test-driven-development

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

Test-Driven Development (TDD) for React Native

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.

Testing Stack

• Test runner: Jest via jest-expo preset (standard for Expo / React Native). Do NOT install Jest 30+ directly — use the Jest version bundled with jest-expo. Run tests with npx jest (the jest-expo preset is configured in package.json).
• Component testing: @testing-library/react-native (React Native Testing Library)
  • render, screen — render components and query the output
  • fireEvent, userEvent — simulate user interactions
  • waitFor, findBy* queries — handle async state updates
  • renderHook — test custom hooks in isolation
  • For complete RNTL API reference, query patterns, and version-specific behavior (v13 vs v14), use the react-native-testing skill. It contains up-to-date API docs, query variant tables, interaction guides, and anti-patterns that override potentially stale training data.
• Assertions: Jest matchers (built into @testing-library/react-native v13+) + @testing-library/jest-native extended matchers
• Device testing: For on-device UI validation after tests pass, use the agent-device skill to automate iOS simulators and Android emulators.

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.

```typescript test('displays welcome message with user name', () => { render();

expect(screen.getByText('Welcome, Alice!')).toBeOnTheScreen(); });

Clear name, tests user-visible behavior, one thing
</Good>

<Bad>
```typescript
test('component works', () => {
  const { root } = render(<WelcomeScreen userName="Alice" />);
  expect(root.props.userName).toBe('Alice');
});

Vague name, tests props/internals instead of rendered output

Requirements:

• One behavior
• Clear name
• Test what the user sees and interacts with (not implementation details)

Verify RED - Watch It Fail

MANDATORY. Never skip.

npx jest path/to/component.test.tsx

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.

```typescript function WelcomeScreen({ userName }: { userName: string }) { return ( Welcome, {userName}! ); } ``` Just enough to pass ```typescript function WelcomeScreen({ userName, theme, onDismiss, animationConfig, analyticsTracker, }: WelcomeScreenProps) { // YAGNI - none of this is tested yet } ``` Over-engineered

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

Verify GREEN - Watch It Pass

MANDATORY.

npx jest path/to/component.test.tsx

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 / shared components

Keep tests green. Don't add behavior.

Repeat

Next failing test for next feature.

React Native Testing Patterns

Testing Component Rendering

import { render, screen } from '@testing-library/react-native';

test('shows error message when validation fails', () => {
  render(<LoginForm errors={{ email: 'Invalid email' }} />);

  expect(screen.getByText('Invalid email')).toBeOnTheScreen();
});

Testing User Interactions

import { render, screen, userEvent } from '@testing-library/react-native';

test('calls onSubmit with email when button pressed', async () => {
  const user = userEvent.setup();
  const onSubmit = jest.fn();
  render(<LoginForm onSubmit={onSubmit} />);

  await user.type(screen.getByPlaceholderText('Email'), '[e-mail]');
  await user.press(screen.getByRole('button', { name: 'Sign In' }));

  expect(onSubmit).toHaveBeenCalledWith({ email: '[e-mail]' });
});

Testing Async Operations

import { render, screen, waitFor } from '@testing-library/react-native';

test('displays profile data after loading', async () => {
  render(<ProfileScreen userId="123" />);

  expect(screen.getByText('Loading...')).toBeOnTheScreen();

  // waitFor retries until assertion passes or times out
  await waitFor(() => {
    expect(screen.getByText('Alice Johnson')).toBeOnTheScreen();
  });

  expect(screen.queryByText('Loading...')).not.toBeOnTheScreen();
});

// Or use findBy* queries (built-in waitFor):
test('displays profile data after loading', async () => {
  render(<ProfileScreen userId="123" />);

  const name = await screen.findByText('Alice Johnson');
  expect(name).toBeOnTheScreen();
});

Testing Custom Hooks

import { renderHook, waitFor } from '@testing-library/react-native';

test('useCounter increments count', () => {
  const { result } = renderHook(() => useCounter(0));

  expect(result.current.count).toBe(0);

  act(() => {
    result.current.increment();
  });

  expect(result.current.count).toBe(1);
});

test('useFetchUser returns user data', async () => {
  const { result } = renderHook(() => useFetchUser('123'));

  await waitFor(() => {
    expect(result.current.data).toEqual({ id: '123', name: 'Alice' });
  });
});

Testing Navigation (Expo Router)

import { render, screen, userEvent } from '@testing-library/react-native';
import { useRouter } from 'expo-router';

jest.mock('expo-router', () => ({
  useRouter: jest.fn(),
}));

test('navigates to details on item press', async () => {
  const push = jest.fn();
  (useRouter as jest.Mock).mockReturnValue({ push });
  const user = userEvent.setup();

  render(<ItemList items={[{ id: '1', title: 'Item 1' }]} />);

  await user.press(screen.getByText('Item 1'));

  expect(push).toHaveBeenCalledWith('/items/1');
});

Testing with Providers (Context, State)

function renderWithProviders(ui: React.ReactElement) {
  return render(
    <ThemeProvider>
      <AuthProvider>
        {ui}
      </AuthProvider>
    </ThemeProvider>
  );
}

test('shows logout button when authenticated', () => {
  // Mock auth state or provide test values
  renderWithProviders(<SettingsScreen />);

  expect(screen.getByRole('button', { name: 'Log Out' })).toBeOnTheScreen();
});

Query Priority

Prefer queries that reflect how users find elements:

Priority	Query	Use For
1st	getByRole	Buttons, headings, text inputs
2nd	getByText	Static text content
3rd	getByPlaceholderText	Text inputs
4th	getByDisplayValue	Filled inputs
5th	getByLabelText	Labeled form elements
Last	getByTestID	Only when no semantic query works

getByTestID is a last resort. If you reach for it first, rethink your component's accessibility.

Good Tests

Quality	Good	Bad
Minimal	One thing. "and" in name? Split it.	test('validates email and domain and whitespace')
Clear	Name describes behavior	test('test1')
Shows intent	Demonstrates desired API	Obscures what code should do
User-centric	Tests what user sees/does	Tests component internals or state

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" does not equal 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 does not equal TDD. You get coverage, lose proof tests work.

Common Rationalizations

Excuse	Reality
"Too simple to test"	Simple code breaks. Test takes 30 seconds.
"I'll test after"	Tests passing immediately prove nothing.
"Tests after achieve same goals"	Tests-after = "what does this do?" Tests-first = "what should this do?"
"Already manually tested"	Ad-hoc does not equal systematic. No record, can't re-run.
"Deleting X hours is wasteful"	Sunk cost fallacy. Keeping unverified code is technical debt.
"Keep as reference, write tests first"	You'll adapt it. That's testing after. Delete means delete.
"Need to explore first"	Fine. Throw away exploration, start with TDD.
"Test hard = design unclear"	Listen to test. Hard to test = hard to use.
"TDD will slow me down"	TDD faster than debugging. Pragmatic = test-first.
"Manual test faster"	Manual doesn't prove edge cases. You'll re-test every change.
"Existing code has no tests"	You're improving it. Add tests for existing code.
"React Native is hard to test"	React Native Testing Library makes it straightforward. No excuses.

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..."
• Testing component state directly instead of rendered output
• Reaching for getByTestID before trying semantic queries

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

Example: Bug Fix (React Native)

Bug: Empty email accepted in login form

RED

test('shows error when email is empty and submit pressed', async () => {
  const user = userEvent.setup();
  render(<LoginForm />);

  await user.press(screen.getByRole('button', { name: 'Sign In' }));

  expect(screen.getByText('Email required')).toBeOnTheScreen();
});

Verify RED

$ npx jest LoginForm.test.tsx
FAIL: Unable to find an element with text: Email required

GREEN

function LoginForm() {
  const [error, setError] = useState('');

  const handleSubmit = (email: string) => {
    if (!email?.trim()) {
      setError('Email required');
      return;
    }
    // ...
  };

  return (
    <View>
      <TextInput placeholder="Email" onChangeText={setEmail} />
      {error ? <Text>{error}</Text> : null}
      <Pressable role="button" accessibilityLabel="Sign In" onPress={() => handleSubmit(email)}>
        <Text>Sign In</Text>
      </Pressable>
    </View>
  );
}

Verify GREEN

$ npx jest LoginForm.test.tsx
PASS

REFACTOR Extract validation logic, add accessibility labels if missing.

Verification Checklist

Before marking work complete:

• Every new function/method/component 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, act() warnings)
• Tests use real components where possible (mocks only if unavoidable)
• Tests query by user-visible attributes (role, text, placeholder), not testID
• Async operations use waitFor or findBy* (no arbitrary delays)
• Edge cases and errors covered

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

When Stuck

Problem	Solution
Don't know how to test	Write wished-for API. Write assertion first. Ask your human partner.
Test too complicated	Design too complicated. Simplify interface.
Must mock everything	Code too coupled. Use dependency injection.
Test setup huge	Extract render helpers with providers. Still complex? Simplify design.
act() warnings	Wrap state updates in act(), or use waitFor/findBy* queries.
Native module errors	Mock the native module in jest.setup.js or per-test.
Navigation hard to test	Mock expo-router hooks (useRouter, useLocalSearchParams).

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
• Testing implementation details instead of user-visible behavior
• Snapshot testing overuse (prefer behavioral assertions)
• Not using waitFor for async state updates

Additionally, the react-native-testing skill includes RNTL-specific anti-patterns (wrong query variants, waitFor misuse, unnecessary act(), legacy accessibility props, v14 async pitfalls) — invoke it when writing or reviewing RNTL tests.

Final Rule

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

No exceptions without your human partner's permission.