[Vuejs]-How to use Intersection Observer API immediately?

its better to be in mounted lifecycle hook because you are using refs, and that means onMounted in composition api. but the is in your callback and here it is:

    const ready = ref(false);
    const myRef = ref(null);

   onMounted(() => {
  const vm = getCurrentInstance();
  let options = {
   root: vm.$refs.myRef
   rootMargin: '0px',
   threshold: 1.0
  }

   let observer = new IntersectionObserver(([entry]) => {
      if (entry && entry.isIntersecting) {
        ready.value = true;
      }
    }, options);
     observer.observe(myRef.value)
 })

  return { ready, myRef }