html element becomes visible

onVisible.js

function onVisible(el, once=true) {
  return new Promise((done, fail) => {
    new IntersectionObserver((entries, observer) => {
      entries.forEach(entry => {
        if (entry.intersectionRatio > 0) {
          done(el, observer, entry)
          if (once) {observer.disconnect()}
        }
      })
    }).observe(el)
  })
}