What causes this failure to detect scrolling to the bottom of an HTML element in pure JavaScript?

Question

I am working on a pure JavaScript infinite scroll.

I have a 4 list of posts. I am trying to load more posts (clones of the "original" ones) when I reach the bottom of the container.

class InfiniteScroll {
  constructor() {
    this.observer = null;
    this.isLoading = false;
    this.postsContainer = document.querySelector('#postsContainer');
    this.postsArary = postsContainer.querySelectorAll('.post');
    this.hasNextPage = this.postsContainer?.dataset?.hasNextPage === 'true';
    this.currentPage = this.postsContainer?.dataset?.currentPage;
    this.nextPage = this.hasNextPage ? this.currentPage + 1 : null;
  }

  loadMorePosts() {
    if (this.hasNextPage) {
      this.postsArary.forEach(item => {
         let postClone = item.cloneNode(true);
         this.postsContainer.appendChild(postClone);
      });
    }
  }

  bindLoadMoreObserver() {
    if (this.postsContainer) {
      this.observer = new IntersectionObserver((entries, observer) => {
        entries.forEach(entry => {
          if (entry && entry.isIntersecting) {
            this.loadMorePosts();
          }
        });
      });

      this.observer.observe(this.postsContainer);
    }
  }

  init() {
    this.bindLoadMoreObserver();
  }
}

const infiniteScroll = new InfiniteScroll();
infiniteScroll.init();

body, body * {
  margin: 0;
  padding: 0;
}

body {
  font-family: Arial, Helvetica, sans-serif;
}

.post {
  margin: 20px;
  padding: 15px;
  border: 1px solid #ccc;
  border-radius: 5px;
}

p {
  line-height: 1.5;
}

<div id="postsContainer" data-has-next-page="true" data-current-page="1" data-max-count="11">
  <div class="post">
    <h2>Title 1</h2>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Neque dolorum consequatur nostrum sapiente ipsa! Veniam, laudantium accusantium, odio maxime quo adipisci possimus enim quam, voluptate quidem animi perferendis delectus aliquam?</p>
  </div>
  <div class="post">
    <h2>Title 2</h2>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Neque dolorum consequatur nostrum sapiente ipsa! Veniam, laudantium accusantium, odio maxime quo adipisci possimus enim quam, voluptate quidem animi perferendis delectus aliquam?</p>
  </div>
  <div class="post">
    <h2>Title 3</h2>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Neque dolorum consequatur nostrum sapiente ipsa! Veniam, laudantium accusantium, odio maxime quo adipisci possimus enim quam, voluptate quidem animi perferendis delectus aliquam?</p>
  </div>
  <div class="post">
    <h2>Title 4</h2>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Neque dolorum consequatur nostrum sapiente ipsa! Veniam, laudantium accusantium, odio maxime quo adipisci possimus enim quam, voluptate quidem animi perferendis delectus aliquam?</p>
  </div>
</div>

The problem

The post clones, instead of being appended to the container after scrolling to the last post, are appended when the page loads.

I do not want to use rootMargin, because it does not seem versatile.

What am I doing wrong?

Answer 1

Ok, maybe it will help someone:

class InfiniteScroll {
    constructor() {
        this.observer = null;
        this.isLoading = false;
        this.postsContainer = document.querySelector('#postsContainer');
        this.postsArary = postsContainer.querySelectorAll('.post');
        this.hasNextPage = this.postsContainer ? .dataset ? .hasNextPage === 'true';
        this.currentPage = this.postsContainer ? .dataset ? .currentPage;
        this.nextPage = this.hasNextPage ? this.currentPage + 1 : null;
    }

    loadMorePosts() {
        if (this.hasNextPage) {
            this.postsArary.forEach(item => {
                let postClone = item.cloneNode(true);
                this.postsContainer.appendChild(postClone);
            });
        } else {
            if (this.observer) this.observe.disconnect();
        }
    }

    getLastPost() {
        const allPosts = [...this.postsContainer.querySelectorAll('.post')];
        return allPosts[allPosts.length - 1];
    }
    
    bindLoadMoreObserver() {
        if (this.postsContainer) {
            this.observer = new IntersectionObserver((entries, observer) => {
                entries.forEach(entry => {
                    if (entry && entry.isIntersecting) {
                        observer.unobserve(entry.target);
                        this.loadMorePosts();    
                        observer.observe(this.getLastPost());
                    }
                });
            });

            this.observer.observe(this.getLastPost());
        }
    }

    init() {
        this.bindLoadMoreObserver();
    }
}

const infiniteScroll = new InfiniteScroll();
infiniteScroll.init();

1. I made the function getLastPost - it gets the last of your posts (included added by js).

2. I observe last post, not the whole container

3. then when this item is intersecting I made 2 things: unobserve current entry element, then after appending new elements I observe the last post- so this will be very last post, added by js.

4. as bonus in loadMorePosts I saw that you want add posts just when there is next page available.But when not your IO still works, but it's not neccesary (performance). So... disconnect-- disonnect is killing IO process also unobserve all elements in this IO.