Next.js 15 App Router: A Complete Guide

Next.js 15 brings significant improvements to the App Router, making it more stable and performant than ever. ## Server vs Client Components Understanding when to use Server Components vs Client Components is crucial. By default, all components in the App Router are Server Components — they render on the server and send HTML to the client. Use Client Components (`'use client'`) when you need: - Event handlers (onClick, onChange) - React hooks (useState, useEffect) - Browser-only APIs - Real-time updates ## Parallel Routes Parallel Routes let you simultaneously render multiple pages in the same layout: ``` app/ layout.tsx @dashboard/ page.tsx @analytics/ page.tsx ``` ## Server Actions Server Actions let you run server-side code directly from your components without creating an API route: ```typescript async function createPost(formData: FormData) { 'use server'; const title = formData.get('title'); await db.posts.create({ data: { title } }); revalidatePath('/posts'); } ``` ## Caching Strategy Next.js 15 changes the caching defaults — fetch requests are no longer cached by default. Be explicit about your caching strategy using the `cache` option. ## Conclusion The App Router is the future of Next.js development. Embrace Server Components where possible for better performance and start building full-stack apps with confidence.