Testing Strategies

Testing Strategies Skill

Test the behavior, not the implementation. Test the boundaries, not the happy path.

Testing Pyramid

Anti-pattern: Inverted pyramid (too many E2E, few unit) → slow CI, flaky tests, hard to debug. Anti-pattern: Ice cream cone (manual testing on top of everything) → doesn't scale.

Unit Test Pattern (AAA)

test('should calculate discount when order exceeds $100', () => {
    // Arrange
    const order = createOrder({ subtotal: 150, customerTier: 'gold' });
    
    // Act
    const discount = calculateDiscount(order);
    
    // Assert
    expect(discount).toBe(15); // 10% for gold tier
});

Naming convention: should [expected behavior] when [condition] — reads as a specification.

Test Types Beyond the Pyramid

What to Mock (and What Not To)

Coverage Philosophy

The real metric: Coverage of changed code in each PR, not overall percentage.

What coverage doesn't tell you: That your tests assert the right things. 100% coverage with no assertions is useless.

What NOT to Test

Test Quality Signals

Mission-Critical Testing (NASA Standards)

For safety-critical or high-reliability projects, apply NASA/JPL Power of 10 testing patterns:

Bounded Behavior Testing

Assertion Coverage Testing

// Test that assertions fire on invalid input
test('validateUser throws on undefined', () => {
    expect(() => validateUser(undefined)).toThrow('assertion failed');
});

// Test that bounds are enforced
test('processItems rejects oversized batch', () => {
    const items = new Array(10001).fill({});
    expect(() => processItems(items)).toThrow('exceeds MAX_BATCH_SIZE');
});

Critical Path Coverage

TDD Cycle

TDD is not always the right choice: It works best for well-understood requirements. For exploratory code, write tests after the design stabilizes.

Flaky Test Triage