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.

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

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) 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, however, you can edit it (but you don't need to).

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

By default, Next.js reports TypeScript errors during development for pages you are actively working on. TypeScript errors for inactive pages do not block the development process.

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

Pages

For function components the NextPage type is exported, here's how to use it:

import { NextPage } from 'next'

interface Props {
  userAgent?: string;
}

const Page: NextPage<Props> = ({ userAgent }) => (
  <main>Your user agent: {userAgent}</main>
)

Page.getInitialProps = async ({ req }) => {
  const userAgent = req ? req.headers['user-agent'] : navigator.userAgent
  return { userAgent }
}

export default Page

And for React.Component you can use NextPageContext:

import React from 'react'
import { NextPageContext } from 'next'

interface Props {
  userAgent?: string;
}

export default class Page extends React.Component<Props> {
  static async getInitialProps({ req }: NextPageContext) {
    const userAgent = req ? req.headers['user-agent'] : navigator.userAgent
    return { userAgent }
  }

  render() {
    const { userAgent } = this.props
    return <main>Your user agent: {userAgent}</main>
  }
}

API Routes

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

import { 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 { NextApiRequest, NextApiResponse } from 'next'

type Data = {
  name: string
}

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

Custom App using _app.tsx

If you’d like to customize the App component in TypeScript, create pages/_app.tsx:

import { AppProps } from 'next/app'

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

export default MyApp