Test JSON responses.

app/tests/Controller/ControllerTest.php

@@ -53,4 +53,26 @@ class ControllerTest extends TestCase
         $this->assertEquals('<p>Oh no!</p>', $response->getBody());
         $this->assertEquals('text/html', $response->getHeaderLine('Content-Type'));
     }
+
+    public function testJsonResponse(): void
+    {
+        $testController = new class extends Controller {
+            public function respond(): Response
+            {
+                return $this->json([ 'data' => 'hello' ]);
+            }
+        };
+
+        $app = $this->getAppInstance();
+        $app->get('/test', $testController);
+
+        $request = $this->createRequest('GET', '/test');
+        $response = $app->handle($request);
+
+        $this->assertEquals('{"data":"hello"}', $response->getBody());
+        $this->assertEquals(
+            'application/json',
+            $response->getHeaderLine('Content-Type')
+        );
+    }
 }