Fix res.render using the outer app's views for mounted sub-apps

src/response.js

@@ -679,7 +679,8 @@ module.exports = class Response extends Writable {
             this.send(str);
         });
 
-        this.app.render(view, options, done);
+        // use req.app like express does, so mounted sub-apps resolve views with their own settings
+        this.req.app.render(view, options, done);
     }
     cookie(name, value, options) {
         const opt = {...(options ?? {})}; // create a new ref because we change original object (https://github.com/dimdenGD/ultimate-express/issues/68)

src/router.js

@@ -241,7 +241,9 @@ module.exports = class Router extends EventEmitter {
                             [...chainPrefix, ...pathToMount, {
                                 ...route,
                                 callbacks: [],
-                                keepMount: true
+                                keepMount: true,
+                                // mounted sub-apps become req.app during their dispatch, like express
+                                mountApp: route.callbacks[0].constructor.name === 'Application' ? route.callbacks[0] : undefined
                             }]
                         );
                     }
@@ -495,6 +497,7 @@ module.exports = class Router extends EventEmitter {
                 return false;
             }
             // on optimized routes, there can be more routes, so we have to use unoptimized routing and skip until we find route we stopped at
+            req.app = this; // restore app in case the optimized path swapped it to a mounted sub-app
             return this._routeRequest(req, res, 0, this._routes, false, skipUntil);
         }
         let callbackindex = 0;
@@ -507,6 +510,11 @@ module.exports = class Router extends EventEmitter {
 
         const strictRouting = this.get('strict routing');
         if(route.use) {
+            if(route.mountApp) {
+                // optimized chain: normal dispatch swaps req.app when it enters a mounted Application,
+                // but the compiled mount route has no callback to do it, so swap it here
+                req.app = route.mountApp;
+            }
             req._stack.push(route.path);
             const fullMountpath = this.getFullMountpath(req);
             req._opPath = fullMountpath !== EMPTY_REGEX ? req._originalPath.replace(fullMountpath, '') : req._originalPath;

tests/parts/subapp/index.ejs

@@ -0,0 +1,2 @@
+<h1>subapp <%= message %></h1>
+<span><%= asdf %></span>

tests/tests/res/res-render-subapp.js

@@ -0,0 +1,50 @@
+// res.render must use the views and engine of the sub-app handling the request
+
+const express = require("express");
+
+const app = express();
+app.set('view engine', 'ejs');
+app.set('views', 'tests/parts');
+app.set('env', 'production');
+
+const subApp = express();
+subApp.set('view engine', 'ejs');
+subApp.set('views', 'tests/parts/subapp');
+subApp.set('env', 'production');
+
+subApp.get('/page', (req, res) => {
+    res.locals.asdf = 'locals test';
+    res.render('index.ejs', { message: 'from subapp' });
+});
+
+subApp.get('/fallthrough', (req, res, next) => {
+    next();
+});
+
+app.use('/sub', subApp);
+
+app.use((req, res) => {
+    res.locals.asdf = 'locals test';
+    res.render('index.ejs', { title: 'Outer', message: 'from outer' });
+});
+
+app.use((err, req, res, next) => {
+    console.log(err.message.split('\n')[0]);
+    res.status(500).send('whoops!');
+});
+
+app.listen(13333, async () => {
+    console.log('Server is running on port 13333');
+
+    // before the router optimization kicks in
+    console.log(await fetch('http://localhost:13333/sub/page').then(res => res.text()));
+    console.log(await fetch('http://localhost:13333/sub/fallthrough').then(res => res.text()));
+
+    await new Promise(resolve => setTimeout(resolve, 1000));
+
+    // after the router optimization kicks in
+    console.log(await fetch('http://localhost:13333/sub/page').then(res => res.text()));
+    console.log(await fetch('http://localhost:13333/sub/fallthrough').then(res => res.text()));
+
+    process.exit(0);
+});