Using intersectionObserver to display an element when NOT in viewport

inView.js

// setting up options for IntersectionObserver
// setting threshold to .9, in this instance, means when 90% of the element is out of view
let options = {
  root: null,
  threshold: [.9]
}

// function with ternary operator to check if threshold is met then display none but if not then update display
function showToATC(entryArray){
   let div = document.getElementById('scroll-to-atc');
   entryArray[0].isIntersecting ? div.style.display = 'none' : div.style.display = "block";
 }

// initialize the function
 let observer = new IntersectionObserver(showToATC, options);
 observer.observe(document.getElementById('atc-btn'));

// credit - handy video: https://www.youtube.com/watch?v=orP9zwnE6PA