🔧 fix: merge set.headers without duplicating Response

src/adapter/utils.ts

@@ -338,6 +338,7 @@ export const createResponseHandler = (handler: CreateHandlerParameter) => {
 	return (response: Response, set: Context['set'], request?: Request) => {
 		let isCookieSet = false
 
+		// Merge headers: Response headers take precedence, set.headers fill in non-conflicting ones
 		if (set.headers instanceof Headers)
 			for (const key of set.headers.keys()) {
 				if (key === 'set-cookie') {
@@ -347,14 +348,21 @@ export const createResponseHandler = (handler: CreateHandlerParameter) => {
 
 					for (const cookie of set.headers.getSetCookie())
 						response.headers.append('set-cookie', cookie)
-				} else response.headers.append(key, set.headers?.get(key) ?? '')
+				} else if (!response.headers.has(key))
+					response.headers.set(key, set.headers?.get(key) ?? '')
 			}
 		else
 			for (const key in set.headers)
-				(response as Response).headers.append(
-					key,
-					set.headers[key] as any
-				)
+				if (key === 'set-cookie')
+					(response as Response).headers.append(
+						key,
+						set.headers[key] as any
+					)
+				else if (!response.headers.has(key))
+					(response as Response).headers.set(
+						key,
+						set.headers[key] as any
+					)
 
 		const status = set.status ?? 200
 

test/response/custom-response.test.ts

@@ -36,4 +36,29 @@ describe('Custom Response Type', () => {
 
         expect(await response.text()).toBe('Shuba Shuba')
     })
+
+    it('Response headers take precedence, set.headers merge non-conflicting', async () => {
+        const app = new Elysia()
+            .onRequest(({ set }) => {
+                set.headers['Content-Type'] = 'application/json'
+                set.headers['X-Framework'] = 'Elysia'
+            })
+            .get('/', () => {
+                return new Response('{"message":"hello"}', {
+                    headers: {
+                        'Content-Type': 'text/plain',
+                        'X-Custom': 'custom-value'
+                    }
+                })
+            })
+
+        const response = await app.handle(req('/'))
+
+        // Response's Content-Type takes precedence
+        expect(response.headers.get('Content-Type')).toBe('text/plain')
+        // set.headers adds non-conflicting headers
+        expect(response.headers.get('X-Framework')).toBe('Elysia')
+        // Response's own headers are preserved
+        expect(response.headers.get('X-Custom')).toBe('custom-value')
+    })
 })