Javascript — Is in viewport

is-in-viewport.js

function isInViewport (el) {
  var rect = el.getBoundingClientRect();

  return (
    rect.left >= 0 && rect.right <= (window.innerWidth || document.documentElement.clientWidth) ||
    rect.left < 0 && rect.right >= 0 ||
    rect.left >= 0 && rect.left < (window.innerWidth || document.documentElement.clientWidth) ||
    rect.top >= 0 && rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) ||
    rect.top < 0 && rect.bottom >= 0 ||
    rect.top >= 0 && rect.top < (window.innerHeight || document.documentElement.clientHeight)
  );
}

isInViewport(document.querySelector('.element'))