Test-Driven Development (TDD) Workflow

A disciplined approach to development where tests drive design and implementation.

The TDD Mantra

"Never write a line of code without a failing test."

The RED-GREEN-REFACTOR Cycle

RED Phase: Write a Failing Test

Goal: Define the expected behavior BEFORE implementation.

Rules:

1. Write the smallest test that fails
2. Test must fail for the RIGHT reason
3. Test should clearly express intent
4. Don't write implementation yet

Example:

// RED: Test the behavior we want
describe("Calculator", () => {
  it("should add two numbers", () => {
    const calc = new Calculator();
    expect(calc.add(2, 3)).toBe(5);
  });
});

// Run: npm test
// Result: FAIL - Calculator is not defined
// This is RED ✓

Checklist:

• Test is written
• Test fails when run
• Failure message is clear
• Test name describes expected behavior

---

GREEN Phase: Make the Test Pass

Goal: Write the MINIMUM code to pass the test.

Rules:

1. Do the simplest thing that works
2. Don't add extra features
3. Don't optimize
4. Just make it green

Example:

// GREEN: Minimum implementation to pass
class Calculator {
  add(a: number, b: number): number {
    return a + b; // Simplest thing that works
  }
}

// Run: npm test
// Result: PASS
// This is GREEN ✓

Checklist:

• Test passes
• No extra code added
• Implementation is minimal

---

REFACTOR Phase: Improve the Code

Goal: Clean up while keeping tests green.

Rules:

1. Only refactor with passing tests
2. Run tests after each change
3. Improve design, not behavior
4. Small, incremental changes

Examples of refactoring:

• Extract methods
• Rename for clarity
• Remove duplication
• Improve performance
• Add types/documentation

Checklist:

• Tests still pass
• Code is cleaner
• No behavior changed
• Ready for next RED

---

TDD in Practice

Starting a New Feature

1. Write high-level acceptance test (may not run yet)
2. Write first unit test (RED)
3. Implement minimum code (GREEN)
4. Refactor if needed (REFACTOR)
5. Repeat 2-4 until feature complete
6. Verify acceptance test passes

Test Structure (AAA Pattern)

it("should [behavior] when [condition]", () => {
  // Arrange - Set up test data and dependencies
  const user = createTestUser({ role: "admin" });
  const service = new UserService();

  // Act - Execute the code under test
  const result = service.getPermissions(user);

  // Assert - Verify expected outcomes
  expect(result).toContain("delete");
  expect(result).toContain("edit");
});

Test Naming Convention

[Unit]_[Scenario]_[ExpectedResult]

Examples:

• add_withPositiveNumbers_returnsSum
• login_withInvalidPassword_throwsAuthError
• getUser_whenNotFound_returnsNull

---

Test Categories

Unit Tests

• Single function/class in isolation
• Mock all dependencies
• Fast (<10ms per test)
• Run constantly during development

Integration Tests

• Multiple components together
• Real database (test instance)
• Slower but more realistic
• Run before commits

End-to-End Tests

• Full system through UI
• Slowest, most realistic
• Run in CI/CD pipeline
• Cover critical user paths

---

TDD Best Practices

DO:

• Start with the simplest case
• Write one test at a time
• Keep tests independent
• Test behavior, not implementation
• Use descriptive test names
• Commit after each green

DON'T:

• Write code before tests
• Test private methods directly
• Test framework code
• Overfit tests to implementation
• Skip the refactor phase

---

Edge Cases to Test

Always test:

• Empty inputs (null, undefined, [], {}, '')
• Boundary values (0, -1, MAX_INT, min/max dates)
• Error conditions (network fail, invalid input)
• Permission boundaries
• Concurrent access
• Unicode/special characters

---

Test Coverage Guidelines

Metric	Minimum	Target
Statements	70%	85%
Branches	70%	80%
Functions	80%	90%
Lines	70%	85%

Coverage is a metric, not a goal. 100% coverage doesn't mean bug-free.

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Quick Reference

RED    → Write failing test (define behavior)
GREEN  → Minimum code to pass (make it work)
REFACTOR → Clean up (make it right)
COMMIT → Save progress (make it permanent)

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Common TDD Mistakes

Mistake	Problem	Solution
Testing implementation	Brittle tests	Test behavior/outcomes
Tests too large	Hard to debug	Smaller, focused tests
Shared state	Flaky tests	Isolate each test
Slow tests	Skipped tests	Mock external deps
Testing obvious code	Wasted time	Focus on logic