Why This Matters

TDD with Copilot produces better code than implementation-first. Tests force you to specify behavior; Copilot's implementation is grounded by tests; iterations are safer.

Concept Explained

TDD with Copilot: write tests first (describing desired behavior), then ask Copilot to implement to make tests pass. Iterate red (failing) → green (passing) → refactor.

How It Works

Describe behavior as tests. Run them (red). Ask Copilot to implement. Run again (green). Refactor with Copilot. Re-run to verify still green.

Step-by-Step Tutorial

1. Write tests

2. Run (red)

3. Ask Copilot to implement

4. Run (green)

5. Refactor

Real-World Example

A developer wrote 8 tests for a date parser, then asked Copilot to implement. First implementation passed 5/8. Iterated: 'these 3 tests fail. Fix the implementation.' Second attempt passed all 8. Refactored for readability; tests still green. Higher quality than implementation-first.

Example Prompts / Commands / Code

# Step 1: Write tests (with Copilot help):
'Write 8 vitest tests for a function parseDate(s: string): Date | null.
Cover: valid ISO, valid US, invalid format, empty string, null, far future, far past, leap day.'

# Step 2: Run (red):
$ npm test
# All 8 fail — no implementation.

# Step 3: Ask Copilot to implement:
'Implement parseDate to pass these tests: [paste tests]'

# Step 4: Run (green or iterate):
$ npm test
# If green: done. If red: 'These tests fail: [list]. Fix.'

# Step 5: Refactor:
'Refactor parseDate for readability. Preserve behavior.'
$ npm test  # Verify still green.

Common Mistakes

• Writing tests after implementation — defeats TDD's purpose.
• Not iterating when tests fail — accept first attempt.
• Refactoring without re-running tests.
• Tests too vague — implementation can pass without correct behavior.

Best Practices

• Tests first; implementation second.
• Use Copilot to help write tests.
• Iterate red → green → refactor.
• Re-run tests after every refactor.
• Write specific tests — vague tests pass with wrong behavior.

Troubleshooting

Practical Exercise

Professional Challenge

Frequently Asked Questions

Further Reading

• Writing tests with Copilot

Official References

• Writing tests with Copilot

Related lessons: BE-25, IN-32