What Is app TSX in Next JS

What Is app TSX in Next JS

In this in‑depth guide, we’ll explore what _app.tsx is, how it works, why it’s essential, and how to use it effectively in your Next.js applications. We’ll also relate the concepts to real development scenarios and provide examples so you can confidently apply them in your own projects.

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Next.js has rapidly grown into one of the most popular React frameworks because of its powerful features—server-side rendering, static site generation, API routes, file-based routing, and more. As projects scale, developers often need a way to manage global state, apply global styles, or wrap all pages with layout components. That’s where _app.tsx comes in.

What Is _app.tsx in Next.js?

In a traditional React application, you have a single top‑level component—often called App.js—that wraps the entire component tree. Next.js follows a similar idea but introduces a special file: _app.tsx, located inside the pages directory.

The Role of _app.tsx

_app.tsx is a custom App component that Next.js uses to initialize every page. It allows developers to:

• Persist layout across page transitions
• Keep UI state when navigating
• Inject global CSS
• Add global providers (Redux, Context API, Theme providers, etc.)
• Execute code on page changes
• Override and extend the default Next.js App behavior

In simpler terms:
_app.tsx is the shared parent component for all pages in your application.

Where _app.tsx Lives

The file must be placed at:

/pages/_app.tsx

or, if using JavaScript:

/pages/_app.js

If this file does not exist, Next.js uses its own default App implementation. You only create it when you need customization.

The Default Structure of _app.tsx

Below is a minimal example of what a custom App component looks like:

import type { AppProps } from 'next/app';

export default function MyApp({ Component, pageProps }: AppProps) {
  return <Component {...pageProps} />;
}

Key Elements Explained

1. Component

This is the active page being rendered. If the route is /about, then Component refers to pages/about.tsx.

2. pageProps

These are the props that Next.js preloads for the page via functions like:

• getStaticProps
• getServerSideProps
• getInitialProps

They get passed into the page component via _app.tsx.

Together, Component and pageProps allow _app.tsx to serve as a wrapper around every page while still passing necessary data through.

Why _app.tsx Is Important

### 1. Consistent Layouts Across Pages

Without _app.tsx, you might duplicate layout code in every page—header, footer, navigation, etc. _app.tsx lets you define global layouts once.

Example:

return (
  <Layout>
    <Component {...pageProps} />
  </Layout>
);

2. Adding Global Providers

State management libraries such as Redux, MobX, Zustand, or React Context need a global wrapper.

Example:

<ThemeProvider>
  <Component {...pageProps} />
</ThemeProvider>

3. Loading Global CSS

Next.js only supports importing global styles inside _app.tsx.

import '../styles/globals.css';

This keeps CSS structured and prevents accidental global scope pollution.

4. Tracking Page Transitions

You can hook into Next.js Router events inside _app.tsx to:

• create loading indicators
• track page analytics
• show progress bars

Example:

import Router from 'next/router';

Router.events.on('routeChangeStart', () => console.log('Loading...'));
Router.events.on('routeChangeComplete', () => console.log('Done!'));

Advanced Use Cases of _app.tsx

1. Maintaining State Across Navigation

Some applications—especially dashboards or media platforms—require persistent state.

Example: keeping music playing while navigating pages.

Since _app.tsx persists across pages, global components remain mounted.

2. Multiple Layouts

Next.js supports dynamic layouts using the getLayout pattern.

const getLayout = Component.getLayout || ((page) => page);

return getLayout(<Component {...pageProps} />);

Pages can define their own layouts:

About.getLayout = function(page) {
  return <SidebarLayout>{page}</SidebarLayout>;
}

This is powerful for apps with varying page designs.

3. Analytics Integration

Tools like Google Analytics, Segment, and Plausible are often added inside _app.tsx.

4. Authentication State

Auth providers like:

• NextAuth.js
• Firebase
• Auth0

are commonly initialized in _app.tsx.

Common Mistakes and How to Avoid Them

Mistake 1: Importing Global CSS Outside _app.tsx

Next.js throws an error:

Global CSS cannot be imported from files other than your Custom <App>.

Keep all global styles in _app.tsx.

Mistake 2: Forgetting to Return <Component />

If you wrap incorrectly, your pages may not render.

Mistake 3: Using getInitialProps Unnecessarily

Declaring this method forces every page to use server-side rendering, removing static optimization. Use it only when required.

_app.tsx vs. App Router (Next.js 13+)

With the introduction of the App Router in Next.js 13, layouts and providers are handled differently using:

• layout.tsx
• template.tsx
• providers.tsx

However, _app.tsx remains essential for projects using the Pages Router.

As of today:

• Pages Router → uses _app.tsx
• App Router → does not use _app.tsx

Many enterprise apps still rely on Pages Router for legacy, migration, or SEO reasons, making _app.tsx highly relevant.

Example: A Complete _app.tsx With Providers and Layout

Here’s a richer example combining common features:

import type { AppProps } from 'next/app';
import '../styles/globals.css';
import { ThemeProvider } from 'next-themes';
import Layout from '../components/Layout';

export default function MyApp({ Component, pageProps }: AppProps) {
  const getLayout = (Component as any).getLayout || ((page: any) => (
    <Layout>{page}</Layout>
  ));

  return (
    <ThemeProvider attribute="class">
      {getLayout(<Component {...pageProps} />)}
    </ThemeProvider>
  );
}

This setup enables:

• global CSS
• theme support
• custom per-page layouts
• a default layout wrapper

This structure is scalable and professional for real‑world apps.

When You Should Create _app.tsx

You should add a custom App component when you need:

✔ Global state management

✔ Authentication context

✔ Theme providers

✔ Third‑party libraries

✔ Global CSS imports

✔ Consistent layouts

✔ Page transition tracking

If your app requires none of the above, the default Next.js behavior is enough.

Best Practices for Using _app.tsx

1. Keep It Clean and Focused

Avoid clutter—put business logic elsewhere.

2. Use Composition Over Complexity

Let pages define their own layouts when necessary.

3. Limit Use of getInitialProps

Prefer getServerSideProps and getStaticProps.

4. Organize Your Providers

Sometimes a dedicated providers.tsx file is cleaner.

5. Use TypeScript

_app.tsx benefits greatly from type safety.

Conclusion

_app.tsx plays a foundational role in the Next.js Pages Router. It acts as the global wrapper for all pages, enabling:

• global state
• layout persistence
• global CSS
• third‑party providers
• custom page transitions
• shared application logic

Understanding its responsibilities is essential for building scalable and maintainable Next.js applications—especially in enterprise or large‑scale projects.

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