Transform/Manipulate index html before sending it

[Jonas Kugelmann]

Is there an existing issue that is already proposing this?

• I have searched the existing issues

Is your feature request related to a problem? Please describe it

Expose a function to transform the index.html before sending it to the client to inject some variables or replace certain parts, without setting up a complete view engine setup.

Describe the solution you'd like

It would be great to expose a function transformIndexHtml?: (indexHtml: string)=> string | Promise<string>
With this function any transform can be handled.
Here is the diff file I use to patch-package this feature in our code base:

diff --git a/node_modules/@nestjs/serve-static/dist/interfaces/serve-static-options.interface.d.ts b/node_modules/@nestjs/serve-static/dist/interfaces/serve-static-options.interface.d.ts
index a996aff..e1dd974 100644
--- a/node_modules/@nestjs/serve-static/dist/interfaces/serve-static-options.interface.d.ts
+++ b/node_modules/@nestjs/serve-static/dist/interfaces/serve-static-options.interface.d.ts
@@ -5,6 +5,7 @@ export interface ServeStaticModuleOptions {
     renderPath?: string | RegExp;
     serveRoot?: string;
     exclude?: string[];
+    transformIndexHtml?: (indexHtml: string) => string;
     serveStaticOptions?: {
         cacheControl?: boolean;
         dotfiles?: string;
diff --git a/node_modules/@nestjs/serve-static/dist/loaders/express.loader.js b/node_modules/@nestjs/serve-static/dist/loaders/express.loader.js
index 75c9f9b..d447522 100644
--- a/node_modules/@nestjs/serve-static/dist/loaders/express.loader.js
+++ b/node_modules/@nestjs/serve-static/dist/loaders/express.loader.js
@@ -29,7 +29,19 @@ let ExpressLoader = class ExpressLoader extends abstract_loader_1.AbstractLoader
                         const stat = fs.statSync(indexFilePath);
                         options.serveStaticOptions.setHeaders(res, indexFilePath, stat);
                     }
-                    res.sendFile(indexFilePath);
+                    if(options.transformIndexHtml != null) {
+                      fs.readFile(indexFilePath, 'utf8', (err, data) => {
+                        if (err) {
+                            next(err);
+                            return;
+                        }
+                        const transformed = options.transformIndexHtml(data);
+                        res.set('Content-Type', 'text/html');
+                        res.send(transformed);
+                      })
+                    } else {
+                     res.sendFile(indexFilePath);
+                    }
                 }
                 else {
                     next();

Teachability, documentation, adoption, migration strategy

No response

What is the motivation / use case for changing the behavior?

We want to inject a variables into the index.html before sending it to the client. In our case we want to replace the <base href="/"/> with the correct baseUrl in case the application runs on a pathname e.g. http://localhost:3001/app. Additionally we want to set a variable on the window object. We don't want to setup the whole view engine and install something like ejs to do these simple transformations.