React 17 introduced a new [JSX Transform](https://reactjs.org/blog/2020/09/22/introducing-the-new-jsx-transform.html) that brings a long-time Next.js feature to the wider React ecosystem: Not having to `import React from 'react'` when using JSX. When using React 17 Next.js will automatically use the new transform. This transform does not make the `React` variable global, which was an unintended side-effect of the previous Next.js implementation. A [codemod is available](/docs/advanced-features/codemods#add-missing-react-import) to automatically fix cases where you accidentally used `React` without importing it.
If you previously configured `routes` in your `vercel.json` file for dynamic routes, these rules can be removed when leveraging Next.js 9's new [Dynamic Routing feature](/docs/routing/dynamic-routes.md).
#### `@zeit/next-typescript` is no longer necessary
Next.js will now ignore usage `@zeit/next-typescript` and warn you to remove it. Please remove this plugin from your `next.config.js`.
Remove references to `@zeit/next-typescript/babel` from your custom `.babelrc` (if present).
Usage of [`fork-ts-checker-webpack-plugin`](https://github.com/Realytics/fork-ts-checker-webpack-plugin/issues) should also be removed from your `next.config.js`.
TypeScript Definitions are published with the `next` package, so you need to uninstall `@types/next` as they would conflict.
The following types are different:
> This list was created by the community to help you upgrade, if you find other differences please send a pull-request to this list to help other users.
From:
```tsx
import { NextContext } from 'next'
import { NextAppContext, DefaultAppIProps } from 'next/app'
import { NextDocumentContext, DefaultDocumentIProps } from 'next/document'
```
to
```tsx
import { NextPageContext } from 'next'
import { AppContext, AppInitialProps } from 'next/app'
import { DocumentContext, DocumentInitialProps } from 'next/document'
```
#### The `config` key is now a special export on a page
You may no longer export a custom variable named `config` from a page (i.e. `export { config }` / `export const config ...`).
This exported variable is now used to specify page-level Next.js configuration like Opt-in AMP and API Route features.
You must rename a non-Next.js-purposed `config` export to something different.
#### `next/dynamic` no longer renders "loading..." by default while loading
Dynamic components will not render anything by default while loading. You can still customize this behavior by setting the `loading` property:
Pages in `./pages/api/` are now considered [API Routes](https://nextjs.org/blog/next-9#api-routes).
Pages in this directory will no longer contain a client-side bundle.
## Deprecated Features
#### `next/dynamic` has deprecated loading multiple modules at once
The ability to load multiple modules at once has been deprecated in `next/dynamic` to be closer to React's implementation (`React.lazy` and `Suspense`).
Updating code that relies on this behavior is relatively straightforward! We've provided an example of a before/after to help you migrate your application: