Flexible Testing

Overview

Write tests for critical paths and complex logic. Test-first is encouraged but not mandatory.

Core principle: Tests should give you confidence, not slow you down.

When to Use

Strong recommendation (test-first):

• Complex business logic
• Edge cases and error handling
• Security-critical code
• Bug fixes (test reproduces bug first)
• Public API surfaces

Optional (test-after or skip):

• UI/view code that changes frequently
• Prototype/exploration phase
• Configuration and setup
• Generated code
• Simple CRUD operations

Two Modes

Production Mode (Default)

Write tests for critical functionality. Test-first encouraged.

1. Write failing test for critical behavior
2. Implement minimal code to pass
3. Add tests for edge cases
4. Refactor with confidence

Prototype Mode

Exploring ideas? Skip tests temporarily.

1. Build the prototype
2. Get feedback, iterate
3. Once design is stable: write tests
4. Or: throw away prototype, rebuild with tests

To enter prototype mode: Tell Claude "I'm prototyping" or "skip tests for now"

What to Test

Always Test

Area	Why
Business logic	Core value of your app
Data transformations	Easy to get wrong
API responses	Contract with clients
Auth/permissions	Security critical
Error handling	Users will hit edge cases

Consider Testing

Area	When
UI components	When logic is complex
Integration points	When external APIs involved
Performance	When SLAs matter

Usually Skip

Area	Why
UI layout	Changes frequently, hard to test
Third-party code	Already tested
Trivial getters/setters	No logic to test
Generated code	Trust the generator

Test-First Pattern

When you do write test-first, follow RED-GREEN-REFACTOR:

RED - Failing Test

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);
});

GREEN - Make It Pass

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');
}

REFACTOR - Clean Up

Once tests pass, improve the code while keeping tests green.

Swift/XCTest Specifics

Basic Test Structure

import XCTest
@testable import YourApp

final class GradingServiceTests: XCTestCase {

    func test_gradeReturnsScoreForValidImage() async throws {
        let service = GradingService()
        let result = try await service.grade(image: testImage)

        XCTAssertNotNil(result.score)
        XCTAssertGreaterThan(result.score!, 0)
    }

    func test_gradeThrowsForInvalidImage() async {
        let service = GradingService()

        do {
            _ = try await service.grade(image: Data())
            XCTFail("Expected error")
        } catch {
            // Expected
        }
    }
}

Testing Actors

func test_actorStateIsolated() async {
    let service = SyncService()

    // Actor methods are async
    await service.sync(item: testItem)

    let synced = await service.pendingItems
    XCTAssertTrue(synced.isEmpty)
}

Testing @Observable

func test_authStateUpdatesOnSignIn() async throws {
    let authService = AuthService()

    XCTAssertFalse(authService.isAuthenticated)

    try await authService.signIn(with: mockCredentials)

    XCTAssertTrue(authService.isAuthenticated)
}

TypeScript/Vitest Specifics

Basic Test Structure

import { describe, it, expect } from 'vitest'
import { gradeImage } from './grading'

describe('gradeImage', () => {
  it('returns score for valid image', async () => {
    const result = await gradeImage(testImageBase64)

    expect(result.score).toBeDefined()
    expect(result.score).toBeGreaterThan(0)
  })

  it('throws for invalid image', async () => {
    await expect(gradeImage('')).rejects.toThrow()
  })
})

Testing Edge Functions

import { createClient } from '@supabase/supabase-js'

describe('process-item function', () => {
  it('returns 401 without auth', async () => {
    const response = await fetch(`${FUNCTION_URL}/process-item`, {
      method: 'POST',
      body: JSON.stringify({ itemId: '123' })
    })

    expect(response.status).toBe(401)
  })
})

Good Tests

Quality	Good	Bad
Focused	One behavior per test	test('validates, saves, and notifies')
Readable	Clear name describes what's tested	test('test1')
Independent	No shared state between tests	Tests fail in isolation
Fast	Milliseconds, not seconds	Slow tests get skipped

When Tests Are Hard

If testing is painful, the design might be the problem:

Symptom	Likely Cause	Fix
Need many mocks	Too coupled	Dependency injection
Huge setup	Complex object graph	Simplify design
Flaky tests	Hidden state/timing	Make dependencies explicit
Can't test in isolation	God objects	Break into smaller units

Checklist Before Shipping

[ ] Critical paths have tests
[ ] Edge cases covered
[ ] Error cases handled
[ ] Tests pass
[ ] No flaky tests

Don't need 100% coverage. Need confidence in critical functionality.