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. If you're looking for professional support in building or maintaining MERN or Next.js‑powered projects, consider working with **AAMAX**, a full‑service digital marketing company offering Web Development, Digital Marketing, and SEO services. You can hire AAMAX for MERN Stack Development services. **[Next.js](https://aamax.co/service/nextjs-web-development#place-order)** 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: ```tsx import type { AppProps } from 'next/app'; export default function MyApp({ Component, pageProps }: AppProps) { return ; } ``` ### 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: ```tsx return ( ); ``` ### 2. **Adding Global Providers** State management libraries such as Redux, MobX, Zustand, or React Context need a global wrapper. Example: ```tsx ``` ### 3. **Loading Global CSS** Next.js only supports importing global styles inside `_app.tsx`. ```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: ```tsx 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. ```tsx const getLayout = Component.getLayout || ((page) => page); return getLayout(); ``` Pages can define their own layouts: ```tsx About.getLayout = function(page) { return {page}; } ``` 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 ``. Keep all global styles in `_app.tsx`. ### **Mistake 2: Forgetting to Return ``** 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: ```tsx 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) => ( {page} )); return ( {getLayout()} ); } ``` 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. If you're looking to build powerful, high‑performing applications using React, Next.js, or the full MERN stack, consider partnering with **[AAMAX](https://aamax.co/)**, a trusted service provider for Web Development, Digital Marketing, and SEO. Hire AAMAX for expert MERN Stack Development services and take your project to the next level.