Building a Back to Top button with IntersectionObserver and Svelte

main.js
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import BackToTop from './components/BackToTop.svelte'

// Back to Top button
if ('IntersectionObserver' in window) {
  new BackToTop({
    target: document.querySelector('#backtotop'),
    props: {
      track: document.querySelector("#search")
    }
  })
}
BackToTop.svelte
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<script>
  import { fade } from 'svelte/transition'
  export let track

  let visible = false

  new IntersectionObserver(entries => {
    entries[0].intersectionRatio > 0 ?
      visible = false :
      visible = true
  }).observe(track)

  function scrollUp() {
    document.body.scrollIntoView({behavior: "smooth"})
  }
</script>

{#if visible}
  <button
    transition:fade
    on:click={scrollUp}
    class="btn bg-orange-700 hover:bg-orange-800 hover:shadow-lg text-white fixed bottom-0 right-0 mb-3 mr-3"
  >
    Back to Top
  </button>
{/if}