Testing with Claude

Claude Code can generate comprehensive tests, but you need to guide it correctly.

Testing Philosophy

Claude should write tests that:

Cover edge cases you might miss
Document behavior through test names
Catch regressions before production
Run fast for quick feedback

Unit Tests

Basic Pattern

> Write unit tests for the validateEmail function
> Cover valid emails, invalid formats, and edge cases

Claude will generate:

describe('validateEmail', () => {
  it('accepts valid email addresses', () => {
    expect(validateEmail('user@example.com')).toBe(true);
  });
 
  it('rejects emails without @', () => {
    expect(validateEmail('invalid')).toBe(false);
  });
 
  it('handles edge cases', () => {
    expect(validateEmail('')).toBe(false);
    expect(validateEmail(null)).toBe(false);
  });
});

Comprehensive Coverage

> Generate unit tests for src/services/user.ts
> Include:
> - Happy path for all functions
> - Error handling paths
> - Edge cases (null, undefined, empty)
> - Boundary conditions

Integration Tests

API Testing

> Write integration tests for the /api/users endpoint
> Test all CRUD operations with real database
> Use test fixtures for setup

Service Integration

> Create integration tests for the payment flow
> Mock Stripe but test our service layer end-to-end

E2E Tests

User Flows

> Write Playwright E2E tests for the login flow
> Cover:
> - Successful login
> - Invalid credentials
> - Password reset
> - Remember me functionality

Test Quality Checklist

Ask Claude to verify:

> Review the tests you wrote:
> 1. Do they test behavior, not implementation?
> 2. Are test names descriptive?
> 3. Is each test independent?
> 4. Are mocks minimal and appropriate?

Coverage Strategy

> Check our test coverage and suggest
> additional tests for uncovered code paths

Best Practices

Name Tests Descriptively

// Bad
it('works', () => {})

// Good
it('rejects login attempts after 5 failures within 15 minutes', () => {})

Test Behavior, Not Implementation

// Bad - tests implementation
expect(user._validatePassword).toHaveBeenCalled()

// Good - tests behavior
expect(result.success).toBe(true)

Keep Tests Fast

> Optimize these tests to run in parallel
> and mock slow external services

Next Steps

In the next lesson, you'll learn to set up CI/CD pipelines that run these tests automatically.