test-driven-development

Test-Driven Development

Write tests first, then implementation. No production code without a failing test.

Purpose

TDD ensures code correctness through disciplined test-first development. Tests written after implementation prove nothing - they pass immediately, providing no evidence the code works correctly. This skill enforces the RED-GREEN-REFACTOR cycle as a non-negotiable practice.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST.

If you write code before the test, you must delete it and start over. The test drives the implementation, not the other way around.

The Cycle

RED: Write a Failing Test

Write ONE minimal test that demonstrates the required behavior:

1. Test the public interface, not internals
2. Never mock what you can use for real
3. Name the test to describe the behavior
4. Run the test - it MUST fail

Mandatory verification:

Run the test. Confirm it fails for the RIGHT reason:
- Missing function/method (expected)
- Wrong return value (expected)
- NOT: Syntax error
- NOT: Import error
- NOT: Test framework misconfiguration

If the test passes immediately, you've written it wrong or the feature already exists. Investigate before proceeding.

GREEN: Write Minimal Code

Write the SIMPLEST code that makes the test pass:

1. No extra features
2. No premature optimization
3. No "while I'm here" additions
4. Just enough to satisfy the test

Mandatory verification:

Run the test. Confirm:
- The new test passes
- All other tests still pass
- No new warnings or errors

REFACTOR: Improve Without Breaking

Improve code quality while keeping tests green:

1. Remove duplication
2. Improve names
3. Extract helpers
4. Simplify logic

After each change:

Run all tests. They must still pass.
If any test fails, revert the refactor.

Cycle Example

Requirement: Function that validates email addresses

RED:
  Write test: expect(isValidEmail("[email protected]")).toBe(true)
  Run test: FAIL - isValidEmail is not defined
  Correct failure reason: function doesn't exist yet

GREEN:
  Write: function isValidEmail(email) { return true; }
  Run test: PASS
  All tests pass

RED:
  Write test: expect(isValidEmail("invalid")).toBe(false)
  Run test: FAIL - Expected false, got true
  Correct failure reason: no validation logic yet

GREEN:
  Write: function isValidEmail(email) { return email.includes("@"); }
  Run test: PASS
  All tests pass

REFACTOR:
  Extract regex pattern to constant
  Run tests: PASS
  Continue improving...