typescript.md

description
Next.js supports TypeScript by default and has built-in types for pages and the API. You can get started with TypeScript in Next.js here.

TypeScript

Examples

• TypeScript

Next.js provides an integrated TypeScript experience out of the box, similar to an IDE.

create-next-app support

You can create a TypeScript project with create-next-app using the --ts, --typescript flag like so:

npx create-next-app --ts
# or
yarn create next-app --typescript

Existing projects

To get started in an existing project, create an empty tsconfig.json file in the root folder:

touch tsconfig.json

Next.js will automatically configure this file with default values. Providing your own tsconfig.json with custom compiler options is also supported.

Next.js uses Babel to handle TypeScript, which has some caveats, and some compiler options are handled differently.

Then, run next (normally npm run dev or yarn dev) and Next.js will guide you through the installation of the required packages to finish the setup:

npm run dev

# You'll see instructions like these:
#
# Please install TypeScript, @types/react, and @types/node by running:
#
#         yarn add --dev typescript @types/react @types/node
#
# ...

You're now ready to start converting files from .js to .tsx and leveraging the benefits of TypeScript!.

A file named next-env.d.ts will be created in the root of your project. This file ensures Next.js types are picked up by the TypeScript compiler. You cannot remove it or edit it as it can change at any time.

TypeScript strict mode is turned off by default. When you feel comfortable with TypeScript, it's recommended to turn it on in your tsconfig.json.

Instead of editing next-env.d.ts, you can include additional types by adding a new file e.g. additional.d.ts and then referencing it in the include array in your tsconfig.json.

By default, Next.js will do type checking as part of next build. We recommend using code editor type checking during development.

If you want to silence the error reports, refer to the documentation for Ignoring TypeScript errors.

Static Generation and Server-side Rendering

For getStaticProps, getStaticPaths, and getServerSideProps, you can use the GetStaticProps, GetStaticPaths, and GetServerSideProps types respectively:

import { GetStaticProps, GetStaticPaths, GetServerSideProps } from 'next'

export const getStaticProps: GetStaticProps = async (context) => {
  // ...
}

export const getStaticPaths: GetStaticPaths = async () => {
  // ...
}

export const getServerSideProps: GetServerSideProps = async (context) => {
  // ...
}

If you're using getInitialProps, you can follow the directions on this page.

API Routes

The following is an example of how to use the built-in types for API routes:

import type { NextApiRequest, NextApiResponse } from 'next'

export default (req: NextApiRequest, res: NextApiResponse) => {
  res.status(200).json({ name: 'John Doe' })
}

You can also type the response data:

import type { NextApiRequest, NextApiResponse } from 'next'

type Data = {
  name: string
}

export default (req: NextApiRequest, res: NextApiResponse<Data>) => {
  res.status(200).json({ name: 'John Doe' })
}

Custom App

If you have a custom App, you can use the built-in type AppProps and change file name to ./pages/_app.tsx like so:

// import App from "next/app";
import type { AppProps /*, AppContext */ } from 'next/app'

function MyApp({ Component, pageProps }: AppProps) {
  return <Component {...pageProps} />
}

// Only uncomment this method if you have blocking data requirements for
// every single page in your application. This disables the ability to
// perform automatic static optimization, causing every page in your app to
// be server-side rendered.
//
// MyApp.getInitialProps = async (appContext: AppContext) => {
//   // calls page's `getInitialProps` and fills `appProps.pageProps`
//   const appProps = await App.getInitialProps(appContext);

//   return { ...appProps }
// }

export default MyApp

Path aliases and baseUrl

Next.js automatically supports the tsconfig.json "paths" and "baseUrl" options.

You can learn more about this feature on the Module Path aliases documentation.

Type checking next.config.js

The next.config.js file must be a JavaScript file as it does not get parsed by Babel or TypeScript, however you can add some type checking in your IDE using JSDoc as below:

// @ts-check

/**
 * @type {import('next').NextConfig}
 **/
const nextConfig = {
  /* config options here */
}

module.exports = nextConfig

Incremental type checking

Since v10.2.1 Next.js supports incremental type checking when enabled in your tsconfig.json, this can help speed up type checking in larger applications.

It is highly recommended to be on at least v4.3.2 of TypeScript to experience the best performance when leveraging this feature.