TDD Workflow

Test-driven development methodology.

The TDD Cycle

code

RED -> GREEN -> REFACTOR

RED Phase

Write a failing test that defines expected behavior.

GREEN Phase

Write the minimum code to make the test pass.

REFACTOR Phase

Improve the code while keeping tests green.

Test Structure

Arrange-Act-Assert

typescript

test("should return user when found", async () => {
  // Arrange: Set up test data
  const userId = "user-123"
  const expectedUser = { id: userId, name: "Test" }
  mockDb.getUser.mockResolvedValue(expectedUser)

  // Act: Execute the function
  const result = await userService.getUser(userId)

  // Assert: Verify the result
  expect(result).toEqual(expectedUser)
})

Test Categories

Unit Tests

•Test single functions in isolation
•Mock all external dependencies
•Fast (< 1ms per test)

Integration Tests

•Test multiple components together
•Use real database/service when appropriate
•Slower (< 100ms per test)

E2E Tests

•Test full user workflows
•Use Playwright
•Slowest but most comprehensive

Best Practices

1. Descriptive Test Names

typescript

// Reads like a requirement
it("should return null when user is not found", async () => {})

// Vague - avoid
it("test user lookup", async () => {})

2. Test Behavior, Not Implementation

typescript

// Tests behavior
test("filterUsers returns only adults", () => {
  const result = filterUsers(users, 18)
  expect(result.every((u) => u.age >= 18)).toBe(true)
})

3. Test Edge Cases

typescript

test("should handle empty array", () => {
  expect(processItems([])).toEqual([])
})

test("should handle null input", () => {
  expect(processItems(null)).toEqual([])
})

test("should handle boundary value", () => {
  expect(isAdult(17)).toBe(false)
  expect(isAdult(18)).toBe(true)
})

4. Test Error Cases

typescript

test("should throw when user not found", async () => {
  mockDb.getUser.mockResolvedValue(null)

  await expect(getUser("invalid-id")).rejects.toThrow(NotFoundError)
})

Running Tests

bash

# Run all tests
bun test

# Run specific file
bun test test/users.test.ts

# Run with coverage
bun test --coverage

# Watch mode
bun test --watch

Coverage Requirements

Code Type	Coverage	Priority
Business Logic	90%+	Critical
API Routes	80%+	High
Utilities	95%+	High
Types	N/A	N/A
Config	N/A	N/A

Testing Checklist

Before marking code complete: