frontend-testing-best-practices

Testing Best Practices

Guidelines for writing effective, maintainable tests that provide real confidence. Contains 6 rules focused on preferring E2E tests, minimizing mocking, and testing behavior over implementation.

Core Philosophy

1. Prefer E2E tests over unit tests - Test the whole system, not isolated pieces
2. Minimize mocking - If you need complex mocks, write an E2E test instead
3. Test behavior, not implementation - Test what users see and do
4. Avoid testing React components directly - Test them through E2E

When to Apply

Reference these guidelines when:

• Deciding what type of test to write
• Writing new E2E or unit tests
• Reviewing test code
• Refactoring tests

Rules Summary

Testing Strategy (CRITICAL)

prefer-e2e-tests - @rules/prefer-e2e-tests.md

Default to E2E tests. Only write unit tests for pure functions.

// E2E test (PREFERRED) - tests real user flow
test("user can place an order", async ({ page }) => {
  await createTestingAccount(page, { account_status: "active" });
  await page.goto("/catalog");
  await page.getByRole("heading", { name: "Example Item" }).click();
  await page.getByRole("link", { name: "Buy" }).click();
  // ... complete flow
  await expect(page.getByAltText("Thank you")).toBeVisible();
});

// Unit test - ONLY for pure functions
test("formatCurrency formats with two decimals", () => {
  expect(formatCurrency(1234.5)).toBe("$1,234.50");
});

avoid-component-tests - @rules/avoid-component-tests.md

Don't unit test React components. Test them through E2E or not at all.

// BAD: Component unit test
describe("OrderCard", () => {
  test("renders amount", () => {
    render(<OrderCard amount={100} />);
    expect(screen.getByText("$100")).toBeInTheDocument();
  });
});

// GOOD: E2E test covers the component naturally
test("order history shows orders", async ({ page }) => {
  await page.goto("/orders");
  await expect(page.getByText("$100")).toBeVisible();
});