Loading & Error UI
Implement loading states with `loading.js` and gracefully handle errors using `error.js` boundaries in your application.
Loading & Error UI is a free Next.js 15 Fullstack (App Router + Server Actions) 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 Next.js 15 Fullstack (App Router + Server Actions) learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Loading States Matter
When your Next.js application fetches data, it can take some time. Without any visual feedback, users might think the app is frozen or broken.
- Improve User Experience: Keep users informed.
- Reduce Frustration: Prevent perceived slowness.
- Indicate Progress: Show that something is happening in the background.
Introducing `loading.js`
Next.js App Router provides a special file convention, loading.js, to automatically create a loading UI for a route segment.
When data is being fetched for a page.js or layout.js file, Next.js automatically displays the UI defined in its nearest loading.js sibling or ancestor.
Creating a Simple Loading UI
To create a loading UI, simply add a loading.js file inside a route segment folder. It should export a default React component.
This component will be rendered instantly when the segment is loading.
export default function Loading() {
return (
<div style={{ padding: '20px', textAlign: 'center' }}>
<h2>Loading...</h2>
<p>Please wait while we fetch the content.</p>
</div>
);
}How `loading.js` Works
Under the hood, loading.js works with React Suspense. Next.js wraps the corresponding page.js and its nested children in a Suspense Boundary.
When any data fetching inside that boundary is pending, the loading.js component is shown as a fallback.
Nested Loading States
You can have multiple loading.js files at different levels of your route. For example, a global loading UI and a more specific one for a sub-route.
Next.js will show the closest ancestor loading.js for the segment that is currently loading.
Handling Application Errors
Just like loading states, errors are a part of any application. Uncaught errors can crash your app, leading to a poor user experience.
Next.js App Router provides a way to gracefully handle errors and display a custom error UI using error.js.
Introducing `error.js`
The error.js file convention allows you to define an error UI that automatically wraps a route segment and its children in a React Error Boundary.
This boundary catches errors that occur during rendering, in event handlers, and in data fetching within its scope.
Creating a Basic Error UI
Similar to loading.js, create an error.js file in a route segment. It must be a React Client Component (use 'use client'; at the top).
It receives error (the error object) and a reset function as props.
'use client';
import { useEffect } from 'react';
export default function Error({ error, reset }) {
useEffect(() => {
// Log the error to an error reporting service
console.error(error);
}, [error]);
return (
<div style={{ padding: '20px', textAlign: 'center', color: 'red' }}>
<h2>Something went wrong!</h2>
<button
onClick={() => reset()} // Attempt to re-render the segment
style={{ marginTop: '10px', padding: '8px 15px' }}
>
Try again
</button>
</div>
);
}Error Recovery with `reset()`
The reset() function provided to your error.js component is crucial. When called, it attempts to re-render the entire segment that threw the error.
This allows users to try again without navigating away, potentially resolving transient issues.
Quick Check on UI Files
Which statement accurately describes the primary purpose of loading.js in Next.js App Router?
Recap: Loading & Error UI
In this lesson, you learned how to enhance your Next.js application's user experience by implementing visual feedback for loading and error states.
loading.js: Displays a temporary UI while data is being fetched, using React Suspense.error.js: Catches errors in a segment and displays a fallback UI, allowing users to attempt recovery with thereset()function.
These conventions help create more robust and user-friendly Next.js applications.
Frequently asked questions
Is the “Loading & Error UI” lesson free?
Yes — the full text of “Loading & Error UI” is free to read here on the web, and the Next.js 15 Fullstack (App Router + Server Actions) 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 Next.js 15 Fullstack (App Router + Server Actions) course, upgrade to CoddyKit PRO.
What will I learn in “Loading & Error UI”?
Implement loading states with `loading.js` and gracefully handle errors using `error.js` boundaries in your application. You practise Next.js 15 Fullstack (App Router + Server Actions) 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 Next.js 15 Fullstack (App Router + Server Actions)?
No prior experience is required. Next.js 15 Fullstack (App Router + Server Actions) 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 “Loading & Error UI” 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 Next.js 15 Fullstack (App Router + Server Actions) lesson?
Yes. Every Next.js 15 Fullstack (App Router + Server Actions) 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.