Add experimental config for basePath testing

packages/next/build/webpack-config.ts

@@ -1408,6 +1408,9 @@ export default async function getBaseWebpackConfig(
             isEdgeServer ? 'edge' : 'nodejs'
           ),
         }),
+        'process.env.__NEXT_MANUAL_CLIENT_BASE_PATH': JSON.stringify(
+          config.experimental.manualClientBasePath
+        ),
         'process.env.__NEXT_NEW_LINK_BEHAVIOR': JSON.stringify(
           config.experimental.newNextLinkBehavior
         ),

packages/next/client/page-loader.ts

@@ -156,7 +156,8 @@ export default class PageLoader {
         '.json'
       )
       return addBasePath(
-        `/_next/data/${this.buildId}${dataRoute}${ssg ? '' : search}`
+        `/_next/data/${this.buildId}${dataRoute}${ssg ? '' : search}`,
+        true
       )
     }
 

packages/next/server/config-shared.ts

@@ -79,6 +79,7 @@ export interface NextJsWebpackConfig {
 }
 
 export interface ExperimentalConfig {
+  manualClientBasePath?: boolean
   newNextLinkBehavior?: boolean
   disablePostcssPresetEnv?: boolean
   swcMinify?: boolean

packages/next/shared/lib/router/router.ts

@@ -205,12 +205,23 @@ export function hasBasePath(path: string): boolean {
   return hasPathPrefix(path, basePath)
 }
 
-export function addBasePath(path: string): string {
+export function addBasePath(path: string, required?: boolean): string {
+  if (process.env.__NEXT_MANUAL_CLIENT_BASE_PATH) {
+    if (!required) {
+      return path
+    }
+  }
   // we only add the basepath on relative urls
   return addPathPrefix(path, basePath)
 }
 
 export function delBasePath(path: string): string {
+  if (process.env.__NEXT_MANUAL_CLIENT_BASE_PATH) {
+    if (!hasBasePath(path)) {
+      return path
+    }
+  }
+
   path = path.slice(basePath.length)
   if (!path.startsWith('/')) path = `/${path}`
   return path
@@ -1120,7 +1131,7 @@ export default class Router implements BaseRouter {
 
       if (process.env.__NEXT_HAS_REWRITES && as.startsWith('/')) {
         const rewritesResult = resolveRewrites(
-          addBasePath(addLocale(cleanedAs, nextState.locale)),
+          addBasePath(addLocale(cleanedAs, nextState.locale), true),
           pages,
           rewrites,
           query,
@@ -1747,7 +1758,7 @@ export default class Router implements BaseRouter {
       ;({ __rewrites: rewrites } = await getClientBuildManifest())
 
       const rewritesResult = resolveRewrites(
-        addBasePath(addLocale(asPath, this.locale)),
+        addBasePath(addLocale(asPath, this.locale), true),
         pages,
         rewrites,
         parsed.query,

test/e2e/manual-client-base-path/app/next.config.js

@@ -0,0 +1,6 @@
+module.exports = {
+  basePath: '/docs-proxy',
+  experimental: {
+    manualClientBasePath: true,
+  },
+}

test/e2e/manual-client-base-path/app/pages/another.js

@@ -0,0 +1,50 @@
+import Link from 'next/link'
+import { useRouter } from 'next/router'
+import { useEffect, useState } from 'react'
+
+export default function Page(props) {
+  const router = useRouter()
+  const [mounted, setMounted] = useState(false)
+
+  useEffect(() => {
+    setMounted(true)
+  }, [])
+
+  return (
+    <>
+      <p id="page">another page</p>
+      <p id="props">{JSON.stringify(props)}</p>
+      <p id="router">
+        {JSON.stringify(
+          mounted
+            ? {
+                basePath: router.basePath,
+                pathname: router.pathname,
+                asPath: router.asPath,
+                query: router.query,
+              }
+            : {}
+        )}
+      </p>
+
+      <Link href="/">
+        <a id="to-index">to /index</a>
+      </Link>
+      <br />
+
+      <Link href="/dynamic/first">
+        <a id="to-dynamic">to /dynamic/first</a>
+      </Link>
+      <br />
+    </>
+  )
+}
+
+export function getServerSideProps() {
+  return {
+    props: {
+      hello: 'world',
+      now: Date.now(),
+    },
+  }
+}

test/e2e/manual-client-base-path/app/pages/dynamic/[slug].js

@@ -0,0 +1,58 @@
+import Link from 'next/link'
+import { useRouter } from 'next/router'
+import { useEffect, useState } from 'react'
+
+export default function Page(props) {
+  const router = useRouter()
+  const [mounted, setMounted] = useState(false)
+
+  useEffect(() => {
+    setMounted(true)
+  }, [])
+
+  return (
+    <>
+      <p id="page">dynamic page</p>
+      <p id="props">{JSON.stringify(props)}</p>
+      <p id="router">
+        {JSON.stringify(
+          mounted
+            ? {
+                basePath: router.basePath,
+                pathname: router.pathname,
+                asPath: router.asPath,
+                query: router.query,
+              }
+            : {}
+        )}
+      </p>
+
+      <Link href="/">
+        <a id="to-index">to /index</a>
+      </Link>
+      <br />
+
+      <Link href="/dynamic/second">
+        <a id="to-dynamic">to /dynamic/second</a>
+      </Link>
+      <br />
+    </>
+  )
+}
+
+export function getStaticPaths() {
+  return {
+    paths: ['/dynamic/first'],
+    fallback: true,
+  }
+}
+
+export function getStaticProps({ params }) {
+  return {
+    props: {
+      params,
+      hello: 'world',
+      now: Date.now(),
+    },
+  }
+}

test/e2e/manual-client-base-path/app/pages/index.js

@@ -0,0 +1,41 @@
+import Link from 'next/link'
+import { useRouter } from 'next/router'
+import { useEffect, useState } from 'react'
+
+export default function Page(props) {
+  const router = useRouter()
+  const [mounted, setMounted] = useState(false)
+
+  useEffect(() => {
+    setMounted(true)
+  }, [])
+
+  return (
+    <>
+      <p id="page">index page</p>
+      <p id="props">{JSON.stringify(props)}</p>
+      <p id="router">
+        {JSON.stringify(
+          mounted
+            ? {
+                basePath: router.basePath,
+                pathname: router.pathname,
+                asPath: router.asPath,
+                query: router.query,
+              }
+            : {}
+        )}
+      </p>
+
+      <Link href="/another">
+        <a id="to-another">to /another</a>
+      </Link>
+      <br />
+
+      <Link href="/dynamic/first">
+        <a id="to-dynamic">to /dynamic/first</a>
+      </Link>
+      <br />
+    </>
+  )
+}