fix(router): handle rejected promise in custom router

src/http-proxy-middleware.ts

@@ -49,8 +49,12 @@ export class HttpProxyMiddleware {
     next: express.NextFunction
   ) => {
     if (this.shouldProxy(this.config.context, req)) {
-      const activeProxyOptions = await this.prepareProxyRequest(req);
-      this.proxy.web(req, res, activeProxyOptions);
+      try {
+        const activeProxyOptions = await this.prepareProxyRequest(req);
+        this.proxy.web(req, res, activeProxyOptions);
+      } catch (err) {
+        next(err);
+      }
     } else {
       next();
     }

test/e2e/router.spec.ts

@@ -1,4 +1,5 @@
 import { createProxyMiddleware, createApp, createAppWithPath } from './_utils';
+import { ErrorRequestHandler } from 'express';
 import * as request from 'supertest';
 import { getLocal, generateCACertificate, Mockttp } from 'mockttp';
 
@@ -100,6 +101,27 @@ describe('E2E router', () => {
       expect(response.text).toBe('C');
     });
 
+    it('should handle promise rejection in router', async () => {
+      const app = createApp(
+        createProxyMiddleware({
+          target: 'https://localhost:6001',
+          secure: false,
+          changeOrigin: true,
+          router: async req => {
+            throw new Error('An error thrown in the router');
+          }
+        })
+      );
+      const errorHandler: ErrorRequestHandler = (err: Error, req, res, next) => {
+        res.status(502).send(err.message);
+      };
+      app.use(errorHandler);
+
+      const agent = request(app);
+      const response = await agent.get('/api').expect(502);
+      expect(response.text).toBe('An error thrown in the router');
+    });
+
     it('missing a : will cause it to use http', async () => {
       const app = createApp(
         createProxyMiddleware({