Write Tests

Ask which file or feature needs tests, then:

For backend service functions (unit tests):

// Pattern: describe > describe > it
describe('AuthService', () => {
  describe('validateCredentials', () => {
    it('should return user when credentials are valid', () => {
      // Arrange
      const username = 'freddy'
      const password = 'ElmStreet2019'

      // Act
      const result = validateCredentials(username, password)

      // Assert
      expect(result).toEqual({ id: 1, username: 'freddy' })
    })

    it('should return null when password is wrong', () => { ... })
    it('should return null when username does not exist', () => { ... })
  })
})

For API endpoints (integration tests with Supertest):

describe('POST /login', () => {
  it('should return 200 with tokens for valid credentials', async () => {
    const res = await request(app)
      .post('/login')
      .send({ username: 'freddy', password: 'ElmStreet2019' })

    expect(res.status).toBe(200)
    expect(res.body).toHaveProperty('access_token')
    expect(res.body).toHaveProperty('refresh_token')
  })

  it('should return 401 for invalid credentials', async () => { ... })
  it('should return 400 when fields are missing', async () => { ... })
})