fix(event-handler): handle repeated queryString values

packages/event-handler/src/rest/converters.ts

@@ -44,16 +44,15 @@ const createBody = (body: string | null, isBase64Encoded: boolean) => {
 };
 
 /**
- * Converts an API Gateway proxy event to a Web API Request object.
+ * Populates headers from single and multi-value header entries.
  *
+ * @param headers - The Headers object to populate
  * @param event - The API Gateway proxy event
- * @returns A Web API Request object
  */
-const proxyEventV1ToWebRequest = (event: APIGatewayProxyEvent): Request => {
-  const { httpMethod, path } = event;
-  const { domainName } = event.requestContext;
-
-  const headers = new Headers();
+const populateV1Headers = (
+  headers: Headers,
+  event: APIGatewayProxyEvent
+): void => {
   for (const [name, value] of Object.entries(event.headers ?? {})) {
     if (value !== undefined) headers.set(name, value);
   }
@@ -66,15 +65,21 @@ const proxyEventV1ToWebRequest = (event: APIGatewayProxyEvent): Request => {
       }
     }
   }
-  const hostname = headers.get('Host') ?? domainName;
-  const protocol = headers.get('X-Forwarded-Proto') ?? 'https';
-
-  const url = new URL(path, `${protocol}://${hostname}/`);
+};
 
+/**
+ * Populates URL search parameters from single and multi-value query string parameters.
+ *
+ * @param url - The URL object to populate
+ * @param event - The API Gateway proxy event
+ */
+const populateV1QueryParams = (url: URL, event: APIGatewayProxyEvent): void => {
   for (const [name, value] of Object.entries(
     event.queryStringParameters ?? {}
   )) {
-    if (value != null) url.searchParams.append(name, value);
+    if (value != null && !event.multiValueQueryStringParameters?.[name]) {
+      url.searchParams.append(name, value);
+    }
   }
 
   for (const [name, values] of Object.entries(
@@ -84,6 +89,27 @@ const proxyEventV1ToWebRequest = (event: APIGatewayProxyEvent): Request => {
       url.searchParams.append(name, value);
     }
   }
+};
+
+/**
+ * Converts an API Gateway proxy event to a Web API Request object.
+ *
+ * @param event - The API Gateway proxy event
+ * @returns A Web API Request object
+ */
+const proxyEventV1ToWebRequest = (event: APIGatewayProxyEvent): Request => {
+  const { httpMethod, path } = event;
+  const { domainName } = event.requestContext;
+
+  const headers = new Headers();
+  populateV1Headers(headers, event);
+
+  const hostname = headers.get('Host') ?? domainName;
+  const protocol = headers.get('X-Forwarded-Proto') ?? 'https';
+
+  const url = new URL(path, `${protocol}://${hostname}/`);
+  populateV1QueryParams(url, event);
+
   return new Request(url.toString(), {
     method: httpMethod,
     headers,

packages/event-handler/tests/unit/rest/converters.test.ts

@@ -298,6 +298,53 @@ describe('Converters', () => {
       expect(url.searchParams.getAll('multi')).toEqual(['value1', 'value2']);
     });
 
+    it('handles same parameter in both queryStringParameters and multiValueQueryStringParameters without duplication', () => {
+      // Prepare
+      const event = {
+        ...baseEvent,
+        queryStringParameters: {
+          filter: 'published', // Last value (API Gateway behavior)
+        },
+        multiValueQueryStringParameters: {
+          filter: ['active', 'published'], // All values (API Gateway behavior)
+        },
+      };
+
+      // Act
+      const request = proxyEventToWebRequest(event);
+
+      // Assess
+      expect(request).toBeInstanceOf(Request);
+      const url = new URL(request.url);
+      expect(url.searchParams.getAll('filter')).toEqual([
+        'active',
+        'published',
+      ]);
+    });
+
+    it('handles mixed single and multi-value query parameters correctly', () => {
+      // Prepare
+      const event = {
+        ...baseEvent,
+        queryStringParameters: {
+          single: 'value1', // Only in single-value
+          multi: 'last', // Also in multi-value (should be ignored)
+        },
+        multiValueQueryStringParameters: {
+          multi: ['first', 'last'], // Should take precedence
+        },
+      };
+
+      // Act
+      const request = proxyEventToWebRequest(event);
+
+      // Assess
+      expect(request).toBeInstanceOf(Request);
+      const url = new URL(request.url);
+      expect(url.searchParams.get('single')).toBe('value1');
+      expect(url.searchParams.getAll('multi')).toEqual(['first', 'last']);
+    });
+
     it('skips undefined queryStringParameter values', () => {
       // Prepare
       const event = {