Use vanilla javascript to add / remove class to a different element after clicking on another one

document.addEventListener("DOMContentLoaded", function() {
  var faqContainers = document.getElementsByClassName('faq-container');
  var faqToggle = document.getElementsByTagName('body')[0];
  for (var i = 0; i < faqContainers.length; i++) {

    faqContainers[i].addEventListener('click', function() {

      if (faqToggle.classList.contains('faq-display')) {
        faqToggle.classList.remove('faq-display');
        // alert("remove faq display!");
      } else {
        faqToggle.classList.add('faq-display');
        // alert("add faq display!");
      }

    });
  }


});

You need to loop:

[...document.querySelectorAll('.faq-container')]
  .forEach(faq => faq.addEventListener('click', () => 
  document.querySelector('body').classList.toggle('faq-display')))

---

NOTE: if you have MANY of these divs, you are better off having one event handler for a parent and test what was clicked.

document.getElementById('faqContainerParent')
  .addEventListener('click', e => {
  const tgt = e.target;
  if (tgt.classList.contains("faq-container") || tgt.closest("div.faq-container")) { // is it the faq-container or one of the decendants
    document.querySelector('body').classList.toggle('faq-display')
  }
});