test("divide by zero", () => { expect(() => divide(10, 0)).toThrow(RangeError); });

test("divide negative", () => { expect(divide(-10, 2)).toBe(-5); });


## T2: Use a Coverage Tool

Coverage tools report gaps in your testing strategy. Don't ignore them.

```bash
# Run with coverage
vitest run --coverage

# Aim for meaningful coverage, not 100%

T3: Don't Skip Trivial Tests

Trivial tests document behavior and catch regressions. They're worth more than their cost.

// Worth having - documents expected behavior
test("user default role", () => {
  const user = new User("Alice");
  expect(user.role).toBe("member");
});

T4: An Ignored Test Is a Question About an Ambiguity

Don't use test.skip to hide problems. Either fix the test or delete it.

// Bad - hiding a problem
test.skip("async operation", () => {
  // flaky, fix later
});

// Good - either fix it or document why it's skipped
test.skip("cache invalidation - requires Redis (see CONTRIBUTING.md)", () => {
});

T5: Test Boundary Conditions

Bugs congregate at boundaries. Test them explicitly.

test("pagination boundaries", () => {
  const items = Array.from({ length: 100 }, (_, i) => i);

  // First page
  expect(paginate(items, 1, 10)).toEqual(items.slice(0, 10));

  // Last page
  expect(paginate(items, 10, 10)).toEqual(items.slice(90, 100));

  // Beyond last page
  expect(paginate(items, 11, 10)).toEqual([]);

  // Page zero (invalid)
  expect(() => paginate(items, 0, 10)).toThrow(RangeError);

  // Empty list
  expect(paginate([], 1, 10)).toEqual([]);
});

T6: Exhaustively Test Near Bugs

When you find a bug, write tests for all similar cases. Bugs cluster.

// Found bug: off-by-one in date calculation
// Now test ALL date boundaries
test("month boundaries", () => {
  expect(lastDayOfMonth(2024, 1)).toBe(31); // January
  expect(lastDayOfMonth(2024, 2)).toBe(29); // Leap year February
  expect(lastDayOfMonth(2023, 2)).toBe(28); // Non-leap February
  expect(lastDayOfMonth(2024, 4)).toBe(30); // 30-day month
  expect(lastDayOfMonth(2024, 12)).toBe(31); // December
});

T7: Patterns of Failure Are Revealing

When tests fail, look for patterns. They often point to deeper issues.

// If all async tests fail intermittently,
// the problem isn't the tests—it's the async handling

T8: Test Coverage Patterns Can Be Revealing

Look at which code paths are untested. Often they reveal design problems.

// If you can't easily test a function, it probably does too much
// Refactor for testability

T9: Tests Should Be Fast

Slow tests don't get run. Keep unit tests under 100ms each.

// Bad - hits real database
test("user creation", async () => {
  const db = await connectToDatabase(); // Slow!
  const user = await db.createUser("Alice");
  expect(user.name).toBe("Alice");
});

// Good - uses mock or in-memory
test("user creation", async () => {
  const db = new InMemoryDatabase();
  const user = await db.createUser("Alice");
  expect(user.name).toBe("Alice");
});

Test Organization

F.I.R.S.T. Principles

Fast: Tests should run quickly
Independent: Tests shouldn't depend on each other
Repeatable: Same result every time, any environment
Self-Validating: Pass or fail, no manual inspection
Timely: Written before or with the code, not after

One Concept Per Test

// Bad - testing multiple things
test("user", () => {
  const user = new User("Alice", "alice@example.com");
  expect(user.name).toBe("Alice");
  expect(user.email).toBe("alice@example.com");
  expect(user.isValid()).toBe(true);
  user.activate();
  expect(user.isActive).toBe(true);
});

// Good - one concept each
test("user stores name", () => {
  const user = new User("Alice", "alice@example.com");
  expect(user.name).toBe("Alice");
});

test("user stores email", () => {
  const user = new User("Alice", "alice@example.com");
  expect(user.email).toBe("alice@example.com");
});

test("new user is valid", () => {
  const user = new User("Alice", "alice@example.com");
  expect(user.isValid()).toBe(true);
});

test("user can be activated", () => {
  const user = new User("Alice", "alice@example.com");
  user.activate();
  expect(user.isActive).toBe(true);
});

Quick Reference

Rule	Principle
T1	Test everything that could break
T2	Use coverage tools
T3	Don't skip trivial tests
T4	Ignored test = ambiguity question
T5	Test boundary conditions
T6	Exhaustively test near bugs
T7	Look for patterns in failures
T8	Check coverage when debugging
T9	Tests must be fast (<100ms)

clean-tests

Clean Tests

T1: Insufficient Tests