What's changed in Asynchronous Request APIs in next.js15?

In the new version, certain APIs in Next.js that handle request-specific data—like reading cookies, headers, URL parameters, or search parameters—are becoming asynchronous. These APIs were synchronous in previous versions, meaning they would wait for the request to complete before moving forward.

Why This Change Matters

Traditionally, when a server handles a request, it waits for all the required data to arrive before rendering the webpage. While this works, it’s slow because the server is idle while waiting for the data. With asynchronous APIs, the server can do multiple things simultaneously, like preparing components that don’t depend on the request, even before the request is fully received.

How It Works

Imagine you have an admin panel that shows different data based on cookies. Here’s how it changes in Next.js 15:

1. Synchronous API: It reads cookies directly, which can block other tasks until the data arrives.
2. Asynchronous API: It "awaits" the cookie data, allowing the server to handle other tasks in parallel.

Before Asynchronous APIs

After Asynchronous APIs

Code

Before Migration

In Next.js 14, the code reads cookies synchronously:

import { cookies } from 'next/headers';

export function AdminPanel() {
  const cookieStore = cookies();
  const token = cookieStore.get('token');
  // Do something with the token
}

• Here, the cookies() function runs synchronously, making the code wait until the cookies are retrieved before moving forward.

After Migration

In Next.js 15, the code becomes asynchronous:

import { cookies } from 'next/headers';

export async function AdminPanel() {
  const cookieStore = await cookies();
  const token = cookieStore.get('token');
  // Do something with the token
}

• Now, the cookies() function is awaited, allowing the server to process other tasks while waiting for the cookies to load.

Benefits

• Better Performance: Asynchronous APIs make your app faster by reducing waiting time.
• Efficient Coding: You don’t need to wait for everything to load before starting the rendering process.
• Easier Scaling: Apps built with asynchronous APIs are better prepared for handling more users and requests simultaneously.

Step-by-Step Migration Guide

FAQs

Q1: What does "asynchronous" mean?

Asynchronous means that the server can handle multiple tasks at once. It doesn't wait for a single task to complete before moving on to the next.

Q2: Do I have to update my code immediately?

No, you can temporarily use synchronous APIs in Next.js 15, but they will show warnings. It’s better to update as soon as possible to avoid future compatibility issues.

Q3: Why do I need to use

await is necessary because asynchronous APIs don’t return results instantly. await pauses the execution until the data arrives, ensuring your code functions correctly.

Summary

The shift to asynchronous APIs in Next.js 15 helps improve performance, makes your app faster, and aligns with modern coding practices. By following this guide, you can smoothly transition your codebase and ensure compatibility with the new changes.