diff --git a/app.test.js b/app.test.js
index f1b561d..64fc38a 100644
--- a/app.test.js
+++ b/app.test.js
@@ -25,6 +25,17 @@ describe('given correct username and password', () => {
         expect(response.body.userId).toBeDefined();
     })
 
+    test('returns correct message, userId value and content-type', async () => {
+        const response = await request(app).post('/users').send({
+            username: 'Username',
+            password: 'Password123',
+            email: 'student@example.com'
+        })
+        expect(response.body.userId).toBe('1')
+        expect(response.body.message).toBe('Valid User')
+        expect(response.headers['content-type']).toMatch(/application\/json/)
+    })
+
     // test response content type?
     // test response message
     // test response user id value
@@ -40,10 +51,72 @@ describe('given incorrect or missing username and password', () => {
         })
         expect(response.statusCode).toBe(400)
     })
-
+    test('returns error message and does not include userId', async () => {
+        const response = await request(app).post('/users').send({
+            username: 'user',
+            password: 'password',
+            email: 'not-an-email'
+        })
+        expect(response.body.error).toBe('Invalid User')
+        expect(response.body.userId).toBeUndefined()
+        expect(response.headers['content-type']).toMatch(/application\/json/)
+    })
     // test response message
     // test that response does NOT have userId
     // test incorrect username or password according to requirements
     // test missing username or password
     // ...
+})
+
+describe('edge cases and performance-friendly tests', () => {
+    test('missing username returns 400 and no userId', async () => {
+        const validateUsernameMock = (u) => !!u && u.length >= 6
+        const validatePasswordMock = (p) => !!p && p.length >= 8
+        const validateEmailMock = jest.fn().mockReturnValue(true)
+
+        const appFast = createApp(validateUsernameMock, validatePasswordMock, validateEmailMock)
+
+        const response = await request(appFast).post('/users').send({
+            password: 'Password123',
+            email: 'student@example.com'
+        })
+
+        expect(response.statusCode).toBe(400)
+        expect(response.body.userId).toBeUndefined()
+        // app validates username, password and email in sequence, so email validator is called
+        expect(validateEmailMock).toHaveBeenCalled()
+    })
+
+    test('missing password returns 400 and no userId', async () => {
+        const validateUsernameMock = (u) => !!u && u.length >= 6
+        const validatePasswordMock = jest.fn().mockReturnValue(false)
+        const validateEmailMock = jest.fn().mockReturnValue(true)
+
+        const appFast = createApp(validateUsernameMock, validatePasswordMock, validateEmailMock)
+
+        const response = await request(appFast).post('/users').send({
+            username: 'Username',
+            email: 'student@example.com'
+        })
+
+        expect(response.statusCode).toBe(400)
+        expect(response.body.userId).toBeUndefined()
+    })
+
+    test('uses mocked email validator so tests stay fast', async () => {
+        const validateUsernameMock = jest.fn().mockReturnValue(true)
+        const validatePasswordMock = jest.fn().mockReturnValue(true)
+        const validateEmailMock = jest.fn().mockReturnValue(true)
+
+        const appFast = createApp(validateUsernameMock, validatePasswordMock, validateEmailMock)
+
+        const response = await request(appFast).post('/users').send({
+            username: 'Username',
+            password: 'Password123',
+            email: 'student@example.com'
+        })
+
+        expect(response.statusCode).toBe(200)
+        expect(validateEmailMock).toHaveBeenCalled()
+    })
 })
\ No newline at end of file
diff --git a/test-runtime.txt b/test-runtime.txt
new file mode 100644
index 0000000..e8e1ed4
--- /dev/null
+++ b/test-runtime.txt
@@ -0,0 +1,3 @@
+duration_ms:22216
+duration_s:22.22
+exit_code:0