Test Driven Development

Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write minimal code to pass.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

Violating the letter of the rules is violating the spirit of the rules.

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask your human partner):

• Throwaway prototypes (but delete and TDD it properly after the spike)
• Generated code
• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Don't look at it
• Delete means delete

Implement fresh from tests. Period.

Red-Green-Refactor

RED - Write Failing Test

One minimal test showing what should happen.

test('retries failed operations 3 times', async () => {
  let attempts = 0;
  const operation = () => {
    attempts++;
    if (attempts < 3) throw new Error('fail');
    return 'success';
  };

  const result = await retryOperation(operation);

  expect(result).toBe('success');
  expect(attempts).toBe(3);
});

Requirements: one behavior, clear name, real code (no mocks unless unavoidable).

Verify RED - Watch It Fail

MANDATORY. Never skip.

npm test path/to/test.test.ts

Confirm:

• Test fails (not errors)
• Failure message is expected
• Fails because feature missing (not typos)

Test passes? You're testing existing behavior. Fix test. Test errors? Fix error, re-run until it fails correctly.

GREEN - Minimal Code

Write simplest code to pass the test. Nothing more. Write the complete solution in one pass — not incrementally (partial code → test → add more → test again wastes cycles).

async function retryOperation<T>(fn: () => Promise<T>): Promise<T> {
  for (let i = 0; i < 3; i++) {
    try {
      return await fn();
    } catch (e) {
      if (i === 2) throw e;
    }
  }
  throw new Error('unreachable');
}

Don't add features, refactor other code, or "improve" beyond the test.

Stop when green. Once tests pass, you are done. Do not refactor, polish, or improve passing code unless explicitly asked. Passing tests = stop.

Verify GREEN - Watch It Pass

MANDATORY.

npm test path/to/test.test.ts

Confirm: test passes, other tests still pass, output pristine (no errors/warnings).

Test fails? Fix code, not test. Other tests fail? Fix now.

REFACTOR - Clean Up

After green only: remove duplication, improve names, extract helpers. Keep tests green. Don't add behavior.

Then: next failing test for next behavior.

Good Tests

Common Rationalizations

Red Flags - STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• Rationalizing "just this once"
• "I already manually tested it"
• "Tests after achieve the same purpose"
• "It's about spirit not ritual"
• "Keep as reference" or "adapt existing code"
• "Already spent X hours, deleting is wasteful"
• "TDD is dogmatic, I'm being pragmatic"
• "This is different because..."

All of these mean: Delete code. Start over with TDD.

Verification Checklist

Before marking work complete:

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for expected reason (feature missing, not typo)
• Wrote minimal code to pass each test
• All tests pass
• Output pristine (no errors, warnings)
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered
• Every test exercises a decision point, transformation, or behavior path (not config/settings)
• All tests target production code (no tests for test helpers/fixtures/utilities)

Can't check all boxes? You skipped TDD. Start over.

When Stuck

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression. Never fix bugs without a test.

Regression Test Pattern

Formalized workflow for bug-driven test creation:

1. Reproduce — Write a failing test with the exact scenario that triggered the bug
2. Name it — test_regression_{description}_{scenario} (e.g., test_regression_null_user_concurrent_login)
3. Fix — Minimal code change to make the test pass
4. Harden — Add boundary variants around the fix

Key rule: The regression test must fail without the fix and pass with it. If you can't demonstrate both states, you're not testing the right thing.

Testing Anti-Patterns

If you are unsure whether your mock approach introduces a known anti-pattern, read references/testing-anti-patterns.md:

• Testing mock behavior instead of real behavior
• Adding test-only methods to production classes
• Mocking without understanding dependencies
• Testing configuration instead of logic (tautology tests)

Final Rule

Production code -> test exists and failed first
Otherwise -> not TDD

No exceptions without your human partner's permission.

Converted and distributed by TomeVault — claim your Tome and manage your conversions.