fix: preserve multiple set-cookie headers

src/utils.ts

@@ -92,27 +92,50 @@ export function mergeHeaders(
 	responseHeaders: Headers,
 	setHeaders: Context['set']['headers']
 ) {
-	// @ts-ignore
-	const headers = new Headers(Object.fromEntries(responseHeaders.entries()))
+	const headers = new Headers()
+
+	for (const [key, value] of responseHeaders.entries()) {
+		if (key === 'set-cookie') continue
+		headers.append(key, value)
+	}
+
+	for (const cookie of responseHeaders.getSetCookie()) {
+		headers.append('set-cookie', cookie)
+	}
 
 	// Merge headers: Response headers take precedence, set.headers fill in non-conflicting ones
-	if (setHeaders instanceof Headers)
+	if (setHeaders instanceof Headers) {
 		// @ts-ignore
 		for (const key of setHeaders.keys()) {
 			if (key === 'set-cookie') {
 				if (headers.has('set-cookie')) continue
 
-				for (const cookie of setHeaders.getSetCookie())
+				for (const cookie of setHeaders.getSetCookie()) {
 					headers.append('set-cookie', cookie)
-			} else if (!responseHeaders.has(key))
+				}
+			} else if (!responseHeaders.has(key)) {
 				headers.set(key, setHeaders?.get(key) ?? '')
+			}
 		}
-	else
-		for (const key in setHeaders)
-			if (key === 'set-cookie')
-				headers.append(key, setHeaders[key] as any)
-			else if (!responseHeaders.has(key))
+	} else {
+		for (const key in setHeaders) {
+			if (key === 'set-cookie') {
+				if (headers.has('set-cookie')) continue
+
+				const cookies = setHeaders[key]
+
+				if (Array.isArray(cookies)) {
+					for (const cookie of cookies) {
+						headers.append('set-cookie', cookie)
+					}
+				} else {
+					headers.append('set-cookie', cookies as string)
+				}
+			} else if (!responseHeaders.has(key)) {
 				headers.set(key, setHeaders[key] as any)
+			}
+		}
+	}
 
 	return headers
 }