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.

Red-Green-Refactor

digraph tdd_cycle {
    rankdir=LR;
    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
    verify_red [label="Verify fails\ncorrectly", shape=diamond];
    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
    verify_green [label="Verify passes\nAll green", shape=diamond];
    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
    next [label="Next", shape=ellipse];

    red -> verify_red;
    verify_red -> green [label="yes"];
    verify_red -> red [label="wrong\nfailure"];
    green -> verify_green;
    verify_green -> refactor [label="yes"];
    verify_green -> green [label="no"];
    refactor -> verify_green [label="stay\ngreen"];
    verify_green -> next;
    next -> red;
}

RED - Write Failing Test

Write one minimal test showing what should happen.

const result = await retryOperation(operation);

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

Clear name, tests real behavior, one thing
</Good>

<Bad>
```typescript
test('retry works', async () => {
  const mock = jest.fn()
    .mockRejectedValueOnce(new Error())
    .mockRejectedValueOnce(new Error())
    .mockResolvedValueOnce('success');
  await retryOperation(mock);
  expect(mock).toHaveBeenCalledTimes(3);
});

Vague name, tests mock not code </Bad>

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.

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

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.

Repeat

Next failing test for next feature.

Good Tests

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

test('rejects empty email', async () => {
  const result = await submitForm({ email: '' });
  expect(result.error).toBe('Email required');
});

Verify RED

$ npm test
FAIL: expected 'Email required', got undefined

GREEN

function submitForm(data: FormData) {
  if (!data.email?.trim()) {
    return { error: 'Email required' };
  }
  // ...
}

Verify GREEN

$ npm test
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

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Testing Anti-Patterns

When adding mocks or test utilities, read @testing-anti-patterns.md to avoid common pitfalls:

• Testing mock behavior instead of real behavior
• Adding test-only methods to production classes
• Mocking without understanding dependencies

Final Rule

Production code → test exists and failed first
Otherwise → not TDD

No exceptions without your human partner's permission.

Godot/GDScript TDD Guide

This section extends TDD principles for Godot 4.x and GDScript development using the GUT (Godot Unit Test) framework.

GUT Testing Framework

This project uses GUT (Godot Unit Test) for testing. Tests are located in tests/ directory.

Running Tests

# Run all tests (headless)
.\Godot_v4.5.1-stable_win64.exe --headless --script tests/run_tests.gd

# Run specific test file
.\Godot_v4.5.1-stable_win64.exe --headless --script tests/run_tests.gd --test tests/test_inventory.gd

# Via MCP (if available)
mcp__godot__run_tests

GUT Test Structure

class TestHealthSystem: extends GutTest

var health_system: HealthSystem

func before_each():
    health_system = HealthSystem.new()

func after_each():
    health_system.free()

func test_take_damage_reduces_health():
    # RED: Write failing test
    health_system.max_health = 100
    health_system.current_health = 100
    
    # Act
    health_system.take_damage(25)
    
    # Assert
    assert_eq(health_system.current_health, 75, "Health should be reduced by damage amount")

Clear test name, setup/teardown, one behavior
</Good>

## Godot RED-GREEN-REFACTOR Examples

### Example 1: Player Movement

**RED - Write Failing Test**
```gdscript
# tests/test_player_movement.gd
class TestPlayerMovement:
    extends GutTest
    
    var player: Player
    
    func before_each():
        player = Player.new()
        player.speed = 100.0
    
    func after_each():
        player.free()
    
    func test_moves_right_when_input_pressed():
        # Arrange
        var delta = 0.016  # One frame at 60fps
        
        # Act
        player.handle_movement(Vector2.RIGHT, delta)
        
        # Assert
        assert_gt(player.position.x, 0.0, "Player should move right")
        assert_eq(player.position.y, 0.0, "Player should not move vertically")

Verify RED

$ .\Godot_v4.5.1-stable_win64.exe --headless --script tests/run_tests.gd --test tests/test_player_movement.gd
>> test_moves_right_when_input_pressed: FAILED
>> Expected X position > 0.0 but got 0.0

GREEN - Minimal Implementation

# game/player/player.gd
class_name Player
extends CharacterBody2D

@export var speed: float = 100.0

func handle_movement(direction: Vector2, delta: float) -> void:
    velocity = direction * speed
    move_and_slide()

Verify GREEN

$ .\Godot_v4.5.1-stable_win64.exe --headless --script tests/run_tests.gd --test tests/test_player_movement.gd
>> test_moves_right_when_input_pressed: PASSED

Example 2: Inventory System

RED - Write Failing Test

# tests/test_inventory.gd
class TestInventory:
    extends GutTest
    
    var inventory: Inventory
    
    func before_each():
        inventory = Inventory.new()
        inventory.max_slots = 10
    
    func after_each():
        inventory.free()
    
    func test_add_item_increases_count():
        var herb = Item.new()
        herb.id = "herb"
        herb.stackable = true
        
        inventory.add_item(herb)
        
        assert_eq(inventory.get_item_count("herb"), 1, "Should have 1 herb")

GREEN - Minimal Implementation

# game/systems/inventory/inventory.gd
class_name Inventory
extends Resource

@export var max_slots: int = 10
var _items: Dictionary = {}  # item_id -> count

func add_item(item: Item) -> void:
    if item.stackable:
        _items[item.id] = _items.get(item.id, 0) + 1

func get_item_count(item_id: String) -> int:
    return _items.get(item_id, 0)

Example 3: Signal Testing

var health: HealthSystem
var signal_received: bool = false
var damage_received: int = 0

func before_each():
    health = HealthSystem.new()
    health.max_health = 100
    health.current_health = 100
    signal_received = false
    damage_received = 0
    
    # Connect signal
    health.health_changed.connect(_on_health_changed)

func _on_health_changed(new_health: int, damage: int):
    signal_received = true
    damage_received = damage

func test_emits_signal_on_damage():
    health.take_damage(25)
    
    assert_true(signal_received, "Signal should be emitted")
    assert_eq(damage_received, 25, "Damage should be passed in signal")

Tests real signal behavior
</Good>

## Godot-Specific Testing Patterns

### Testing Scene Instancing

```gdscript
func test_scene_instantiates_correctly():
    var scene = load("res://game/enemies/enemy.tscn")
    var instance = scene.instantiate()
    add_child_autofree(instance)  # GUT will free this automatically
    
    assert_not_null(instance, "Scene should instantiate")
    assert_is(instance, Enemy, "Instance should be Enemy type")

Testing with Autoloads (Singletons)

func before_each():
    # Store original state
    _original_inventory = GameState.inventory
    
    # Use test double
    GameState.inventory = Inventory.new()

func after_each():
    # Restore original state
    GameState.inventory = _original_inventory

Testing Node Tree Operations

func test_find_player_in_scene():
    var level = Level.new()
    add_child_autofree(level)
    
    var player = Player.new()
    player.name = "Player"
    level.add_child(player)
    
    var found = level.find_child("Player", true, false)
    
    assert_not_null(found, "Should find player node")
    assert_is(found, Player, "Found node should be Player")

Testing Physics/Process

func test_gravity_applies_over_time():
    var player = Player.new()
    add_child_autofree(player)
    player.position = Vector2.ZERO
    player.velocity = Vector2.ZERO
    
    # Simulate multiple frames
    await wait_frames(10)
    
    assert_gt(player.velocity.y, 0.0, "Gravity should increase Y velocity")

Godot TDD Verification Checklist

Before marking work complete:

• Every new GDScript class has corresponding GUT tests
• Watched each test fail before implementing (RED)
• Wrote minimal GDScript code to pass test (GREEN)
• Refactored while keeping tests green (REFACTOR)
• All GUT tests pass: .\Godot*.exe --headless --script tests/run_tests.gd
• No Godot errors or warnings in output
• Scene tests use add_child_autofree() for cleanup
• Signal tests verify both emission and data
• Node tree tests verify correct hierarchy
• Resource tests use temporary instances

Common Godot Testing Mistakes

Testing Anti-Patterns (Godot)

When adding mocks or test utilities, read @testing-anti-patterns.md. Godot-specific notes:

• Don't test mock scenes: Test real scenes with add_child_autofree()
• Don't add test-only methods: Use test utilities in tests/utils/
• Don't mock Godot internals: Mock your own systems, not Engine/SceneTree
• Use real Resources: Don't mock simple data containers

Related Skills

• systematic-debugging - Find root cause before fixing bugs
• verification-before-completion - Verify tests pass before claiming complete