Why Next.js App Router

Next.js is the leading production framework for React. With version 13, Next.js introduced the App Router, a brand-new architecture built on top of React Server Components. Let us explore why the industry has shifted to this model.

1. Pages Router vs App Router

The legacy Pages Router mapped routes to files inside the pages directory. While simple, it suffered from limitations:

• Blocking Hydration: Every component on a page had to be hydrated in the browser, even static navigation bars or footer elements.
• Co-location Limitations: You could not place tests, components, or style sheets inside the pages directory without them accidentally becoming routing entry points.
• Layout Re-rendering: Navigating between pages forced the entire page layout tree to destroy and recreate.

The App Router resolves these issues by using the app directory and adopting React Server Components as the default.

2. Server Components by Default

In the App Router, every component you create is a Server Component unless you explicitly add the "use client" directive.

This design delivers massive performance benefits:

• Zero Client-side JavaScript: Static sections of your page (such as documentation texts or layout frames) do not ship any JavaScript bundles to the browser.
• Reduced Hydration Cost: The browser only hydrates interactive components, leading to faster Time to Interactive (TTI) scores.

3. Nested Layout System

The App Router structures layouts hierarchically.

• layout.tsx: Defines the shared UI wrapper for a route and its children.
• page.tsx: Defines the unique content of a specific page path.

When navigating between sibling pages, parent layouts remain fully interactive and do not re-render. This allows you to persist scroll positions, input values, and component state easily.

4. Simplified Data Fetching

In the legacy Pages Router, you had to use page-level APIs like getServerSideProps or getStaticProps to fetch data, passing the results down via props.

In the App Router, data fetching is unified. Since components render on the server, you can use standard async-await functions directly inside any server component to fetch database query results or API values.