testing-typescript

name	testing-typescript
description	TypeScript testing with Vitest/Jest: file structure, mocking strategy, async patterns, and coverage targets

Testing — TypeScript

Framework

• Vitest for new projects (fast, native ESM, Jest-compatible API).
• Jest only if already configured — don't mix frameworks.
• Run: vitest run / vitest run --coverage

File Conventions

• Co-locate: src/utils/format.ts → src/utils/format.test.ts
• Shared fixtures: tests/fixtures/ or src/__tests__/helpers/

Test Structure

describe('formatDate', () => {
  it('should return ISO date string when given a valid Date', () => {
    // Arrange
    const date = new Date('2026-01-15');

    // Act
    const result = formatDate(date);

    // Assert
    expect(result).toBe('2026-01-15');
  });
});

Mocking

// Module mock — top of file
vi.mock('../services/emailService');

// Spy without full mock
const spy = vi.spyOn(mailer, 'send').mockResolvedValue(undefined);

// Restore after each test
afterEach(() => vi.restoreAllMocks());

// Time control
beforeEach(() => vi.useFakeTimers());
afterEach(() => vi.useRealTimers());

Async Tests

// Always await; use assertions count for safety
it('should reject with AuthError when token is expired', async () => {
  expect.assertions(1);
  await expect(verifyToken('expired')).rejects.toThrow('Token expired');
});

Coverage Targets

• Business logic: 85%+ branch coverage.
• Utility functions: 100% line coverage.
• Do not write tests just to hit numbers — test behavior.