test: Email Service - Convert integration tests to unit tests with proper mocking

[rezwana-karim]

Context

Part of the Test Suite Improvement Roadmap to increase test pass rate from 67.7% to 95%+.

Related: See docs/test-suite-improvement-roadmap.md for full context.

Problem

Failing Tests: 42 tests in tests/unit/services/email-service.test.ts

Files Affected:

• tests/unit/services/email-service.test.ts

Root Cause:
Tests use CommonJS require() in ES module context and attempt integration-style testing with real API calls instead of proper unit testing with mocks.

Specific Issues

1. Line 42: const { renderTemplate } = require('@/services/email-service')

   • Uses require() instead of ES module import
   • Fails in Vitest ES module environment

2. Resend API Mocking: Tests attempt to use real Resend API

   • Not properly mocked
   • Causes network-dependent test failures

3. Date/Time Handling: Tests fail due to timezone inconsistencies

   • Password reset expiration tests
   • Verification email expiration tests

4. Template Rendering: Tests for XSS protection and variable substitution fail

   • Module import issues prevent testing

Proposed Solution

Changes Required

1. Convert to ES Module Imports:

   // Remove this:
   const { renderTemplate } = require('@/services/email-service');
   
   // Replace with:
   import { 
     renderTemplate, 
     sendEmail, 
     sendOrderConfirmation,
     sendShippingConfirmation,
     sendPasswordReset 
   } from '@/services/email-service';

2. Add Proper Resend Mocking:

   vi.mock('resend', () => ({
     Resend: vi.fn(() => ({
       emails: {
         send: vi.fn().mockResolvedValue({ 
           id: 'test-email-id',
           from: 'noreply@stormcom.test',
           to: 'test@example.com',
         }),
       },
     })),
   }));

3. Implement Consistent Date Mocking:

   beforeEach(() => {
     // Mock system time for consistent test results
     vi.setSystemTime(new Date('2025-01-01T00:00:00Z'));
   });
   
   afterEach(() => {
     vi.useRealTimers();
   });

4. Refactor from Integration to Unit Tests:

   • Test renderTemplate() function in isolation
   • Mock all external dependencies (Resend, database)
   • Verify function behavior, not side effects
   • Test error handling paths

Expected Outcome

• All 42 tests passing
• No real API calls (fully mocked)
• Tests run in < 2 seconds
• TypeScript strict mode compliant
• Deterministic results (no timezone dependencies)

Implementation Checklist

Development

• Read docs/test-suite-improvement-roadmap.md
• Review ES module import patterns in passing tests
• Set up local environment
• Run failing tests: npm run test -- tests/unit/services/email-service.test.ts
• Convert all require() to ES imports
• Add proper Resend mocking at file top
• Add date/time mocking in beforeEach
• Refactor tests to be unit tests (not integration)
• Run npm run type-check
• Run npm run lint
• Verify all 42 tests passing

Testing

• All 42 email service tests passing
• No new test failures introduced
• Tests complete in < 2 seconds
• Tests pass consistently (run 5 times)
• No real network calls

Documentation

• Add JSDoc comments to complex test helpers
• Document Resend mocking pattern
• Update roadmap progress
• Add example to tests/README.md

Code Review

• Self-review completed
• No functional code changes (test-only PR)
• Follows patterns from security-headers.test.ts
• Proper commit messages
• PR description includes before/after test results

Acceptance Criteria

• All 42 tests passing (was: 42 failing)
• Test suite pass rate increased by ~4%
• No regressions in other tests
• TypeScript/ESLint passing
• Reusable Resend mock pattern documented

Test Results

Before

❌ FAIL tests/unit/services/email-service.test.ts (42 tests)
  Error: Cannot find module '@/services/email-service'
  
Test Files  1 failed (1)
     Tests  42 failed (42)

After

✅ PASS tests/unit/services/email-service.test.ts (42 tests)
  
Test Files  1 passed (1)
     Tests  42 passed (42)
   Duration  1.8s

Related Issues

• Part of: Test Suite Improvement Roadmap
• Blocks: #XXX (Notification Service tests - similar pattern)
• See also: tests/unit/lib/security-headers.test.ts for ES module mock example

Priority

• P1 - Critical (blocks production deployment)
• P2 - Important (blocks features)
• P3 - Nice to have (quality improvement)

Estimated Effort

• Small (< 4 hours)
• Medium (4-8 hours)
• Large (1-2 days)
• X-Large (> 2 days)

Notes

Key Files to Reference

• tests/setup.ts - ES module import patterns
• tests/unit/lib/security-headers.test.ts - next-auth mocking example
• tests/unit/components/gdpr/cookie-consent.test.tsx - date mocking example

Testing Gotchas

• Must use vi.setSystemTime() not vi.useFakeTimers() for date mocking
• Resend mock must return proper structure with id, from, to fields
• Template rendering tests need actual HTML string validation

Success Metrics

• Test execution time: < 2 seconds (currently fails to complete)
• Pass rate contribution: +4% to overall suite
• Code coverage: Email service should reach 80%+ coverage

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