Performant, 60FPS smooth scrolling in Vanilla JavaScript using requestAnimationFrame

smooth-scrolling.js

/**
 * @param {number} yPos Pixels from the top of the screen to scroll to
 * @param {number} duration Time of animation in milliseconds
 */
const scrollTo = (yPos, duration = 600) => {
  const startY = window.scrollY;
  const difference = yPos - startY;
  const startTime = performance.now();

  const step = () => {
    const progress = (performance.now() - startTime) / duration;
    const amount = easeOutCubic(progress);
    window.scrollTo({ top: startY + amount * difference });
    if (progress < 0.99) {
      window.requestAnimationFrame(step);
    }
  };

  step();
}

// Easing function from https://gist.github.com/gre/1650294
const easeOutCubic = t => --t * t * t + 1;

Adapted this slightly, mainly to be promise based. Have found it to be a reliable way to know when the scroll has finished:

export const smoothScrollTo = (
  y: number,
  {duration = 400, offset = 0} = {}
) => {
  const easeOutCubic = (t: number) => --t * t * t + 1;
  const startY = window.scrollY;
  const difference = y - startY;
  const startTime = performance.now();

  if (y === startY + offset) {
    return Promise.resolve(undefined);
  }

  return new Promise((resolve) => {
    const step = () => {
      const progress = (performance.now() - startTime) / duration;
      const amount = easeOutCubic(progress);
      window.scrollTo({top: startY + amount * difference - offset});
      if (progress < 0.99) {
        window.requestAnimationFrame(step);
      } else {
        resolve(undefined);
      }
    };
    step();
  });
};