IntersectionObserver setup 4

intersection-observer-setup-4.js

  function observe(me) { // observe new element, me
    if (!me) {
      return;
    }
 
    const index = me.getAttribute('data-idx');
 
    if (index == indexToObserve) {
      elementToObserve = me;
      intersectionObserver.observe(me);
      console.info('elementToObserve', index);
    } else {
      console.log('observe(' + index + ' !== ' + indexToObserve + ')');
    }
  }
 
  function unobserve() { // unobserve current element
    if (elementToObserve) {
      intersectionObserver.unobserve(elementToObserve);
      intersectionRatio = undefined;
      console.info('unobserve [' + indexToObserve + ']');
    } else {
      console.log('null elementToObserve');
    }
  }