Fixes #970 by adding middleware dev config option. (#1062)

Co-authored-by: Josh Wilson <[email protected]>

snowpack/src/commands/dev.ts

@@ -881,7 +881,15 @@ ${err}`);
 
   const server = createServer(async (req, res) => {
     try {
-      return await requestHandler(req, res);
+      // Allow users to supply Express-style middleware (or a full Express app) to handle requests
+      // before Snowpack receives them. Snowpack will handle anything the middleware doesn't.
+      const middleware = config.devOptions.middleware;
+      if (typeof middleware === 'function') {
+        const next = () => requestHandler(req, res);
+        middleware(req, res, next);
+      } else {
+        await requestHandler(req, res);
+      }
     } catch (err) {
       logger.error(`[500] ${req.url}`);
       logger.error(err.toString() || err);

snowpack/src/types/snowpack.ts

@@ -1,4 +1,5 @@
 import type HttpProxy from 'http-proxy';
+import type * as http from 'http';
 import type {InstallOptions} from 'esinstall';
 
 export type DeepPartial<T> = {
@@ -129,6 +130,7 @@ export interface SnowpackConfig {
     fallback: string;
     open: string;
     hmr?: boolean;
+    middleware?: (req: http.IncomingMessage, res: http.ServerResponse, next: () => void) => unknown;
     hmrDelay: number;
     hmrPort: number;
   };