Vanilla JS Sticky Nav Observer

watch-sticky.js

const stickyNav = document.querySelector('#site-navigation')
const mutateOptions = {
  attributes: true
}

function callback(mutationList, observer) {
  mutationList.forEach(function(mutation) {
    if (mutation.type === 'attributes' && mutation.attributeName === 'id') {
      // handle ID change
      if (mutation.target.id == 'sticky-navigation') {
          document.body.classList.add("nav-is-sticky")
      } else {
          document.body.classList.remove("nav-is-sticky")
      }
    }
  })
}

const observer = new MutationObserver(callback)
observer.observe(stickyNav, mutateOptions)