Infinite Scroll Implementation
Load more content automatically as the user scrolls.
Infinite Scroll Implementation is a free HTML Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the HTML Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
The Pattern
Infinite scroll loads more content as the user nears the bottom of the list. The cleanest implementation uses IntersectionObserver: place a "sentinel" element at the end of the list and observe it. When the sentinel enters the viewport, fetch the next page.
Sentinel Element
Append an invisible <div> (or a "loading..." placeholder) after the last list item. Observe it with IntersectionObserver. When it intersects, that signals the user has scrolled near the bottom — time to fetch more.
<ul id="feed">
<li>Post 1</li>
<li>Post 2</li>
</ul>
<div id="sentinel" aria-hidden="true"></div>The Observer Loop
On intersection: fetch the next page, render the new items, and re-position the sentinel (it stays at the end of the list automatically if you append items before it). Unobserve and re-observe is not needed — the sentinel is still observed.
const sentinel = document.getElementById("sentinel");
const feed = document.getElementById("feed");
let page = 1;
let loading = false;
const observer = new IntersectionObserver(async (entries) => {
if (!entries[0].isIntersecting || loading) return;
loading = true;
const items = await fetchPage(page++);
items.forEach((it) => feed.insertAdjacentHTML("beforeend", renderItem(it)));
loading = false;
});
observer.observe(sentinel);Loading State Guard
A boolean loading flag prevents stacking requests if the user scrolls fast and triggers multiple intersections before the first response arrives. Without it, the same page is fetched repeatedly and items appear duplicated.
End of Data
When the server returns an empty page (or signals "no more"), unobserve the sentinel and replace it with an end-of-list message. Future intersections do not trigger fetches; the user sees they have reached the end.
Error Handling
Wrap the fetch in try/catch. On error, leave loading = true so retries do not fire automatically; show a "Retry" button that resets loading and re-triggers the load. Without this, network failures lead to retry storms.
rootMargin for Eager Loading
rootMargin: "500px" starts the next fetch 500px before the user reaches the sentinel. Tune based on item height and average scroll speed — too small and users hit "no content yet"; too large and you fetch ahead of need.
Scroll Position Restoration
When the user navigates away and returns, the scroll position should be preserved AND the previously-loaded items should re-render. Cache items in sessionStorage or a Zustand-style store; restore on mount and scrollTo the remembered offset.
Memory Considerations
Long infinite scrolls accumulate thousands of DOM nodes. For very long feeds (Twitter, Facebook), combine with windowing: remove items above the viewport from the DOM, restore them on scroll-up. Libraries like react-virtual or @tanstack/virtual handle this efficiently.
Accessibility
Infinite scroll is hostile to keyboard and screen reader users who cannot easily reach a footer. Provide an alternate "Load more" button as a fallback and ensure the page has a real footer accessible via Home/End keys. Aria-live region announcing new items helps too.
Mobile Bounce Scroll
iOS bounce scrolling can fire intersections multiple times near the bottom. The loading guard handles most cases; you may also want to debounce by 300ms to filter out the burst of intersections during the bounce.
Alternative: Cursor-Based Pagination
Server APIs should return a "next cursor" rather than relying on client-tracked page numbers — robust against item insertions/deletions during scroll. The client passes the cursor on each request and stops when the server returns no cursor.
Knowledge Check
Why is a "loading" flag necessary in an infinite-scroll IntersectionObserver callback?
Summary
Infinite scroll uses an IntersectionObserver on a sentinel element at the end of the list. On intersection, fetch the next page, render, and stay observing. Guard with a loading flag, handle end-of-data and errors gracefully, tune rootMargin for prefetch distance. Provide keyboard alternatives (Load more button) and consider windowing for very long feeds.
Frequently asked questions
Is the “Infinite Scroll Implementation” lesson free?
Yes — the full text of “Infinite Scroll Implementation” is free to read here on the web, and the HTML Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the HTML Academy course, upgrade to CoddyKit PRO.
What will I learn in “Infinite Scroll Implementation”?
Load more content automatically as the user scrolls. You practise HTML Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start HTML Academy?
No prior experience is required. HTML Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Infinite Scroll Implementation” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this HTML Academy lesson?
Yes. Every HTML Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- IntersectionObserver API and Thresholds
- Lazy Loading with IntersectionObserver
- Infinite Scroll Implementation
- Scroll-Linked Animations Setup