Question 1

What is a unit test?

Accepted Answer

A unit test is automated code that tests a small, isolated piece of functionality (typically a function or method) in isolation from its dependencies. Good unit tests are fast, isolated, repeatable, and self-validating.

Question 2

What is the AAA pattern in unit testing?

Accepted Answer

AAA stands for Arrange-Act-Assert: Arrange sets up test data and preconditions, Act executes the code being tested, and Assert verifies the expected outcome. This pattern makes tests readable and well-structured.

Question 3

What is the difference between a mock and a stub?

Accepted Answer

A stub provides canned responses to calls made during the test without verifying interactions. A mock is pre-programmed with expectations and verifies that specific methods were called with specific arguments.

Question 4

What are test doubles?

Accepted Answer

Test doubles are objects that replace real dependencies in tests. Types include: dummies (placeholder objects), stubs (provide canned answers), mocks (verify interactions), spies (record calls), and fakes (working implementations).

Question 5

What is test isolation?

Accepted Answer

Test isolation means each test runs independently without affecting or being affected by other tests. Isolated tests can run in any order, don't share state, and their failure points to a specific problem.

Question 6

What is code coverage?

Accepted Answer

Code coverage measures how much of your code is executed during tests. Types include line coverage (lines executed), branch coverage (decision paths taken), and path coverage (complete execution paths tested).

Question 7

What is a good code coverage percentage?

Accepted Answer

Most teams aim for 70-80% coverage. Focus on testing critical business logic thoroughly rather than hitting arbitrary percentages. 100% coverage doesn't guarantee bug-free code—it only means all lines were executed.

Question 8

What is the test pyramid?

Accepted Answer

The test pyramid suggests having many fast unit tests at the base, fewer integration tests in the middle, and even fewer slow end-to-end tests at the top. This provides fast feedback while maintaining confidence.

Question 9

What is dependency injection?

Accepted Answer

Dependency injection passes dependencies to a class instead of creating them internally. This makes code testable by allowing test doubles to be injected, isolating the unit under test from external dependencies.

Question 10

What are the dangers of over-mocking?

Accepted Answer

Over-mocking creates tests tightly coupled to implementation details, making them fragile. Tests may pass even when the real system fails. Mock only external dependencies and collaborators, not internal implementation.

Question 11

How do you test asynchronous code?

Accepted Answer

Use async/await in test functions, return promises from tests, or use done callbacks. Test frameworks like Jest handle async automatically. Ensure you await all async operations and test both success and error paths.

Question 12

How do you test time-dependent code?

Accepted Answer

Use fake timers provided by test frameworks (jest.useFakeTimers()). Inject clock dependencies that can be controlled in tests. Avoid setTimeout/setInterval in business logic—extract time dependencies for testability.

Question 13

What is mutation testing?

Accepted Answer

Mutation testing introduces small changes (mutations) to your code and checks if tests catch them. If a mutation survives (tests still pass), your tests may be weak. It measures test effectiveness beyond code coverage.

Question 14

How do you name unit tests effectively?

Accepted Answer

Use descriptive names following patterns like 'methodName_scenario_expectedResult' or 'should_expectedBehavior_when_condition'. Names should describe what is being tested and what outcome is expected.

Question 15

What makes a test fragile?

Accepted Answer

Fragile tests break when implementation changes but behavior doesn't. Causes include testing implementation details, over-mocking, relying on specific data ordering, and coupling to UI structure. Test behavior, not implementation.

Unit Testing Fundamentals(5 questions)

What is a unit test and what makes a good unit test?

What are the characteristics of a unit test (fast, isolated, repeatable, self-validating)?

What is the AAA pattern (Arrange-Act-Assert)?

What is test isolation and why is it important?

What is the difference between unit tests and integration tests?

Test Doubles(5 questions)

What are test doubles and what types exist (mocks, stubs, fakes, spies, dummies)?

What is the difference between a mock and a stub?

When should you use mocks vs stubs?

What is a spy and when would you use it?

What is a fake and when is it appropriate to use one?

Mocking and Dependencies(5 questions)

How do you handle external dependencies in unit tests?

What is dependency injection and how does it help testing?

How do you mock static methods and singletons?

What are the dangers of over-mocking?

How do you decide what to mock and what not to mock?

Test Organization(5 questions)

How do you name unit tests effectively?

How do you organize test files and test suites?

What is the one assertion per test rule and when should you follow it?

How do you handle test setup and teardown?

What are test fixtures and how do you use them?

Code Coverage(4 questions)

What is code coverage and what types exist (line, branch, path)?

What is a good code coverage percentage to aim for?

Why can 100% code coverage still have bugs?

How do you identify untested code paths?

Testing Edge Cases(4 questions)

How do you test edge cases and boundary conditions?

How do you test error handling and exceptions?

How do you test null and empty values?

What is property-based testing and when would you use it?

Test Quality and Maintainability(5 questions)

What makes a test fragile and how do you avoid it?

What is the test pyramid and why is it important?

How do you refactor tests without breaking them?

What are testing anti-patterns to avoid?

How do you balance test coverage with development speed?

Advanced Unit Testing(4 questions)

How do you test asynchronous code?

How do you test time-dependent code?

How do you test random behavior?

What is mutation testing and how does it improve test quality?