scroll_to( element ) ES6

function scroll_to(element, duration = 500, easing = 'easeInOutQuart') {

  
  var start_pos = document.body.scrollTop,
      end_pos = element.offsetTop,
      distance = end_pos - start_pos,
      start_time = Date.now(),
      end_time = start_time + duration;
 
  var easingFunctions = {
    // https://gist.github.com/gre/1650294
    // no easing, no acceleration
    linear: function (t) { return t },
    // accelerating from zero velocity
    easeInQuad: function (t) { return t*t },
    // decelerating to zero velocity
    easeOutQuad: function (t) { return t*(2-t) },
    // acceleration until halfway, then deceleration
    easeInOutQuad: function (t) { return t<.5 ? 2*t*t : -1+(4-2*t)*t },
    // accelerating from zero velocity 
    easeInCubic: function (t) { return t*t*t },
    // decelerating to zero velocity 
    easeOutCubic: function (t) { return (--t)*t*t+1 },
    // acceleration until halfway, then deceleration 
    easeInOutCubic: function (t) { return t<.5 ? 4*t*t*t : (t-1)*(2*t-2)*(2*t-2)+1 },
    // accelerating from zero velocity 
    easeInQuart: function (t) { return t*t*t*t },
    // decelerating to zero velocity 
    easeOutQuart: function (t) { return 1-(--t)*t*t*t },
    // acceleration until halfway, then deceleration
    easeInOutQuart: function (t) { return t<.5 ? 8*t*t*t*t : 1-8*(--t)*t*t*t },
    // accelerating from zero velocity
    easeInQuint: function (t) { return t*t*t*t*t },
    // decelerating to zero velocity
    easeOutQuint: function (t) { return 1+(--t)*t*t*t*t },
    // acceleration until halfway, then deceleration 
    easeInOutQuint: function (t) { return t<.5 ? 16*t*t*t*t*t : 1+16*(--t)*t*t*t*t }
  }
  
  var requestAnimFrame = (function(){
    return  window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || function( callback ){ window.setTimeout(callback, 1000 / 60); };
  })();
  
  var animate_frame = function() {

    var now = Date.now(),
      percent = easingFunctions[easing]( (now - start_time) / duration ),
      new_top = start_pos + (distance * percent);
    
    document.body.scrollTop = new_top;
    
    if (now >= end_time) {
      document.body.scrollTop = end_pos;
    } else {
      requestAnimFrame(animate_frame);
    }
  };
  
  animate_frame();
  
}