I want bulletproof testing for our finance tracker. Here's what I'm thinking:
 
- Unit tests for all utility functions (currency formatting, date calculations, validation)
- Component tests using React Testing Library for every UI component
- Integration tests for our API endpoints with proper database setup/teardown
- End-to-end tests for critical user flows like adding transactions and viewing reports
- Performance tests to ensure the app stays fast as data grows
 
Set up the testing infrastructure with proper configuration, then write comprehensive tests for our existing features. I want to be confident that changes won't break anything.

// currency.test.ts
describe('formatCurrency', () => {
  it('formats positive amounts correctly', () => {
    expect(formatCurrency(1234.56)).toBe('$1,234.56')
  })
 
  it('handles negative amounts', () => {
    expect(formatCurrency(-50)).toBe('-$50.00')
  })
 
  it('handles edge case of zero', () => {
    expect(formatCurrency(0)).toBe('$0.00')
  })
})

// TransactionForm.test.tsx
describe('TransactionForm', () => {
  it('submits valid transaction data', async () => {
    render(<TransactionForm onSubmit={mockSubmit} />)
 
    await userEvent.type(screen.getByLabelText('Amount'), '100')
    await userEvent.click(screen.getByRole('button', { name: 'Save' }))
 
    expect(mockSubmit).toHaveBeenCalledWith({
      amount: 100,
      type: 'expense'
    })
  })
})

// api/transactions.test.ts
describe('POST /api/transactions', () => {
  beforeEach(async () => {
    await db.reset()
  })
 
  it('creates a new transaction', async () => {
    const response = await request(app)
      .post('/api/transactions')
      .send({ amount: 50, type: 'income' })
 
    expect(response.status).toBe(201)
    expect(response.body.id).toBeDefined()
  })
})

// e2e/add-transaction.spec.ts
test('user can add a transaction', async ({ page }) => {
  await page.goto('/transactions')
  await page.click('button:has-text("Add")')
  await page.fill('[name="amount"]', '100')
  await page.click('button:has-text("Save")')
 
  await expect(page.locator('.transaction-list')).toContainText('$100')
})