test-driven-development

name: test-driven-development description: Use when implementing any feature or bugfix, before writing implementation code

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
• 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.

Good Tests

Red-Green-Refactor

1. RED: Write failing test
2. Watch it fail
3. GREEN: Write minimal code to pass
4. Watch it pass
5. GREEN: Refactor
6. Repeat

RED - Write Failing Test

Write one minimal test showing what should happen.

...
func TestSumAbsOfTwoNumbers(t *testing.T) {
    // Act
    result := calculator.AddAbs(2, 3)
    
    // Assert
    assert.Equal(t, 5, result)
}

Requirements:

• One behavior
• Real code (no mocks unless unavoidable)

Verify RED - Watch It Fail

MANDATORY. Never skip.

just ut ./calculator/...

Confirm:

• Test fails (not errors)

Test passes? You're testing existing behavior. Fix test.

GREEN - Minimal Code

...
func AddAbs(a, b int) int {
    return a + b
}

Verify GREEN - Watch It Pass

MANDATORY.

just ut ./calculator/...

Confirm:

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

Test fails? Fix code, not test.

GREEN - Refactor

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Don't add behavior.

...
func TestSumAbsOfTwoNumbers(t *testing.T) {
    // Act & Assert
    assert.Equal(t, 5, calculator.AddAbs(2, 3))
}

Repeat

Next failing test for next feature.

one more example cycle for AddAbs:

RED Failing Test:

...
func TestSumAbsOfTwoNumbers(t *testing.T) {
    t.Run("sum of two positive numbers", func(t *testing.T) {
        // Act & Assert
        assert.Equal(t, 5, calculator.AddAbs(2, 3))
    })

    t.Run("sum of two negative numbers", func(t *testing.T) {
        // Act & Assert
        assert.Equal(t, 5, calculator.AddAbs(-2, -3))
    })
}

test failed.

GREEN Code:

...
func AddAbs(a, b int) int {
    if a < 0 {
        a = -a
    }
    if b < 0 {
        b = -b
    }
    return a + b
}

test passed.

GREEN Refactor:

...
func abs(a int) int {
  if a < 0 {
    return -a
  }
  return a
}

func AddAbs(a, b int) int {
    return abs(a) + abs(b)
}

test passed.

FINISH:

If there's no more to implement & do refactoring, you're done.

Why Order Matters

"I'll write tests after to verify it works"

Tests written after code pass immediately. Passing immediately proves nothing:

• Might test wrong thing
• Might test implementation, not behavior
• Might miss edge cases you forgot
• You never saw it catch the bug

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

"I already manually tested all the edge cases"

Manual testing is ad-hoc. You think you tested everything but:

• No record of what you tested
• Can't re-run when code changes
• Easy to forget cases under pressure
• "It worked when I tried it" ≠ comprehensive

Automated tests are systematic. They run the same way every time.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. The time is already gone. Your choice now:

• Delete and rewrite with TDD (X more hours, high confidence)
• Keep it and add tests after (30 min, low confidence, likely bugs)

The "waste" is keeping code you can't trust. Working code without real tests is technical debt.

"TDD is dogmatic, being pragmatic means adapting"

TDD IS pragmatic:

• Finds bugs before commit (faster than debugging after)
• Prevents regressions (tests catch breaks immediately)
• Documents behavior (tests show how to use code)
• Enables refactoring (change freely, tests catch breaks)

"Pragmatic" shortcuts = debugging in production = slower.

"Tests after achieve the same goals - it's spirit not ritual"

No. Tests-after answer "What does this do?" Tests-first answer "What should this do?"

Tests-after are biased by your implementation. You test what you built, not what's required. You verify remembered edge cases, not discovered ones.

Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't).

30 minutes of tests after ≠ TDD. You get coverage, lose proof tests work.

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.

Example: Bug Fix

Bug: Empty email accepted

RED

func Test_SendMail(t *testing.T) {
    t.Run("rejects empty email", func(t *testing.T) {
        _, err := SendMail("")
        assert.Error(t, err)
    })
}

Verify RED

$ just ut
FAIL: expected error, got nil

GREEN

func SendMail(email string) (bool, error) {
    if email == "" {
        return false, errors.New("empty email")
    }

    ...
}

Verify GREEN

$ just ut
PASS

REFACTOR Extract validation for multiple fields if needed.

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

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

When Stuck