JAVASCRIPT

addclass js vanilla

  element.classList.add('className');

vanilla javascript add class

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')
      } else {
        faqToggle.classList.add('faq-display')
      }
    })
  }
})