fix(alb): alb not working when header value is not string

src/core/headers.ts

@@ -35,7 +35,7 @@ export function getFlattenedHeadersMap(
 
     if (Array.isArray(headerValue))
       commaDelimitedHeaders[newKey] = headerValue.join(separator);
-    else commaDelimitedHeaders[newKey] = headerValue || '';
+    else commaDelimitedHeaders[newKey] = String(headerValue ?? '');
   }
 
   return commaDelimitedHeaders;

test/core/headers.spec.ts

@@ -12,12 +12,14 @@ describe('getFlattenedHeadersMap', () => {
         'Accept-Language': 'en-US,en;q=0.9',
         Host: undefined,
         'Content-Type': '',
+        'Content-Length': 40 as unknown as string,
       },
       {
         'Accept-Encoding': ['gzip'],
         'Accept-Language': ['en-US', 'en;q=0.9'],
         Host: undefined,
         'Content-Type': '',
+        'Content-Length': [40] as unknown as string[],
       },
     ];
 
@@ -31,6 +33,7 @@ describe('getFlattenedHeadersMap', () => {
       expect(flattenedHeaders).toHaveProperty('Accept-Encoding');
       expect(flattenedHeaders['Accept-Encoding']).toEqual('gzip');
       expect(flattenedHeaders['Accept-Language']).toEqual('en-US,en;q=0.9');
+      expect(flattenedHeaders['Content-Length']).toEqual('40');
     }
   });
 

test/issues/alb-express-static/alb-express-static.spec.ts

@@ -0,0 +1,34 @@
+import { resolve } from 'path';
+import type { ALBResult } from 'aws-lambda';
+import express from 'express';
+import { AlbAdapter } from '../../../src/adapters/aws/index';
+import { ExpressFramework } from '../../../src/frameworks/express/index';
+import { DefaultHandler } from '../../../src/handlers/default/index';
+import { PromiseResolver } from '../../../src/resolvers/promise/index';
+import { ServerlessAdapter } from '../../../src/index';
+import { createAlbEvent } from '../../adapters/aws/utils/alb-event';
+
+describe('ALB rejecting response when uses express.static because', () => {
+  it('returns some headers that are not string', async () => {
+    const app = express();
+
+    app.use(express.static(resolve(__dirname)));
+
+    const handler = ServerlessAdapter.new(app)
+      .setHandler(new DefaultHandler())
+      .setFramework(new ExpressFramework())
+      .setResolver(new PromiseResolver())
+      .addAdapter(new AlbAdapter())
+      .build();
+
+    const albEvent = createAlbEvent('GET', '/robots.txt');
+
+    const response = (await handler(albEvent, {})) as ALBResult;
+
+    for (const header of Object.keys(response.headers || {})) {
+      expect(`typeof ${header}: ${typeof response.headers![header]}`).toBe(
+        `typeof ${header}: string`,
+      );
+    }
+  });
+});

test/issues/alb-express-static/robots.txt

@@ -0,0 +1 @@
+test