Next.js 15 builds upon the app router paradigm, optimizing performance, developer experience, and scalability. Here are some of the essential concepts you should master:

1. App Router (New Default)

Next.js 15 fully embraces the App Router (introduced in v13), using the app/ directory for file-based routing. It supports React Server Components (RSC) and enhances data fetching, layouts, and nesting.

2. Server Components & Client Components

• By default, components in app/ are React Server Components (RSC), improving performance by reducing client-side JavaScript.
• Use 'use client' at the top of a file to opt into Client Components, necessary for interactivity (e.g., event handlers, state).

3. Data Fetching with fetch, getServerSideProps Deprecated

• Fetch data inside components without using getServerSideProps or getStaticProps.
• Use built-in caching and revalidation (fetch with { cache: 'no-store' } for fresh data or revalidatePath for ISR behavior).

4. Loading UI & Streaming

• loading.tsx provides an automatic loading state while fetching data.
• React’s Streaming enables progressive rendering for improved perceived performance.

5. Layouts & Nested Routing

• layout.tsx allows persistent layouts across routes (e.g., navigation bars).
• Next.js supports nested routes, making UI composition more intuitive.

6. Middleware for Edge Functions

Middleware enables server-side logic before requests reach a route, useful for:

• Authentication
• Redirects
• A/B Testing
• Geolocation-based personalization

Defined in middleware.ts and runs at the Edge for low-latency responses.

7. Server Actions (Experimental)

A new way to handle mutations on the server without API routes. Example:

tsxCopy
"use server";
export async function submitForm(formData) { /* process data */ }

They simplify API handling and work well with forms and database updates.

8. New Metadata API

Define SEO metadata inside metadata.ts or within page components:

tsxCopy
export const metadata = {
  title: "Next.js 15 Guide",
  description: "Learn about Next.js 15 concepts."
};

Supports dynamic metadata for better SEO flexibility.

9. useOptimistic for UX Improvements

React 19 introduces useOptimistic, a UI-first state update for smoother experiences (e.g., instant UI updates while awaiting a response).

tsxCopy
const [optimisticLikes, addLike] = useOptimistic(likes, (state) => state + 1);

10. Edge & Serverless Deployment Improvements

Next.js 15 optimizes Edge and Serverless runtimes, with:

• Faster execution at Vercel Edge Functions
• More control over caching & revalidation

11. Improved Image & Font Optimization

• next/image now supports faster LCP optimizations.
• next/font automatically optimizes fonts without layout shifts.

12. Turbopack & Compiler Upgrades

• Turbopack speeds up development by replacing Webpack.
• Better HMR (Hot Module Replacement) improves refresh speeds.
• Rust-based optimizations make builds significantly faster.

Final Thoughts

Next.js 15 continues pushing server-first React while keeping full client flexibility. Mastering Server Components, Streaming, Middleware, and Server Actions will help you build faster and more scalable apps.