Very simple ES6 JavaScript background-position parallax function using requestAnimationFrame

simpe-parallax.js

export default function attachParallax(elm) {
  document.addEventListener('mousemove', (e) => {
    window.requestAnimationFrame(() => {
      const left = (e.clientX / window.innerWidth) * 5;
      const top = (e.clientY / (window.innerHeight)) * 5;
      elm.style.backgroundPosition = `${47.5 + left}% ${47.5 + top}%`;
    });
  }, false);
}

/*
Usage:
attachParallax(document.querySelector('div.mydiv'));

Also needs the following CSS-variables:
background-size: 110% auto;
background-image: url('myfile.png');
background-position: 50% 50%;
*/