Scroll Animator using requestAnimationFrame

animateScrollTo.js

// Add requestAnimationFrame shim
// http://www.paulirish.com/2011/requestanimationframe-for-smart-animating/
window._animate = (function() {
  return (
    window.requestAnimationFrame ||
    window.webkitRequestAnimationFrame ||
    window.mozRequestAnimationFrame ||
    function(callback) {
      window.setTimeout(callback, 1000 / 60);
    }
  );
})();

// The main function
export function animateScrollTo({
  context = window, // what you're trying to scroll, defaults to "window"
  scrollTo = 0, // where you want to scroll to
  speed = 2000, // how fast you want to scroll
  easing = "easeInOutSine" // the animation easing
}) {
  const scrollY = context.scrollY || context.scrollTop;
  let currentTime = 0;
  const scrollTargetY = scrollTo || 0;

  // min time .1, max time .8 seconds
  let time = Math.max(
    0.1,
    Math.min(Math.abs(scrollY - scrollTargetY) / speed, 0.8)
  );

  // easing equations from https://github.com/danro/easing-js/blob/master/easing.js
  const easingEquations = {
    easeOutSine(pos) {
      return Math.sin(pos * (Math.PI / 2));
    },
    easeInOutSine(pos) {
      return -0.5 * (Math.cos(Math.PI * pos) - 1);
    },
    easeInOutQuint(pos) {
      if ((pos /= 0.5) < 1) return 0.5 * Math.pow(pos, 5);
      return 0.5 * (Math.pow(pos - 2, 5) + 2);
    }
  };

  // add animation loop
  function tick() {
    currentTime += 1 / 60;

    let p = currentTime / time;
    let t = easingEquations[easing](p);

    if (p < 1) {
      _animate(tick);
      context.scrollTo(0, scrollY + (scrollTargetY - scrollY) * t);
    } else {
      context.scrollTo(0, scrollTargetY);
    }
  }

  // call it once to get started
  tick();
}

export default animateScrollTo;