Test patterns

A test's job is to fail when the behavior is wrong. Most mediocre tests pass whether the code is right or not.

Rules of thumb

Assert behavior, not implementation

Bad:

expect(spy).toHaveBeenCalled()

Good:

expect(response.body.status).toBe('completed')
expect(db.getUser(id).lastSeenAt).toBe(mockNow)

Spy-assertions fail on refactor and protect nothing. Behavior-assertions fail when the user-visible contract breaks.

One logical check per test

Many expect lines can add up to one assertion of one outcome — that's fine. What's not fine: one test that covers three unrelated behaviors so nobody can tell what broke when it fails.

Arrange-Act-Assert

it('rejects stale tokens', () => {
  // Arrange
  const token = signToken({ exp: yesterday() })
  // Act
  const result = verify(token)
  // Assert
  expect(result.ok).toBe(false)
  expect(result.reason).toBe('expired')
})

Table-driven when the shape repeats

test.each([
  ['empty',       '',         'required'],
  ['too short',   'ab',       'min_length'],
  ['has space',   'a b',      'invalid_char'],
  ['ok',          'alice',    null],
])('validateUsername(%s=%j)', (_, input, expected) => {
  expect(validateUsername(input).error).toBe(expected)
})

Fixtures > inline setup

If three tests set up the same validUser, extract it. If the setup is 20 lines, it's probably doing too much — mock less, use a real test DB.

Name tests as specs

The test name is a sentence the reader can understand without opening the code. it('rejects stale tokens') > it('test2').

Integration > unit for risk hotspots

Unit tests are great for pure logic. For "this endpoint returns the right data for this user" you want an integration test that hits a real database, a real router, and a real serializer. Mocks hide the bugs you actually ship.

Framework-specific pointers

Testing Library (React/Vue/Svelte)

• Query by role/label first, getByTestId last.
• userEvent over fireEvent.
• Avoid testing props/state; test what the user sees.

Pytest

• Use fixtures for setup, not class-level setUp.
• parametrize for table tests.
• -k and markers to target — CI can split suites.

Go

• Table tests with subtests: t.Run(tt.name, func(t *testing.T) { ... }).
• t.Cleanup over defer for teardown.

Jest/Vitest

• describe.each + test.each for tables.
• Avoid toHaveBeenCalled when you can check the effect instead.