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of 32What is unit testing and why is it important?
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Unit testing tests individual components (functions, methods, classes) in isolation to verify they work correctly.
Importance:
- Early bug detection: Catches issues during development
- Documentation: Tests document expected behavior
- Refactoring confidence: Enables safe code changes
- Design improvement: Promotes modular, testable code
- Cost reduction: Fixing bugs early is cheaper
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What are the characteristics of a good unit test?
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Good unit tests follow the FIRST principles:
- Fast: Run quickly (milliseconds)
- Independent: Don't depend on other tests or external systems
- Repeatable: Produce consistent results
- Self-validating: Clear pass/fail result
- Timely: Written alongside production code
Additional characteristics:
- Readable: Clear names and structure
- Focused: Test one specific behavior
- Isolated: Use mocks/stubs for dependencies
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Explain the difference between unit testing, integration testing, and end-to-end testing.
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- Unit Testing: Tests individual components in isolation using mocks. Fast and focused.
- Integration Testing: Tests how components work together. Involves real dependencies like databases.
- End-to-End Testing: Tests complete user workflows through the entire system. Comprehensive but slow.
Test Pyramid: Many unit tests (fast, cheap) → Some integration tests → Few E2E tests (slow, expensive).
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What is the AAA pattern in unit testing?
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AAA stands for Arrange, Act, Assert - a common pattern for structuring unit tests:
- Arrange: Set up test data, mocks, and preconditions
- Act: Execute the method or function being tested
- Assert: Verify the expected outcome
test('calculateDiscount returns correct discount amount', () => {
// Arrange
const price = 100;
const discountPercent = 10;
// Act
const result = calculateDiscount(price, discountPercent);
// Assert
expect(result).toBe(10);
});
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How should unit tests be organized and named?
How do you test error conditions and exceptions?
What is code coverage and what are its different types?
What is dependency injection and how does it help with testing?
What is the difference between a test double, mock, stub, and spy?
Compare different JavaScript testing frameworks (Jest, Mocha, Jasmine).
What are parameterized tests and when would you use them?
What is test isolation and why is it important?
Explain the concept of test fixtures and setup/teardown methods.
When should you use mocks vs real dependencies?
How do you test code that depends on external APIs?
Explain the TDD cycle (Red-Green-Refactor).
What are the benefits and drawbacks of TDD?
How do you handle testing private methods?
What makes a unit test maintainable?
How do you test asynchronous code?
What are flaky tests and how do you prevent them?
What are the limitations of code coverage metrics?
How do you measure and improve test suite performance?
What are some common unit testing anti-patterns?
What is the testing pyramid vs testing trophy and when to use each?
How do you test React components effectively?
What are the challenges of mocking and how do you overcome them?
What is property-based testing and when would you use it?
Explain mutation testing and its benefits.
How do you test legacy code that wasn't designed for testing?
What is contract testing and how does it relate to unit testing?
How do you handle testing in a microservices architecture?
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