dynamic-import.js

if ( 'IntersectionObserver' in window ) {
  const observer = new IntersectionObserver(callback, options)
  observer.observe($el)
} else {
  import(/* webpackChunkName: "intersection-observer" */ 'intersection-observer').then(_ => {
    const observer = new IntersectionObserver(callback, options)
    observer.observe($el)
  })
}