Infinite scroll Svelte component that appends DOM elements in increments of page sizes as the user scrolls beyond a threshold

InfiniteScroll.svelte

<!--
    InfiniteScroll.svelte
    
    Author: Tom Barrasso
    
    Licensed to the Apache Software Foundation (ASF) under one
    or more contributor license agreements.  See the NOTICE file
    distributed with this work for additional information
    regarding copyright ownership.  The ASF licenses this file
    to you under the Apache License, Version 2.0 (the
    "License"); you may not use this file except in compliance
    with the License.  You may obtain a copy of the License at

      http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing,
    software distributed under the License is distributed on an
    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    KIND, either express or implied.  See the License for the
    specific language governing permissions and limitations
    under the License.    
-->

<!--
    // Sample Usage

    <script>
      const THRESHOLD = 100; // px

      function incrementPage() {
        if (pagedItems.length < items.length) {
          dispatch('page', { page: ++page });
        }
      }
    </script>

    <ol class="list">

      {#each pagedItems as subitem, i}
        <li class="list-item">
          <slot {subitem} {i} />
        </li>
      {/each}

    </ol>

    <InfiniteScroll threshold={THRESHOLD} on:loadMore={incrementPage} />
-->

<script>
  // Modified from: https://github.com/andrelmlins/svelte-infinite-scroll
  import { onMount, onDestroy, createEventDispatcher } from "svelte";

  export let threshold = 0;
  export let hasMore = true;

  const dispatch = createEventDispatcher();

  function supportsListenerOption(attr) {
    let optionSupported = false;

    try {
      const options = {
        get [attr]() {
          optionSupported = true;
          return false;
        }
      };

      window.addEventListener("test", null, options);
      window.removeEventListener("test", null, options);
    } catch (err) {
      optionSupported = false;
    }

    return optionSupported;
  }

  let isLoadMore = false;
  const supportsPassive = supportsListenerOption("passive");

  onMount(() => {
    requestAnimationFrame(() => {
      const element = getElement();
      const options = (supportsPassive) ? { passive: true } : false;
      element.addEventListener("scroll", onScroll, options);
      element.addEventListener("resize", onScroll, options);
    });
  });

  const onScroll = (e) => {
    if (!hasMore) return;
    const offset = calcOffset(e);
    if (offset <= threshold) {
      if (!isLoadMore && hasMore) {
        dispatch("loadMore");
      }
      isLoadMore = true;
    } else {
      isLoadMore = false;
    }
  };

  const calcOffset = (e, reverse = false, horizontal = false) => {
    const element = e.target.documentElement
      ? e.target.documentElement
      : e.target;
    if (reverse) {
      return horizontal ? element.scrollLeft : element.scrollTop;
    } else {
      return horizontal
        ? element.scrollWidth - element.clientWidth - element.scrollLeft
        : element.scrollHeight - element.clientHeight - element.scrollTop;
    }
  };

  const getElement = () =>
    (document.getElementById('viewList') || document.body);

  onDestroy(() => {
    const element = getElement();
    if (element) {
      element.removeEventListener("scroll", onScroll);
      element.removeEventListener("resize", onScroll);
    }
  });
</script>