Next.js 15 Fullstack Web Apps · 课时

加载与错误 UI 约定

使用 App Router 的文件约定 loading.js、error.js 和 not-found.js，构建具备韧性、支持流式传输并能优雅回退的路由。

加载与错误 UI 约定 是 CoddyKit 上的免费 Next.js 15 Fullstack Web Apps 课时。这是第 4 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Next.js 15 Fullstack Web Apps 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Next.js 15 Fullstack Web Apps 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Why Loading and Error UI Matter

Advanced routing is not only about where a route lives but about what users see while it resolves or fails. The App Router gives you special files that wrap segments automatically.

loading.js renders an instant fallback while the segment streams.
error.js catches runtime errors in that segment.
not-found.js renders when notFound() is called.

The loading.js Convention

A loading.js file in a segment folder is automatically wrapped around page.js in a React Suspense boundary. While the server component awaits data, the loading UI shows instantly.

export default function Loading() {
  return <div className="spinner">Loading dashboard...</div>;
}

Skeletons Beat Spinners

For perceived performance, render a skeleton that mirrors the final layout instead of a generic spinner. It reduces layout shift and feels faster.

export default function Loading() {
  return (
    <ul>
      {Array.from({ length: 5 }).map((_, i) => (
        <li key={i} className="skeleton-row" />
      ))}
    </ul>
  );
}

Streaming with Suspense

Because loading.js is just Suspense under the hood, the rest of the layout renders immediately while only the slow segment streams in. You can also nest your own Suspense boundaries inside a page for finer control.

import { Suspense } from 'react';

export default function Page() {
  return (
    <section>
      <h1>Reports</h1>
      <Suspense fallback={<p>Loading chart...</p>}>
        <SlowChart />
      </Suspense>
    </section>
  );
}

The error.js Convention

error.js must be a Client Component. It receives the thrown error and a reset function to retry rendering the segment.

'use client';

export default function Error({ error, reset }) {
  return (
    <div>
      <p>Something went wrong: {error.message}</p>
      <button onClick={() => reset()}>Try again</button>
    </div>
  );
}

Error Boundaries Are Scoped

An error.js catches errors in its segment and its children, but not in the layout at the same level. To catch layout errors, place the error file one level up.

Errors bubble up to the nearest parent boundary.
The root layout cannot be caught by a sibling error file.

global-error.js for the Root

To catch errors in the root layout itself, add global-error.js. It replaces the entire document, so it must render its own <html> and <body> tags.

'use client';

export default function GlobalError({ error, reset }) {
  return (
    <html>
      <body>
        <h2>App crashed</h2>
        <button onClick={() => reset()}>Reload</button>
      </body>
    </html>
  );
}

Triggering not-found.js

Call notFound() from next/navigation inside a server component to render the nearest not-found.js and send a 404 status.

import { notFound } from 'next/navigation';

export default async function Page({ params }) {
  const post = await getPost(params.id);
  if (!post) notFound();
  return <article>{post.title}</article>;
}

Custom not-found.js UI

Place not-found.js in any segment to override the default 404 for that part of the route tree. A root-level one acts as the global 404 page.

import Link from 'next/link';

export default function NotFound() {
  return (
    <div>
      <h2>Post not found</h2>
      <Link href="/blog">Back to blog</Link>
    </div>
  );
}

Combining the Conventions

A robust segment folder often contains all four files working together:

page.js — the content
loading.js — streamed fallback
error.js — runtime failure recovery
not-found.js — missing resource

Each is wired up automatically by the App Router with no manual provider setup.

Logging Errors in Production

Use a useEffect inside error.js to report errors to your monitoring service while still showing recovery UI to the user.

'use client';
import { useEffect } from 'react';

export default function Error({ error, reset }) {
  useEffect(() => {
    reportToSentry(error);
  }, [error]);
  return <button onClick={reset}>Retry</button>;
}

Quick Check

Which statement about error.js in the App Router is correct?

Recap

You learned the App Router's resilience conventions:

loading.js wraps segments in Suspense for instant streamed fallbacks.
error.js (Client Component) recovers from runtime errors with reset.
global-error.js catches root-layout failures.
not-found.js renders when notFound() is called.

Together they make advanced routes graceful under load and failure.

常见问题解答

「加载与错误 UI 约定」课时是免费的吗？

是的 — 「加载与错误 UI 约定」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Next.js 15 Fullstack Web Apps 课程的其余内容，请升级到 CoddyKit PRO。 Next.js 15 Fullstack Web Apps 课程共包含 4 节课。

「加载与错误 UI 约定」这节课中我会学到什么？

使用 App Router 的文件约定 loading.js、error.js 和 not-found.js，构建具备韧性、支持流式传输并能优雅回退的路由。你通过在浏览器中直接运行的动手代码来练习 Next.js 15 Fullstack Web Apps，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Next.js 15 Fullstack Web Apps 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Next.js 15 Fullstack Web Apps 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 4 节课，共 4 节。

「加载与错误 UI 约定」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Next.js 15 Fullstack Web Apps 课中编写并运行代码吗？

能。每节 Next.js 15 Fullstack Web Apps 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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