Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write the minimum code to make it pass.

Core principle: If you did not watch the test fail, you do not know whether you are testing the right thing.

Violating the letter of these rules violates their spirit.

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavioral changes

Exceptions (ask the user):

• Throwaway prototypes
• Generated code
• Configuration files

If you are thinking "I will skip TDD just this once" — stop. That is rationalization.

Iron Rule

NO PRODUCTION CODE WITHOUT A FAILING TEST

Wrote code before the test? DELETE IT. Start over.

No exceptions:

• Do not keep it "for reference"
• Do not "adapt" it while writing tests
• Do not even look at it
• Delete means delete

Implement from scratch, starting from tests. Period.

Red-Green-Refactor Cycle

RED (Write a failing test)
  |
  v
Fails correctly? --NO--> Fix the test
  |
  YES
  |
  v
GREEN (Write minimum code)
  |
  v
All tests pass? --NO--> Fix the code
  |
  YES
  |
  v
REFACTOR (Clean up)
  |
  v
Still green? --NO--> Revert the refactor
  |
  YES
  |
  v
Next test --> Return to RED

RED -- Write a Failing Test

Write one minimal test that shows what should happen.

Good:

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

  const result = await retry(operation);

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

Clear name, tests real behavior, tests one thing.

Bad:

test('retry works', async () => {
  const mock = jest.fn()
    .mockRejectedValueOnce(new Error())
    .mockResolvedValueOnce('success');
  await retry(mock);
  expect(mock).toHaveBeenCalledTimes(2);
});

Vague name, tests the mock instead of the code.

Requirements:

• One behavior per test
• Clear, descriptive name
• Real code (avoid mocks unless unavoidable)

RED Verify -- Watch It Fail

MANDATORY. Never skip.

npm test path/to/test.test.ts

Check:

• Test fails (not errors)
• Failure message is what you expected
• It fails because the feature is missing (not because of a typo)

Test passes? You are testing existing behavior. Fix the test.

Test errors? Fix the error, re-run until it fails correctly.

GREEN -- Minimum Code

Write the simplest code that makes the test pass.

Good:

async function retry<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');
}

Just enough to pass.

Bad:

async function retry<T>(
  fn: () => Promise<T>,
  options?: {
    maxRetries?: number;
    backoff?: 'linear' | 'exponential';
    onRetry?: (attempt: number) => void;
  }
): Promise<T> {
  // YAGNI -- do not add things that were not asked for
}

Over-engineering.

Do not add features beyond the test. Do not refactor other code. Do not "improve" anything.

GREEN Verify -- Watch It Pass

MANDATORY.

npm test path/to/test.test.ts

Check:

• Test passes
• Other tests still pass
• Output is clean (no errors, no warnings)

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

REFACTOR -- Clean Up

Only after green:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Do not add behavior.

Repeat

Next failing test for the next feature.

Good Tests

Why Order Matters

"I will write tests afterward to verify my work"

Tests written after pass immediately. Passing immediately proves nothing:

• You might be testing the wrong thing
• You might be testing the implementation, not the behavior
• You will miss edge cases you did not think of
• You never saw the test catch an error

Test-first forces you to see the test fail, proving it actually tests something.

Common Rationalizations

Red Flags -- STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately
• Cannot explain why the test fails
• Tests added "later"
• "Just this once" rationalization
• "I already tested manually"
• "Keep it as reference" or "adapt the existing code"
• "I spent X hours, deleting is wasteful"
• "TDD is dogmatic, I am pragmatic"
• "This is different because..."

All of these mean: Delete the code. Restart with TDD.

Verification Checklist

Before marking work as complete:

• Every new function/method has a test
• I watched every test fail BEFORE implementing
• Every test failed for the expected reason (missing feature, not typo)
• I wrote the minimum code to make each test pass
• All tests pass
• Output is clean (no errors, no warnings)
• Tests use real code (mocks only when unavoidable)
• Edge cases and error paths are covered

Cannot check all boxes? You skipped TDD. Start over.

Debug Integration

Bug found? Write a failing test that reproduces the bug. Follow the TDD cycle. The test proves the fix and prevents regression.

Never fix a bug without a test.

REQUIRED SKILL: systematic-debugging (when the test fails unexpectedly)