intersection-observer-polyfill.js

loadPolyfills()
  .then(() => /* Render React application now that your Polyfills are ready */)

/**
* Do feature detection, to figure out which polyfills needs to be imported.
**/
function loadPolyfills() {
  const polyfills = []

  if (!supportsIntersectionObserver()) {
    polyfills.push(import('intersection-observer'))
  }

  return Promise.all(polyfills)
}

function supportsIntersectionObserver() {
  return (
    'IntersectionObserver' in global &&
    'IntersectionObserverEntry' in global &&
    'intersectionRatio' in IntersectionObserverEntry.prototype
  )
}