What are New Features in Next.js 15? Next.js 15 Upgrade Guide

Next.js 15 builds on previous innovations to deliver significant improvements in performance and developer experience. This Next.js latest version focuses on optimizing Server Actions, React Server Components (RSC), and Dynamic HTML Streaming to streamline data handling, boost rendering efficiency, and reduce JavaScript bundle sizes.

Understanding these new features will help you optimize your apps for better performance and easier maintenance. Notably, some features that were first introduced in Next.js 14 have been refined and extended to deliver a more secure, efficient, and developer-friendly experience.

New Features of Next.js 15

Server Actions

In previous versions, Server Actions allowed you to handle form submissions and data mutations directly within your components without the need for separate API routes. However, there were some challenges:

• Security risks: Predictable endpoints could be exposed.
• Unintended Caching: Sometimes old data was used instead of fresh updates.
• Unused Actions: Extra functions cluttered the codebase.

In Next.js 15, these issues are addressed:

• Enhanced Security: Endpoints are now unguessable, minimizing the risk of unauthorized access.
• Automatic Cleanup: Unused actions are removed during build time, keeping your application lean.
• Refined Caching: The default caching is now set to “no-store” so that data is always fresh unless you opt into caching.
• Seamless Integration: More compatible with middleware and real-time updates, simplifying complex flows.

Example: A Simple Task Submission Form

With Next.js 15, defining a secure server action is easier than ever:

// Server-side Function: server/actions.ts
"use server";
import { db } from "@/lib/db";
export async function addTask(task: string) {
  // Secure, dynamic endpoint generated automatically
  await db.tasks.create({ task });
}

Now, you can call this action directly from a client component without an extra API request:

// Client Component: TaskForm.tsx
"use client";
import { useState } from "react";
import { addTask } from "@/components/AddTask.server";
export default function TaskForm() {
  const [task, setTask] = useState("");
  async function handleSubmit(e: React.FormEvent) {
    e.preventDefault();
    await addTask(task);
    setTask("");
  }
  return (
    <form onSubmit={handleSubmit}>
      <input
        value={task}
        onChange={(e) => setTask(e.target.value)}
        placeholder="Enter a new task"
      />
      <button type="submit">Add Task</button>
    </form>
  );
}

With these improvements, your code is cleaner, your app is safer, and your data updates more predictably. No unnecessary API routes, no guessable endpoints, no caching surprises—just smooth, efficient data handling.

React Server Components (RSC)

React Server Components originally enabled non-interactive parts of your application to be rendered on the server, reducing the client-side JavaScript load and benefiting SEO. In Next.js 15, these components have been refined further:

• Server-Only: Components without interactive features run on the server.
• Client Components: Interactive parts of the page are marked with "use client" and sent to the browser.
• Benefits: Reduced client-side code means that pages load quickly and become interactive sooner.

Example: A product listing page that fetches and renders data on the server:

// app/products/page.server.tsx
export default async function ProductsPage() {
  const res = await fetch("https://api.example.com/products");
  const products = await res.json();
  return (
    <div>
      <h1>Products</h1>
      <ul>
        {products.map((p: any) => (
          <li key={p.id}>{p.name}</li>
        ))}
      </ul>
    </div>
  );
}

With Next.js 15, you can fetch data on the server and send fully-rendered HTML to the browser, eliminating the need for client-side data fetching.

Dynamic HTML Streaming

One of the biggest improvements in Next.js 15 is Dynamic HTML Streaming. It allows parts of a page to load progressively instead of waiting for everything to be ready. This means users can see static content instantly, while dynamic elements (like API-fetched data) load in the background.

• Streaming: Static parts of the page (like text, images, and layout) load immediately, while dynamic parts (like API-fetched data, comments, or dashboards) load as the data is ready.
• Fallbacks: Use React <Suspense> to show a temporary fallback (like a loading message) until the dynamic content is ready.
• Enhanced Responsiveness: Reduces waiting time and makes the page feel faster.

In Next.js 14, you could already use React <Suspense> to handle loading states, but with Next.js 15, streaming is even more optimized, making complex pages feel much more responsive.

Example: A blog post page where comments load dynamically:

// app/blog/[id]/page.server.tsx
import { Suspense } from "react";
import Comments from "@/components/Comments.server"; // Server component
export default async function BlogPost({ params }: { params: { id: string } }) {
  return (
    <div>
      <h1>Blog Post Title</h1>
      <p>Static post content...</p>
      <Suspense fallback={<p>Loading comments...</p>}>
        <Comments postId={params.id} />
      </Suspense>
    </div>
  );
}

Instead of making users wait for everything to load, we show the blog post instantly, while comments stream in later.

Partial Prerendering (Experimental)

Previous Next.js versions asked developers to choose between static and dynamic rendering—either a page was fully pre-rendered at build time (SSG) or fetched data dynamically on request (SSR). Partial Prerendering (PPR) in Next.js 15 changes that by allowing you to mix static and dynamic elements on the same page.

• Hybrid Approach: Combine static and dynamic content seamlessly to optimize performance.
• Tailored Rendering: Static elements load instantly, while dynamic components update as needed.

Example: A landing page with a static header and a dynamic product grid:

// app/landing/page.server.tsx
export const experimental_ppr = true;
import { Suspense } from "react";
import DynamicProducts from "@/components/DynamicProducts.server";
export default function LandingPage() {
  return (
    <div>
      <header>
        <h1>Welcome to Our Store</h1>
        <p>This header is rendered statically for immediate display.</p>
      </header>
      <Suspense fallback={<p>Loading latest products...</p>}>
        <DynamicProducts />
      </Suspense>
    </div>
  );
}

Though still experimental, Partial Prerendering in Next.js 15 is a potential feature for balancing speed and flexibility. If you’re building fast, dynamic apps, this feature lets you pre-render what makes sense and keep the rest fresh—without forcing a hard choice between SSG and SSR.

>> Read more: Optimize Next.js E-commerce Sites for Better SEO and Performance

The New next/after API (Experimental)

Handling background tasks like logging, analytics, and cleanup has always been tricky in server-rendered apps. Previously, running these tasks could slow down page delivery, affecting user experience.

The new next/after API in Next.js 15 fixes this by letting you run non-essential tasks AFTER the main content is sent to the user.

• Post-Response Processing: Offload secondary tasks to run asynchronously once the main HTML is delivered.

Example: Logging an event after the page renders:

// app/layout.server.tsx
import { unstable_after as after } from "next/server";
import { logAnalytics } from "@/lib/analytics";
export default function RootLayout({ children }: { children: React.ReactNode }) {
  // Run secondary tasks after the main response is delivered
  after(() => {
    logAnalytics("Page rendered successfully");
  });
  return <html><body>{children}</body></html>;
}

This ensures that important logging happens in the background without delaying the user’s interaction with the page.

Performance Enhancements in Next.js 15

Key Performance Improvements

Next.js 15 improves performance in several ways:

• Smaller JavaScript Bundles: With RSC and Server Actions, less code is sent to the browser.
• Faster Page Loads: Dynamic HTML Streaming helps pages load faster.
• Lower Memory Use: More efficient hydration means less memory is used on the client.
• Better Time-to-Interactive (TTI): With less client-side JavaScript, pages become interactive more quickly.

Performance Benchmarks

These numbers are approximate and can vary based on your app.

Practical Tips for Using Next.js 15

Getting Started:

• Switch to Server Actions: Replace old API endpoints with Server Actions to simplify data handling.
• Use React Server Components: Move parts of your app that do not need interactivity to the server.
• Implement HTML Streaming: Use <Suspense> to load dynamic content gradually, improving user experience.

Optimization:

• Use Caching: Leverage Next.js’s built-in caching and revalidation features to speed up data fetching.
• Profile Performance: Use tools like Next.js Analytics, Chrome DevTools, and Lighthouse to monitor and improve your app’s performance.
• Keep Server and Client Code Separate: This makes your app easier to maintain and takes full advantage of Next.js 15 features.

With these practical tips in mind, you will have a better experience with this Next.js latest version.

Well, are you ready to upgrade to Next.js 15? If yes, follow a structured process to avoid issues. The checklist below will guide you through each step, making sure everything is properly updated and working smoothly.

Next.js 15 Upgrade Guide

Step 1: Backup & Version Control

• Backup your entire project and create a snapshot in version control. This ensures you can revert if needed.
• Confirm that a complete backup exists and that your version control system (e.g., Git) has a tagged commit before the upgrade.

git tag -a before-next15 -m "Backup before upgrading to Next.js 15"
git push origin before-next15

Step 2: Review Official Documentation

• Read the Next.js 15 migration guide and release notes to understand breaking changes, new features, and deprecations.
• Compare your current project’s configuration and code against the official migration notes.

Step 3: Check Environment Compatibility

• Verify that your development environment meets the new requirements. Next.js 15 might require a minimum Node.js version (Node.js 18 or later) or updated tools.
• Run node -v and check compatibility with the Next.js 15 requirements stated in the docs.

Step 4: Update Dependencies

• Upgrade Next.js and related packages (React, React-DOM, etc.). Ensure that any third-party plugins or libraries are also compatible. 
• Update package.json, run npm install or yarn, and review the changelogs for each package for compatibility notes.

Run the following command to upgrade Next.js and related dependencies:

npm install next@latest react@latest react-dom@latest

or using Yarn:

yarn add next@latest react@latest react-dom@latest

Step 5: Update Next.js Configuration

• Review and update your next.config.js. Some configuration options might have changed or been deprecated.

Convert next.config.js to TypeScript for better type safety:

import { NextConfig } from "next";

const nextConfig: NextConfig = {
  reactStrictMode: true,
  experimental: {
    serverActions: true, // Enable new Next.js 15 Server Actions
  },
};

export default nextConfig;

• Compare your configuration against examples in the official documentation; run the app locally to catch any warnings or errors.

Start the dev server and check for warnings:

npm run dev

>> Explore more: Using Next.js with TypeScript to Upgrade Your React Development

Step 6: Migrate API Endpoints to Server Actions

Replace legacy API routes with Server Actions. Mark functions with "use server" and adjust client-side calls accordingly.

• Before (API Route in Next.js 14):

export default async function handler(req, res) {
  const data = await db.tasks.create({ task: req.body.task });
  res.status(200).json(data);
}

• After (Server Action in Next.js 15):

"use server";

export async function addTask(task: string) {
  return await db.tasks.create({ data: { task } });
}

Then, test forms and data mutations to ensure that server actions execute correctly without extra API calls.

Step 7: Validate React Server Components (RSC)

Ensure that components meant for the server remain purely server-side and do not contain client-side interactivity.

• Before (Incorrect RSC in Next.js 14):

export default function Dashboard() {
  const [count, setCount] = useState(0); // ❌ This should be in a client component
  return <button onClick={() => setCount(count + 1)}>Increment</button>;
}

• After (Fixed for Next.js 15):

"use client"; // ✅ Mark interactive components explicitly

export default function Counter() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(count + 1)}>Increment</button>;
}

Manually check pages for proper separation, run integration tests, and check the browser console for deprecation warnings.

Step 8: Verify Dynamic HTML Streaming

• Test that dynamic content streams progressively while static content loads immediately. Leverage <Suspense> fallbacks where necessary.
• Use tools like Lighthouse to measure TTFB, and simulate slow network conditions to ensure that fallbacks appear as expected.

Step 9: Run Linting & Type Checks

• Update ESLint, Babel, and TypeScript configurations if needed, to account for new Next.js 15 features and syntax changes.
• Run npm run lint and tsc --noEmit (if using TypeScript) to ensure no new errors or warnings arise.

Step 10: Test Static & Server-Side Rendered Pages

• Review all pages built with static generation (SSG) or server-side rendering (SSR). Ensure routing, data fetching, and hydration work as expected.
• Run your full test suite, and manually verify key pages for content, SEO tags, and performance metrics.

Step 11: Run Integration Tests & End-to-End Tests

• Perform comprehensive testing covering user flows and interactions. This is critical if your application uses a mix of static and dynamic content.
• Use testing frameworks (e.g., Jest, Cypress) to run your test suite; ensure that all tests pass and report any unsual behavior.

Step 12: Monitor Performance Metrics

• Compare pre-upgrade and post-upgrade performance.
• Analyze JavaScript bundle sizes, hydration times, and server response times.
• Use performance profiling tools (e.g., Lighthouse, Next.js Analytics) and compare metrics against the documented improvements in Next.js 15.

Step 13: Update CI/CD and Deployment Configurations

• Adjust your build scripts and deployment pipelines to incorporate any new build or runtime changes in Next.js 15.
• Trigger a staging deployment and review logs, environment variable configurations, and any build-time warnings or errors.

Step 14: Validate Production Readiness

• Conduct a smoke test on your production environment after deployment to catch any runtime issues that weren’t evident in staging.    
• Monitor error logs, user feedback, and performance dashboards for at least 48–72 hours post-deployment.

Step 15: Prepare a Rollback Plan

• Ensure you have a clear rollback strategy in case unexpected issues arise after the upgrade.
• Document rollback steps and verify that backups/previous versions are accessible and deployable if needed.

Conclusion

Next.js 15 makes building fast and efficient web apps easier. With features like Server Actions, React Server Components, and Dynamic HTML Streaming, you can reduce your JavaScript load, speed up page rendering, and improve user experience. By following these simple patterns and tips, you can build modern, high-performance web applications with this Next.js latest version.

Further Reading:

• Next.js 15 Documentation
• Server Actions in Next.js
• Streaming and Suspense in Next.js

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