Support async html transformer function

Having an async function to transform the html allows for more complex transformations, such as inserting data fetched from a database

src/main.ts

@@ -32,7 +32,7 @@ const Config = {
     | ((path: string, req: express.Request) => boolean),
   transformer: undefined as
     | undefined
-    | ((html: string, req: express.Request) => string),
+    | ((html: string, req: express.Request) => string | Promise<string>),
 };
 
 type ConfigurationOptions = Partial<typeof Config>;
@@ -50,7 +50,7 @@ function isStaticFilePath(path: string) {
   return path.match(/(\.\w+$)|@vite|@id|@react-refresh/);
 }
 
-function getTransformedHTML(html: string, req: express.Request) {
+async function getTransformedHTML(html: string, req: express.Request) {
   return Config.transformer ? Config.transformer(html, req) : html;
 }
 
@@ -224,8 +224,15 @@ async function injectViteIndexMiddleware(
       if (indexPath === undefined) return next();
 
       const template = fs.readFileSync(indexPath, "utf8");
-      const html = await server.transformIndexHtml(req.originalUrl, template);
-      res.send(getTransformedHTML(html, req));
+      let html = await server.transformIndexHtml(req.originalUrl, template);
+      try {
+        html = await getTransformedHTML(html, req);
+        res.send(html);
+      } catch (e) {
+        console.error(e);
+        res.status(500);
+        return next();
+      }
     }
   });
 }