Remove react root condition and always use concurrent mode

packages/next/bin/next.ts

@@ -95,13 +95,6 @@ if (process.env.NODE_ENV) {
 ;(process.env as any).NODE_ENV = process.env.NODE_ENV || defaultEnv
 ;(process.env as any).NEXT_RUNTIME = 'nodejs'
 
-// In node.js runtime, react has to be required after NODE_ENV is set,
-// so that the correct dev/prod bundle could be loaded into require.cache.
-const { shouldUseReactRoot } = require('../server/utils')
-if (shouldUseReactRoot) {
-  ;(process.env as any).__NEXT_REACT_ROOT = 'true'
-}
-
 // x-ref: https://github.com/vercel/next.js/pull/34688#issuecomment-1047994505
 if (process.versions.pnp === '3') {
   const nodeVersionParts = process.versions.node

packages/next/build/index.ts

@@ -274,8 +274,6 @@ export default async function build(
       setGlobal('phase', PHASE_PRODUCTION_BUILD)
       setGlobal('distDir', distDir)
 
-      const hasReactRoot = !!process.env.__NEXT_REACT_ROOT
-
       const { target } = config
       const buildId: string = await nextBuildSpan
         .traceChild('generate-buildid')
@@ -910,7 +908,6 @@ export default async function build(
         const commonWebpackOptions = {
           buildId,
           config,
-          hasReactRoot,
           pagesDir,
           reactProductionProfiling,
           rewrites,

packages/next/build/webpack-config.ts

@@ -1,3 +1,4 @@
+import React from 'react'
 import ReactRefreshWebpackPlugin from 'next/dist/compiled/@next/react-refresh-utils/dist/ReactRefreshWebpackPlugin'
 import chalk from 'next/dist/compiled/chalk'
 import crypto from 'crypto'
@@ -69,6 +70,10 @@ const NEXT_PROJECT_ROOT_DIST_CLIENT = path.join(
   'client'
 )
 
+if (parseInt(React.version) < 18) {
+  throw new Error('Next.js requires react >= 18.2.0 to be installed.')
+}
+
 const babelIncludeRegexes: RegExp[] = [
   /next[\\/]dist[\\/](esm[\\/])?shared[\\/]lib/,
   /next[\\/]dist[\\/](esm[\\/])?client/,
@@ -161,7 +166,6 @@ export function getDefineEnv({
   distDir,
   isClient,
   hasRewrites,
-  hasReactRoot,
   isNodeServer,
   isEdgeServer,
   middlewareMatchers,
@@ -170,7 +174,6 @@ export function getDefineEnv({
   distDir: string
   isClient?: boolean
   hasRewrites?: boolean
-  hasReactRoot?: boolean
   isNodeServer?: boolean
   isEdgeServer?: boolean
   middlewareMatchers?: MiddlewareMatcher[]
@@ -255,7 +258,6 @@ export function getDefineEnv({
           : false
         : config.reactStrictMode
     ),
-    'process.env.__NEXT_REACT_ROOT': JSON.stringify(hasReactRoot),
     'process.env.__NEXT_OPTIMIZE_FONTS': JSON.stringify(
       !dev && config.optimizeFonts
     ),
@@ -569,7 +571,6 @@ export default async function getBaseWebpackConfig(
     compilerType,
     dev = false,
     entrypoints,
-    hasReactRoot,
     isDevFallback = false,
     pagesDir,
     reactProductionProfiling = false,
@@ -584,7 +585,6 @@ export default async function getBaseWebpackConfig(
     compilerType: CompilerNameValues
     dev?: boolean
     entrypoints: webpack.EntryObject
-    hasReactRoot: boolean
     isDevFallback?: boolean
     pagesDir?: string
     reactProductionProfiling?: boolean
@@ -608,19 +608,8 @@ export default async function getBaseWebpackConfig(
     rewrites.fallback.length > 0
 
   const hasAppDir = !!config.experimental.appDir && !!appDir
-  const hasConcurrentFeatures = hasReactRoot
   const hasServerComponents = hasAppDir
-
-  // Only error in first one compiler (client) once
-  if (isClient) {
-    if (!hasReactRoot) {
-      throw new Error('Next.js requires React 18.2.0 to be installed.')
-    }
-  }
-
-  const disableOptimizedLoading = hasConcurrentFeatures
-    ? true
-    : config.experimental.disableOptimizedLoading
+  const disableOptimizedLoading = true
 
   if (isClient) {
     if (config.experimental.runtime === SERVER_RUNTIME.edge) {
@@ -1973,7 +1962,6 @@ export default async function getBaseWebpackConfig(
           distDir,
           isClient,
           hasRewrites,
-          hasReactRoot,
           isNodeServer,
           isEdgeServer,
           middlewareMatchers,
@@ -1983,7 +1971,7 @@ export default async function getBaseWebpackConfig(
         new ReactLoadablePlugin({
           filename: REACT_LOADABLE_MANIFEST,
           pagesDir,
-          runtimeAsset: hasConcurrentFeatures
+          runtimeAsset: true
             ? `server/${MIDDLEWARE_REACT_LOADABLE_MANIFEST}.js`
             : undefined,
           dev,
@@ -2056,7 +2044,7 @@ export default async function getBaseWebpackConfig(
           buildId,
           rewrites,
           isDevFallback,
-          exportRuntime: hasConcurrentFeatures,
+          exportRuntime: true,
           appDirEnabled: hasAppDir,
         }),
       new ProfilingPlugin({ runWebpackSpan }),

packages/next/client/image.tsx

@@ -794,19 +794,13 @@ export default function Image({
     }
   }
 
-  let imageSrcSetPropName = 'imagesrcset'
-  let imageSizesPropName = 'imagesizes'
-  if (process.env.__NEXT_REACT_ROOT) {
-    imageSrcSetPropName = 'imageSrcSet'
-    imageSizesPropName = 'imageSizes'
-  }
   const linkProps: React.DetailedHTMLProps<
     React.LinkHTMLAttributes<HTMLLinkElement>,
     HTMLLinkElement
   > = {
-    // Note: imagesrcset and imagesizes are not in the link element type with react 17.
-    [imageSrcSetPropName]: imgAttributes.srcSet,
-    [imageSizesPropName]: imgAttributes.sizes,
+    // @ts-expect-error upgrade react types to react 18
+    imageSrcSet: imgAttributes.srcSet,
+    imageSizes: imgAttributes.sizes,
     crossOrigin: rest.crossOrigin,
   }
 

packages/next/client/index.tsx

@@ -1,10 +1,12 @@
 /* global location */
 import '../build/polyfills/polyfill-module'
+import type Router from '../shared/lib/router/router'
 import React from 'react'
+// @ts-expect-error upgrade react types to react 18
+import ReactDOM from 'react-dom/client'
 import { HeadManagerContext } from '../shared/lib/head-manager-context'
 import mitt, { MittEmitter } from '../shared/lib/mitt'
 import { RouterContext } from '../shared/lib/router-context'
-import type Router from '../shared/lib/router/router'
 import {
   AppComponent,
   AppProps,
@@ -35,10 +37,6 @@ import { ImageConfigComplete } from '../shared/lib/image-config'
 import { removeBasePath } from './remove-base-path'
 import { hasBasePath } from './has-base-path'
 
-const ReactDOM = process.env.__NEXT_REACT_ROOT
-  ? require('react-dom/client')
-  : require('react-dom')
-
 /// <reference types="react-dom/experimental" />
 
 declare let __webpack_public_path__: string
@@ -492,26 +490,16 @@ function renderReactElement(
   }
 
   const reactEl = fn(shouldHydrate ? markHydrateComplete : markRenderComplete)
-  if (process.env.__NEXT_REACT_ROOT) {
-    if (!reactRoot) {
-      // Unlike with createRoot, you don't need a separate root.render() call here
-      reactRoot = ReactDOM.hydrateRoot(domEl, reactEl)
-      // TODO: Remove shouldHydrate variable when React 18 is stable as it can depend on `reactRoot` existing
-      shouldHydrate = false
-    } else {
-      const startTransition = (React as any).startTransition
-      startTransition(() => {
-        reactRoot.render(reactEl)
-      })
-    }
+  if (!reactRoot) {
+    // Unlike with createRoot, you don't need a separate root.render() call here
+    reactRoot = ReactDOM.hydrateRoot(domEl, reactEl)
+    // TODO: Remove shouldHydrate variable when React 18 is stable as it can depend on `reactRoot` existing
+    shouldHydrate = false
   } else {
-    // The check for `.hydrate` is there to support React alternatives like preact
-    if (shouldHydrate) {
-      ReactDOM.hydrate(reactEl, domEl)
-      shouldHydrate = false
-    } else {
-      ReactDOM.render(reactEl, domEl)
-    }
+    const startTransition = (React as any).startTransition
+    startTransition(() => {
+      reactRoot.render(reactEl)
+    })
   }
 }
 

packages/next/client/legacy/image.tsx

@@ -969,19 +969,13 @@ export default function Image({
     }
   }
 
-  let imageSrcSetPropName = 'imagesrcset'
-  let imageSizesPropName = 'imagesizes'
-  if (process.env.__NEXT_REACT_ROOT) {
-    imageSrcSetPropName = 'imageSrcSet'
-    imageSizesPropName = 'imageSizes'
-  }
   const linkProps: React.DetailedHTMLProps<
     React.LinkHTMLAttributes<HTMLLinkElement>,
     HTMLLinkElement
   > = {
-    // Note: imagesrcset and imagesizes are not in the link element type with react 17.
-    [imageSrcSetPropName]: imgAttributes.srcSet,
-    [imageSizesPropName]: imgAttributes.sizes,
+    // @ts-expect-error upgrade react types to react 18
+    imageSrcSet: imgAttributes.srcSet,
+    imageSizes: imgAttributes.sizes,
     crossOrigin: rest.crossOrigin,
   }
 

packages/next/package.json

@@ -84,8 +84,8 @@
   "peerDependencies": {
     "fibers": ">= 3.1.0",
     "node-sass": "^6.0.0 || ^7.0.0",
-    "react": "^18.0.0-0",
-    "react-dom": "^18.0.0-0",
+    "react": "^18.2.0",
+    "react-dom": "^18.2.0",
     "sass": "^1.3.0"
   },
   "peerDependenciesMeta": {

packages/next/server/app-render.tsx

@@ -23,7 +23,6 @@ import {
   streamToString,
 } from './node-web-streams-helper'
 import { ESCAPE_REGEX, htmlEscapeJsonString } from './htmlescape'
-import { shouldUseReactRoot } from './utils'
 import { matchSegment } from '../client/components/match-segments'
 import {
   FlightCSSManifest,
@@ -671,7 +670,6 @@ function headersWithoutFlight(headers: IncomingHttpHeaders) {
 }
 
 async function renderToString(element: React.ReactElement) {
-  if (!shouldUseReactRoot) return ReactDOMServer.renderToString(element)
   const renderStream = await ReactDOMServer.renderToReadableStream(element)
   await renderStream.allReady
   return streamToString(renderStream)

packages/next/server/base-server.ts

@@ -1150,10 +1150,8 @@ export default abstract class Server<ServerOptions extends Options = Options> {
       const isBotRequest = isBot(req.headers['user-agent'] || '')
       const isSupportedDocument =
         typeof components.Document?.getInitialProps !== 'function' ||
-        // When concurrent features is enabled, the built-in `Document`
-        // component also supports dynamic HTML.
-        (!!process.env.__NEXT_REACT_ROOT &&
-          NEXT_BUILTIN_DOCUMENT in components.Document)
+        // The built-in `Document` component also supports dynamic HTML for concurrent mode.
+        NEXT_BUILTIN_DOCUMENT in components.Document
 
       // Disable dynamic HTML in cases that we know it won't be generated,
       // so that we can continue generating a cache key when possible.

packages/next/server/dev/hot-reloader.ts

@@ -163,7 +163,6 @@ export default class HotReloader {
   private webpackHotMiddleware?: WebpackHotMiddleware
   private config: NextConfigComplete
   public hasServerComponents: boolean
-  public hasReactRoot: boolean
   public clientStats: webpack.Stats | null
   public serverStats: webpack.Stats | null
   public edgeServerStats: webpack.Stats | null
@@ -216,8 +215,7 @@ export default class HotReloader {
     this.serverPrevDocumentHash = null
 
     this.config = config
-    this.hasReactRoot = !!process.env.__NEXT_REACT_ROOT
-    this.hasServerComponents = this.hasReactRoot && !!this.appDir
+    this.hasServerComponents = !!this.appDir
     this.previewProps = previewProps
     this.rewrites = rewrites
     this.hotReloaderSpan = trace('hot-reloader', undefined, {
@@ -456,7 +454,6 @@ export default class HotReloader {
         dev: true,
         buildId: this.buildId,
         config: this.config,
-        hasReactRoot: this.hasReactRoot,
         pagesDir: this.pagesDir,
         rewrites: this.rewrites,
         runWebpackSpan: this.hotReloaderSpan,
@@ -521,7 +518,6 @@ export default class HotReloader {
           pageExtensions: this.config.pageExtensions,
         })
       ).client,
-      hasReactRoot: this.hasReactRoot,
     })
     const fallbackCompiler = webpack(fallbackConfig)
 

packages/next/server/dev/next-dev-server.ts

@@ -547,7 +547,6 @@ export default class DevServer extends Server {
                     distDir: this.distDir,
                     isClient,
                     hasRewrites,
-                    hasReactRoot: this.hotReloader?.hasReactRoot,
                     isNodeServer,
                     isEdgeServer,
                   })

packages/next/server/next-server.ts

@@ -94,15 +94,10 @@ import { removeTrailingSlash } from '../shared/lib/router/utils/remove-trailing-
 import { getNextPathnameInfo } from '../shared/lib/router/utils/get-next-pathname-info'
 import { getClonableBody } from './body-streams'
 import { checkIsManualRevalidate } from './api-utils'
-import { shouldUseReactRoot } from './utils'
 import ResponseCache from './response-cache'
 import { IncrementalCache } from './lib/incremental-cache'
 import { normalizeAppPath } from '../shared/lib/router/utils/app-paths'
 
-if (shouldUseReactRoot) {
-  ;(process.env as any).__NEXT_REACT_ROOT = 'true'
-}
-
 import { renderToHTMLOrFlight as appRenderToHTMLOrFlight } from './app-render'
 
 export * from './base-server'

packages/next/server/next.ts

@@ -11,7 +11,6 @@ import { PHASE_DEVELOPMENT_SERVER } from '../shared/lib/constants'
 import { PHASE_PRODUCTION_SERVER } from '../shared/lib/constants'
 import { IncomingMessage, ServerResponse } from 'http'
 import { NextUrlWithParsedQuery } from './request-meta'
-import { shouldUseReactRoot } from './utils'
 import {
   loadRequireHook,
   overrideBuiltInReactPackages,
@@ -211,10 +210,6 @@ function createServer(options: NextServerOptions): NextServer {
     )
   }
 
-  if (shouldUseReactRoot) {
-    ;(process.env as any).__NEXT_REACT_ROOT = 'true'
-  }
-
   return new NextServer(options)
 }