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App Router...UpgradingMigrating from Create React App

Migrating from Create React App

This guide will help you migrate an existing Create React App site to Next.js.

Why Switch?

There are several reasons why you might want to switch from Create React App to Next.js:

Slow initial page loading time

Create React App uses purely client-side React. Client-side only applications, also known as single-page applications (SPAs), often experience slow initial page loading time. This happens due to a couple of reasons:

  1. The browser needs to wait for the React code and your entire application bundle to download and run before your code is able to send requests to load data.
  2. Your application code grows with every new feature and dependency you add.

No automatic code splitting

The previous issue of slow loading times can be somewhat managed with code splitting. However, if you try to do code splitting manually, you'll often make performance worse. It's easy to inadvertently introduce network waterfalls when code-splitting manually. Next.js provides automatic code splitting built into its router.

Network waterfalls

A common cause of poor performance occurs when applications make sequential client-server requests to fetch data. One common pattern for data fetching in an SPA is to initially render a placeholder, and then fetch data after the component has mounted. Unfortunately, this means that a child component that fetches data can't start fetching until the parent component has finished loading its own data.

While fetching data on the client is supported with Next.js, it also gives you the option to shift data fetching to the server, which can eliminate client-server waterfalls.

Fast and intentional loading states

With built-in support for streaming through React Suspense, you can be more intentional about which parts of your UI you want to load first and in what order without introducing network waterfalls.

This enables you to build pages that are faster to load and eliminate layout shifts.

Choose the data fetching strategy

Depending on your needs, Next.js allows you to choose your data fetching strategy on a page and component basis. You can decide to fetch at build time, at request time on the server, or on the client. For example, you can fetch data from your CMS and render your blog posts at build time, which can then be efficiently cached on a CDN.

Middleware

Next.js Middleware allows you to run code on the server before a request is completed. This is especially useful to avoid having a flash of unauthenticated content when the user visits an authenticated-only page by redirecting the user to a login page. The middleware is also useful for experimentation and internationalization.

Built-in Optimizations

Images, fonts, and third-party scripts often have significant impact on an application's performance. Next.js comes with built-in components that automatically optimize those for you.

Migration Steps

Our goal with this migration is to get a working Next.js application as quickly as possible, so that you can then adopt Next.js features incrementally. To begin with, we'll keep it as a purely client-side application (SPA) without migrating your existing router. This helps minimize the chances of encountering issues during the migration process and reduces merge conflicts.

Step 1: Install the Next.js Dependency

The first thing you need to do is to install next as a dependency:

Terminal
npm install next@latest

Step 2: Create the Next.js Configuration File

Create a next.config.mjs at the root of your project. This file will hold your Next.js configuration options.

next.config.mjs
/** @type {import('next').NextConfig} */
const nextConfig = {
  output: 'export', // Outputs a Single-Page Application (SPA).
  distDir: './dist', // Changes the build output directory to `./dist/`.
}
 
export default nextConfig

Good to know: You can use either .js or .mjs for your Next.js configuration file.

Step 3: Update TypeScript Configuration

If you're using TypeScript, you need to update your tsconfig.json file with the following changes to make it compatible with Next.js:

tsconfig.json
{
  "compilerOptions": {
    "target": "es5",
    "lib": ["dom", "dom.iterable", "esnext"],
    "allowJs": true,
    "skipLibCheck": true,
    "strict": false,
    "forceConsistentCasingInFileNames": true,
    "noEmit": true,
    "esModuleInterop": true,
    "module": "esnext",
    "moduleResolution": "node",
    "resolveJsonModule": true,
    "isolatedModules": true,
    "jsx": "preserve",
    "baseUrl": ".",
    "incremental": true,
    "plugins": [
      {
        "name": "next"
      }
    ],
    "strictNullChecks": true
  },
  "include": [
    "next-env.d.ts",
    "**/*.ts",
    "**/*.tsx",
    ".next/types/**/*.ts",
    "./dist/types/**/*.ts"
  ],
  "exclude": ["node_modules"]
}

You can find more information about configuring TypeScript on the Next.js docs.

Step 4: Create the Root Layout

A Next.js App Router application must include a root layout file, which is a React Server Component that will wrap all pages in your application. This file is defined at the top level of the app directory.

The closest equivalent to the root layout file in a CRA application is the index.html file, which contains your <html>, <head>, and <body> tags.

In this step, you'll convert your index.html file into a root layout file:

  1. Create a new app directory in your src directory.
  2. Create a new layout.tsx file inside that app directory:
app/layout.tsx
export default function RootLayout({
  children,
}: {
  children: React.ReactNode
}) {
  return null
}

Good to know: .js, .jsx, or .tsx extensions can be used for Layout files.

  1. Copy the content of your index.html file into the previously created <RootLayout> component while replacing the body.div#root and body.script tags with <div id="root">{children}</div>:
app/layout.tsx
export default function RootLayout({
  children,
}: {
  children: React.ReactNode
}) {
  return (
    <html lang="en">
      <head>
        <meta charset="UTF-8" />
        <link rel="icon" href="%PUBLIC_URL%/favicon.ico" />
        <meta name="viewport" content="width=device-width, initial-scale=1" />
        <title>React App</title>
        <meta name="description" content="Web site created..." />
      </head>
      <body>
        <div id="root">{children}</div>
      </body>
    </html>
  )
}

Good to know: We'll ignore the manifest file, additional iconography other than the favicon, and testing configuration, but if these are requirements, Next.js also supports these options.

  1. Next.js already includes by default the meta charset and meta viewport tags, so you can safely remove those from your <head>:
app/layout.tsx
export default function RootLayout({
  children,
}: {
  children: React.ReactNode
}) {
  return (
    <html lang="en">
      <head>
        <link rel="icon" href="%PUBLIC_URL%/favicon.ico" />
        <title>React App</title>
        <meta name="description" content="Web site created..." />
      </head>
      <body>
        <div id="root">{children}</div>
      </body>
    </html>
  )
}
  1. Any metadata files such as favicon.ico, icon.png, robots.txt are automatically added to the application <head> tag as long as you have them placed into the top level of the app directory. After moving all supported files into the app directory you can safely delete their <link> tags:
app/layout.tsx
export default function RootLayout({
  children,
}: {
  children: React.ReactNode
}) {
  return (
    <html lang="en">
      <head>
        <title>React App</title>
        <meta name="description" content="Web site created..." />
      </head>
      <body>
        <div id="root">{children}</div>
      </body>
    </html>
  )
}
  1. Finally, Next.js can manage your last <head> tags with the Metadata API. Move your final metadata info into an exported metadata object:
app/layout.tsx
import type { Metadata } from 'next'
 
export const metadata: Metadata = {
  title: 'React App',
  description: 'Web site created with Next.js.',
}
 
export default function RootLayout({
  children,
}: {
  children: React.ReactNode
}) {
  return (
    <html lang="en">
      <body>
        <div id="root">{children}</div>
      </body>
    </html>
  )
}

With the above changes, you shifted from declaring everything in your index.html to using Next.js' convention-based approach built into the framework (Metadata API). This approach enables you to more easily improve your SEO and web shareability of your pages.

Step 5: Create the Entrypoint Page

On Next.js you declare an entrypoint for your application by creating a page.tsx file. The closest equivalent of this file on CRA is your src/index.tsx file. In this step, you’ll set up the entrypoint of your application.

  1. Create a [[...slug]] directory in your app directory.

Since this guide is aiming to first set up our Next.js as an SPA (Single Page Application), you need your page entrypoint to catch all possible routes of your application. For that, create a new [[...slug]] directory in your app directory.

This directory is what is called an optional catch-all route segment. Next.js uses a file-system based router where directories are used to define routes. This special directory will make sure that all routes of your application will be directed to its containing page.tsx file.

  1. Create a new page.tsx file inside the app/[[...slug]] directory with the following content:
app/[[...slug]]/page.tsx
import '../../src/index.css'
 
export function generateStaticParams() {
  return [{ slug: [''] }]
}
 
export default function Page() {
  return '...' // We'll update this
}

Good to know: .js, .jsx, or .tsx extensions can be used for Page files.

This file is a Server Component. When you run next build, the file is prerendered into a static asset. It does not require any dynamic code.

This file imports our global CSS and tells generateStaticParams we are only going to generate one route, the index route at /.

Now, let's move the rest of our CRA application which will run client-only.

app/[[...slug]]/client.tsx
'use client'
 
import React from 'react'
import dynamic from 'next/dynamic'
 
const App = dynamic(() => import('../../src/App'), { ssr: false })
 
export function ClientOnly() {
  return <App />
}

This file is a Client Component, defined by the 'use client' directive. Client Components are still prerendered to HTML on the server before being sent to the client.

Since we want a client-only application to start, we can configure Next.js to disable prerendering from the App component down.

const App = dynamic(() => import('../../App'), { ssr: false })

Now, update your entrypoint page to use the new component:

app/[[...slug]]/page.tsx
import '../../src/index.css'
import { ClientOnly } from './client'
 
export function generateStaticParams() {
  return [{ slug: [''] }]
}
 
export default function Page() {
  return <ClientOnly />
}

Step 6: Update Static Image Imports

Next.js handles static image imports slightly different from CRA. With CRA, importing an image file will return its public URL as a string:

App.tsx
import image from './img.png'
 
export default function App() {
  return <img src={image} />
}

With Next.js, static image imports return an object. The object can then be used directly with the Next.js <Image> component, or you can use the object's src property with your existing <img> tag.

The <Image> component has the added benefits of automatic image optimization. The <Image> component automatically sets the width and height attributes of the resulting <img> based on the image's dimensions. This prevents layout shifts when the image loads. However, this can cause issues if your app contains images with only one of their dimensions being styled without the other styled to auto. When not styled to auto, the dimension will default to the <img> dimension attribute's value, which can cause the image to appear distorted.

Keeping the <img> tag will reduce the amount of changes in your application and prevent the above issues. You can then optionally later migrate to the <Image> component to take advantage of optimizing images by configuring a loader, or moving to the default Next.js server which has automatic image optimization.

  1. Convert absolute import paths for images imported from /public into relative imports:
// Before
import logo from '/logo.png'
 
// After
import logo from '../public/logo.png'
  1. Pass the image src property instead of the whole image object to your <img> tag:
// Before
<img src={logo} />
 
// After
<img src={logo.src} />

Alternatively, you can reference the public URL for the image asset based on the filename. For example, public/logo.png will serve the image at /logo.png for your application, which would be the src value.

Warning: If you're using TypeScript, you might encounter type errors when accessing the src property. You can safely ignore those for now. They will be fixed by the end of this guide.

Step 7: Migrate the Environment Variables

Next.js has support for .env environment variables similar to CRA.

The main difference is the prefix used to expose environment variables on the client-side. Change all environment variables with the REACT_APP_ prefix to NEXT_PUBLIC_.

Step 8: Update Scripts in package.json

You should now be able to run your application to test if you successfully migrated to Next.js. But before that, you need to update your scripts in your package.json with Next.js related commands, and add .next and next-env.d.ts to your .gitignore:

package.json
{
  "scripts": {
    "dev": "next dev",
    "build": "next build",
    "start": "next start"
  }
}
.gitignore
# ...
.next
next-env.d.ts
dist

Now run npm run dev, and open http://localhost:3000. You should see your application now running on Next.js.

Example: Check out this repository for a working example of a CRA application migrated to Next.js.

Step 9: Clean Up

You can now clean up your codebase from Create React App related artifacts:

  • Delete src/index.tsx
  • Delete public/index.html
  • Delete reportWebVitals setup
  • Uninstall CRA dependencies (react-scripts)

Bundler Compatibility

Create React App and Next.js both default to using webpack for bundling.

When migrating your CRA application to Next.js, you might have a custom webpack configuration you're looking to migrate. Next.js supports providing a custom webpack configuration.

Further, Next.js has support for Turbopack through next dev --turbo to improve your local dev performance. Turbopack supports some webpack loaders as well for compatibility and incremental adoption.

Next Steps

If everything went according to plan, you now have a functioning Next.js application running as a single-page application. However, you aren't yet taking advantage of most of Next.js' benefits, but you can now start making incremental changes to reap all the benefits. Here's what you might want to do next:

Good to know: Using a static export does not currently support using the useParams hook.