua = window.navigator.userAgent;

  // Test values; Uncomment to check result …

  // IE 10
  // ua = 'Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; Trident/6.0)';
  
  // IE 11
  // ua = 'Mozilla/5.0 (Windows NT 6.3; Trident/7.0; rv:11.0) like Gecko';
  
  // Edge 12 (Spartan)
  // ua = 'Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.71 Safari/537.36 Edge/12.0';
  
  // Edge 13
  // ua = 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2486.0 Safari/537.36 Edge/13.10586';

  var msie = ua.indexOf('MSIE ');
  if (msie > 0) {
    // IE 10 or older => return version number
    return parseInt(ua.substring(msie + 5, ua.indexOf('.', msie)), 10);
  }

  var trident = ua.indexOf('Trident/');
  if (trident > 0) {
    // IE 11 => return version number
    var rv = ua.indexOf('rv:');
    return parseInt(ua.substring(rv + 3, ua.indexOf('.', rv)), 10);
  }

  var edge = ua.indexOf('Edge/');
  if (edge > 0) {
    // Edge (IE 12+) => return version number
    return parseInt(ua.substring(edge + 5, ua.indexOf('.', edge)), 10);
  }

  // other browser
  return false;
}
var ie_browser = detectIE();
var isSmoothScrollSupported = 'scrollBehavior' in document.documentElement.style;
if( (!ie_browser || ie_browser > 11) && !isSmoothScrollSupported ){
	ssc_addEvent("mousedown", ssc_mousedown);
	ssc_addEvent("mousewheel", ssc_wheel);
	ssc_addEvent("load", ssc_init);
}

Forked and updated to only load when the browser doesn't natively support smooth scrolling.