Unit Testing Guidelines Skill

Unit Testing Guidelines

Best practices for writing and reviewing unit tests. Contains rules covering test structure, naming, determinism, and code review.

Reference: MetaMask Unit Testing Guidelines

When to Apply

Reference these guidelines when:

Writing new unit tests
Reviewing test code in PRs
Refactoring existing tests
Debugging flaky or brittle tests
Improving test coverage

Rule Categories by Priority

| Priority | Category | Impact | Rule File | | -------- | --------------- | -------- | ---------------------------- | | 1 | Best Practices | CRITICAL | rules/best-practices.md | | 2 | Determinism | HIGH | rules/determinism.md | | 3 | Reviewer Guide | MEDIUM | rules/reviewer-guide.md | | 4 | Anti-patterns | MEDIUM | rules/anti-patterns.md |

Quick Reference

1. Best Practices (CRITICAL)

Use the AAA pattern (Arrange, Act, Assert):

it('indicates expired milk when past due date', () => {
  // Arrange
  const today = new Date('2025-06-01');
  const milk = { expiration: new Date('2025-05-30') };

  // Act
  const result = isMilkGood(today, milk);

  // Assert
  expect(result).toBe(false);
});

Meaningful test names - describe purpose, not implementation:

it('displays an error when input is invalid', () => { ... });

One behavior per test, isolated from others.

See rules/best-practices.md for complete guidance.

2. Determinism (HIGH)

Mock time, randomness, and external systems:

jest.useFakeTimers();
jest.setSystemTime(new Date('2024-01-01'));

Avoid relying on global state or hardcoded values
Only test public behavior, not implementation details

See rules/determinism.md for details.

3. Reviewer Guide (MEDIUM)

Test the test - validate tests fail when code is broken:

// Break the SuT and make sure this test fails
expect(result).toBe(false);

Ensure proper matchers (toBeOnTheScreen vs toBeDefined)
Reject complex test names with multiple conditions

See rules/reviewer-guide.md for details.

4. Anti-patterns (MEDIUM)

Avoid:

Code coverage without real assertions
Weak matchers (toBeDefined, toBeTruthy) for element presence

See rules/anti-patterns.md for examples.

Test Scope and Verification

Do not write tests for reversible, low-impact changes that mirror the implementation. If you do choose to verify your work with tests, make sure that the tests are meaningful and necessary to verify implementation.

Run tests appropriate to the change and complete required checks. Once those pass, broaden or repeat testing only when new changes, failures, or unresolved concerns justify it; otherwise, continue toward completing the task.

PR Checklist

Before submitting a PR with tests:

[ ] Tests use AAA pattern (Arrange, Act, Assert)
[ ] Test names describe the expected behavior
[ ] One behavior per test
[ ] No reliance on global state or implementation details
[ ] Time/randomness mocked where needed
[ ] Proper matchers used for assertions
[ ] Checked for existing *TestUtils/*TestFactory classes before creating new fixtures
[ ] Extracted shared fixtures to utility class if same setup needed in 2+ test files
[ ] All tests pass locally

Agent Skills: Unit Testing Guidelines

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