What Is app JS in Next JS

What Is app JS in Next JS

When working with Next.js, one of the most important files in the entire project structure is _app.js. Whether you are building a simple website or a large‑scale MERN stack application, understanding _app.js is essential. It controls how your pages are rendered, how global state works, and how your entire application initializes.

where it becomes crucial, and how you can use it effectively in real‑world applications. If you're building complex React/Next.js applications and need expert support, you can hire AAMAX for professional MERN Stack Development, Web Development, Digital Marketing, and SEO services.

Introduction: Why _app.js Matters in Next.js

In a traditional React app, you have a single root component---usually App.js---that wraps the entire application. Next.js uses a similar concept, but with a special file named _app.js inside the pages directory.

This file allows you to:

• Control the entire app's layout\
• Inject global styles\
• Share state across pages\
• Implement authentication\
• Persist layouts during navigation\
• Add providers like Redux or Context API\
• Run code before rendering a page\
• Customize error handling\
• Override Next.js default behavior

Without _app.js, your Next.js app would still work, but you would not have access to powerful customization features that make advanced app development easier, faster, and more scalable.

What Is _app.js in Next.js?

_app.js is a special file used by Next.js to initialize pages. It overrides the default App component and lets you run shared logic across every page in your application.

The official definition:

_app.js is used to initialize pages and allows you to override the default Next.js App component.

This means every time a user navigates to a page, Next.js uses _app.js to load it.

By default, Next.js includes its own minimal App component, but when you create _app.js, you gain full control over how your application renders.

Where Is _app.js Located?

The file must be placed inside:

/pages/_app.js

If it's missing, Next.js will fall back to its internal App component.

Basic Structure of _app.js

Here is the simplest example:

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

What do these props mean?

• Component
  The active page being rendered (e.g., /about, /contact, /blog/[slug])

• pageProps
  Props that are passed through getServerSideProps, getStaticProps, or getInitialProps.

_app.js acts as a global wrapper around all pages.

Why Developers Use _app.js

Here are the most common and important uses:

✔ 1. Adding Global CSS

Next.js only allows global CSS imports in _app.js.

Example:

import "../styles/globals.css";

✔ 2. Adding Layouts That Persist Across Pages

Example:

import Layout from "../components/Layout";

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

Without _app.js, every page would need to repeat the same layout manually.

✔ 3. Managing Global State (Redux, Context API, Zustand)

Example with Redux:

import { Provider } from "react-redux";
import store from "../store";

export default function MyApp({ Component, pageProps }) {
  return (
    <Provider store={store}>
      <Component {...pageProps} />
    </Provider>
  );
}

✔ 4. Authentication Wrappers

You can check user roles, permissions, or login status globally.

✔ 5. Custom Themes (Tailwind, Material UI, Chakra UI)

Example:

import { ThemeProvider } from "@mui/material/styles";

✔ 6. Injecting Analytics and Tracking Scripts

Google Analytics
Facebook Pixel
Hotjar
etc.

✔ 7. Overriding Error Handling

You can catch rendering errors and show a fallback UI.

✔ 8. Running Global Logic on Every Page

Loading indicators
API initialization
Cookie/session checks

✔ 9. Persisting State During Page Transitions

Useful for dashboards, logged‑in sessions, and e-commerce carts.

Example: Adding a Global Layout

If your website needs a header and footer everywhere, _app.js solves this easily.

import Layout from "../components/Layout";

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

Now every page automatically has:

• Navigation bar\
• Footer\
• Sidebar\
• Message banner

No need to repeat layout code in every page file.

Example: Adding Global CSS

You cannot import global CSS anywhere else in a Next.js application.

import "../styles/global.css";

This ensures the entire app shares a consistent design system.

Using _app.js with getInitialProps

getInitialProps allows you to fetch data before rendering pages, but it disables automatic static optimization.

Usage:

MyApp.getInitialProps = async ({ Component, ctx }) => {
  const pageProps = Component.getInitialProps
    ? await Component.getInitialProps(ctx)
    : {};

  return { pageProps };
};

Downsides of getInitialProps

• Breaks static optimization\
• Slower performance\
• More server load

Avoid it unless absolutely needed.

Common Use Cases for _app.js

1. Authentication (JWT, Cookies, Sessions)

Check user login status globally.

2. Theme Providers

Dark mode
Material UI
Chakra UI
Emotion

3. State Providers

Redux
Zustand
Recoil
Context API

4. Global Layout

Headers
Footers
Sidebars

5. Analytics Tracking

Google Analytics
Facebook Pixel

6. Custom Loading Screens

Show loaders during route change events.

Difference Between _app.js and _document.js

A common point of confusion is the difference between _app.js and _document.js.

✔ _app.js

Controls page initialization and rendering
Used for layouts, providers, global styles
Runs on client + server

✔ _document.js

Controls HTML document structure
Used for meta tags, fonts, language attributes
Runs only on server

Example tasks:

Task _app.js _document.js

---

Add global CSS ✔ ✖ Add custom <html> or <body> attributes ✖ ✔ Add layout ✔ ✖ Wrap page with providers ✔ ✖ Insert tracking script in <head> ✖ ✔

Both are essential but serve different purposes.

_app.js in Large‑Scale Applications

In real-world enterprise apps, _app.js becomes the main control center for:

• Global configuration\
• State management\
• API initialization\
• User authentication\
• Layout persistence\
• Security settings\
• Internationalization (i18n)\
• Error boundaries

It ensures consistency and reduces code duplication across hundreds of pages.

_app.js in the Next.js App Router (Next.js 13+)

With the introduction of the App Router, developers now use:

• layout.js
• template.js
• Server Components
• Client Components

However, for projects still using the Pages Router, _app.js remains critical.

Even in newer versions, pages‑based routing still relies on _app.js.

Best Practices for Using _app.js

✔ Use it only for global logic

Avoid putting page-specific logic here.

✔ Keep the component clean

Too much logic will slow down page transitions.

✔ Don't use getInitialProps unless necessary

It disables many Next.js optimizations.

✔ Wrap providers smartly

Only include global providers inside _app.js.

✔ Avoid complex conditional logic

It may affect hydration or rendering speed.

✔ Use dynamic imports for heavy components

Boosts performance and load speed.

Example: Complete _app.js Setup

Here is a professional example used in production‑grade MERN stack apps:

import "../styles/globals.css";
import Layout from "../components/Layout";
import { Provider } from "react-redux";
import store from "../store";
import { ThemeProvider } from "@mui/material/styles";
import theme from "../theme";

export default function MyApp({ Component, pageProps }) {
  return (
    <Provider store={store}>
      <ThemeProvider theme={theme}>
        <Layout>
          <Component {...pageProps} />
        </Layout>
      </ThemeProvider>
    </Provider>
  );
}

This example includes:

• Global CSS\
• Global layout\
• Redux state\
• Material UI theme

Exactly how a real production app should be structured.

Why Understanding _app.js Is Important for MERN Developers

MERN stack developers working with:

• MongoDB\
• Express\
• React\
• Node.js

often build large-scale applications that require:

• Authentication\
• State management\
• Protected routes\
• Global UI themes\
• Dynamic dashboards

All of these require heavy use of _app.js to stay organized and maintainable.

Final Thoughts

_app.js is one of the most important files in the Next.js Pages Router. It allows developers to customize how the entire application works, from state management and global styles to authentication and UI layouts. Without _app.js, every page would be isolated and less efficient to manage.

Understanding _app.js is essential for building modern, scalable, and production-ready Next.js applications. Whether you're developing blogs, e-commerce stores, SaaS dashboards, or enterprise-level systems, mastering this file gives you complete control over your app's structure and performance.

If you're building advanced MERN stack or Next.js applications and want professional development, optimization, or scaling support, you can AAMAX---a full-service digital marketing and development company providing MERN Stack Development, Web Development, Digital Marketing, and SEO Services.

Next.js is powerful, and _app.js is one of its strongest tools---when used properly, it becomes the backbone of your entire application.