Simple document.activeElement observation

ActiveElementObserver.js

export default function ActiveElementObserver (handler = function () {}) {
  let prev;
  let raf;

  this.observe = _ => {
    cancelAnimationFrame(raf);
    raf = undefined;
    if (document.activeElement !== prev) {
      handler({ activeElement: document.activeElement, prev });
      prev = document.activeElement;
    }
    if (raf === false)
      return;
    raf = requestAnimationFrame(this.observe);
  };

  this.disconnect = _ => {
    cancelAnimationFrame(raf);
    raf = false;
  };
}