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

Consider Testing

Usually Skip

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

When Tests Are Hard

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

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.