Make sure Next has basePath before remounting baseUrl

server/Server.js

@@ -46,6 +46,18 @@ const passport = require('passport')
 const expressSession = require('express-session')
 const MemoryStore = require('./libs/memorystore')
 
+/**
+ * `next()` for a custom server is a NextCustomServer wrapper. After prepare(),
+ * `.server` is another NextServer wrapper; the Node server with `nextConfig` is
+ * one more `.server` down. Older react clients have no basePath and must keep
+ * Express-stripped URLs.
+ */
+function getPreparedNextBasePath(nextApp) {
+  const nodeServer = nextApp.server?.server
+  const basePath = nodeServer?.nextConfig?.basePath
+  return typeof basePath === 'string' ? basePath : ''
+}
+
 class Server {
   constructor(SOURCE, PORT, HOST, CONFIG_PATH, METADATA_PATH, ROUTER_BASE_PATH) {
     this.Port = PORT
@@ -415,11 +427,18 @@ class Server {
       const nextApp = next({ dev: Logger.isDev, dir: ReactClientPath })
       const handle = nextApp.getRequestHandler()
       await nextApp.prepare()
+      const nextBasePath = getPreparedNextBasePath(nextApp)
+      if (nextBasePath) {
+        Logger.info(`[Server] Next client basePath is "${nextBasePath}"`)
+      } else {
+        Logger.info(`[Server] Next client has no basePath; leaving Express-stripped URLs as-is`)
+      }
       router.all('*', (req, res) => {
         // Next is configured with the same base path as this router, but Express strips the mount
         // prefix from req.url, so put it back. The bare mount path is passed through without a
         // trailing slash, which Next would otherwise redirect away from on every request.
-        if (req.baseUrl) {
+        // Skip when Next has no basePath (older client on master).
+        if (nextBasePath && req.baseUrl) {
           const queryIndex = req.url.indexOf('?')
           const pathname = queryIndex === -1 ? req.url : req.url.slice(0, queryIndex)
           const search = queryIndex === -1 ? '' : req.url.slice(queryIndex)