Next.js 15 Features Every Developer Should Know

Next.js keeps shipping. Version 15 brought some significant changes — and a few breaking ones. Here's the complete rundown.

Major Changes

1. Async Request APIs (Breaking)

cookies(), headers(), and params are now async:

// ❌ Old (sync)
import { cookies } from 'next/headers';
const cookieStore = cookies();
 
// ✅ New (async)
import { cookies } from 'next/headers';
const cookieStore = await cookies();

This change enables better server infrastructure optimization.

2. Turbopack for next dev (Stable)

Turbopack is now the default dev server. Speed improvements:

• Local server startup: 76.7% faster
• Code updates (HMR): 96.3% faster

Finally stable and battle-tested.

3. New Caching Defaults

Fetch requests are NOT cached by default anymore:

// Next.js 14 — cached by default
fetch('/api/data');
 
// Next.js 15 — no cache by default
fetch('/api/data'); // equivalent to { cache: 'no-store' }
 
// Opt in explicitly
fetch('/api/data', { cache: 'force-cache' });

This aligns Next.js with how most developers expect HTTP to work.

4. React 19 Support

Full React 19 compatibility including:

• use() hook for promise unwrapping
• Form Actions
• Optimistic updates with useOptimistic
• New <form> behavior

5. after() API

Run code after the response is sent — perfect for logging and analytics:

import { after } from 'next/server';
 
export async function GET() {
  after(async () => {
    // Runs after response, doesn't block the user
    await logRequest();
  });
  
  return Response.json({ data: 'hello' });
}

Migration Tips

1. Run npx @next/codemod@canary upgrade latest for auto-migrations
2. Update all cookies(), headers(), params to async
3. Audit caching behavior — your app might get more data fetches
4. Test with Turbopack locally before deploying

Next.js 15 is a solid release. The caching changes especially make the mental model cleaner. Worth upgrading.