Testing & Debugging

Question 1

What is Mocha?

Answer 1

Mocha is a JavaScript test framework for Node.js that provides a flexible way to write asynchronous tests using BDD or TDD style.

Question 2

What is Jest?

Answer 2

Jest is a JavaScript testing framework developed by Meta that offers zero-config testing, built-in assertions, mocking, and code coverage.

Question 3

What is Supertest?

Answer 3

Supertest is a Node.js library used for HTTP assertions to test APIs by making requests to an Express app or any HTTP server.

Question 4

What is the Test Pyramid?

Answer 4

The Test Pyramid is a concept that advocates for a balanced test suite: more unit tests, fewer integration tests, and even fewer end-to-end (E2E) tests.

Question 5

What is a mock?

Answer 5

A mock is a simulated object or function used to isolate the behavior of code being tested by replacing real dependencies.

Question 6

What is a spy?

Answer 6

A spy is a wrapper around a function that tracks how it is called, including arguments, return values, and call counts.

Question 7

What is a stub?

Answer 7

A stub is a function that replaces another function and returns controlled outputs for testing purposes.

Question 8

What are Chrome DevTools?

Answer 8

Chrome DevTools are a set of web development tools built into the Chrome browser, useful for debugging, profiling, and inspecting code.

Question 9

What is the Node.js Inspector?

Answer 9

Node Inspector is a debugging tool for Node.js applications that allows step-by-step code execution and inspection using DevTools.

Question 10

What is the advantage of Jest over Mocha?

Answer 10

Jest includes built-in mocking, coverage reports, snapshot testing, and requires zero configuration.

Question 11

What is a disadvantage of using Mocha?

Answer 11

Mocha requires additional libraries like Chai or Sinon for assertions, spies, and mocks.

Question 12

What is the advantage of the Test Pyramid model?

Answer 12

It ensures faster, more maintainable test suites by emphasizing unit tests and limiting slow, flaky end-to-end tests.

Question 13

What is a best practice for mocking?

Answer 13

Mock only external dependencies or services, not the code you’re testing directly.

Question 14

What is a use case for Supertest?

Answer 14

Testing REST APIs in Express applications by simulating HTTP requests and verifying responses.

Question 15

What is a typical example of using Jest for a test?

Answer 15

test(‘adds numbers’, () => { expect(add(2, 3)).toBe(5); });

Question 16

What is the impact of good testing on system design?

Answer 16

It encourages modular, decoupled components and facilitates easier refactoring and maintenance.

Question 17

How does mocking affect architectural decisions?

Answer 17

Heavy use of mocks can indicate tightly coupled systems and may hide integration issues if overused.

Question 18

What is the performance impact of end-to-end tests?

Answer 18

E2E tests are slower and more brittle due to full-stack execution, so they should be minimized in number.

Question 19

What is a common debugging workflow in Node.js?

Answer 19

Add breakpoints using debugger, start Node with --inspect, and connect with Chrome DevTools.

Question 20

What is an example of a stub in Sinon?

Answer 20

const stub = sinon.stub(obj, 'method').returns('value');

Question 21

What is a real-world tradeoff with Jest’s snapshot testing?

Answer 21

Snapshots can become outdated or misleading if developers accept changes without reviewing them carefully.

Question 22

What is a potential gotcha when mocking?

Answer 22

Improper mocking may hide real bugs or give a false sense of correctness in tests.

Question 23

What should you test with unit tests?

Answer 23

Pure functions or isolated logic with no external dependencies like databases or APIs.

Question 24

What should you test with integration tests?

Answer 24

Interactions between components or services such as database queries or API calls.

Question 25

What is a common gotcha with async tests in Jest or Mocha?

Answer 25

Forgetting to use `async/await` or not returning a promise may cause the test to pass before execution completes.

Question 26

What are lifecycle hooks in Jest?

Answer 26

beforeAll, afterAll, beforeEach, afterEach – used to run setup/teardown logic around test execution.

Question 27

What is code coverage?

Answer 27

A metric that indicates how much of your codebase is executed by your test suite.

Question 28

What’s a Jest feature for mocking modules?

Answer 28

`jest.mock('module')` allows mocking entire modules easily.

Question 29

What are spies used for in testing?

Answer 29

To observe if a function was called, how many times, and with what arguments, without replacing it.

Question 30

How can DevTools help in debugging?

Answer 30

They allow stepping through code, watching variables, setting breakpoints, and inspecting the call stack and scopes.

Question 31

What is a disadvantage of snapshot testing?

Answer 31

Snapshots can become brittle and don't provide semantic understanding of changes.

Question 32

How does the test pyramid improve fault tolerance?

Answer 32

By detecting bugs early through unit tests, reducing reliance on flaky E2E tests.

Question 33

What is a best practice for debugging production Node.js apps?

Answer 33

Use logging tools (e.g., Winston), structured logs, and APM tools like New Relic or Datadog instead of breakpoints.

Question 34

How do you run a single test file in Jest?

Answer 34

`jest filename.test.js`

Question 35

How can Supertest be integrated with Jest?

Answer 35

By using Supertest in a Jest test block: `await request(app).get('/endpoint').expect(200);`

Question 36

What is a common interview question about Jest?

Answer 36

What are the differences between Jest and Mocha? How do you mock a function in Jest?

Question 37

What is a common interview question about test design?

Answer 37

Explain the test pyramid and how you structure tests in a large application.

Question 38

What is the benefit of automated testing in CI/CD?

Answer 38

Automated tests ensure code quality, catch regressions, and allow safe deployments.

Question 39

What is a common debugging tool for Node.js in the terminal?

Answer 39

`node inspect` or `node --inspect-brk` for interactive debugging.

Question 40

Why is test isolation important?

Answer 40

Test isolation prevents side effects between tests and makes results reliable and reproducible.

Question 41

What are flaky tests?

Answer 41

Tests that sometimes pass and sometimes fail due to timing, randomness, or external dependencies.

Question 42

What’s the difference between unit and integration tests?

Answer 42

Unit tests test isolated pieces of code; integration tests check how components work together.

Question 43

What is a good rule for when to mock?

Answer 43

Mock when you want to control or observe external dependencies, not internal logic.

Question 44

What’s the role of `expect` in Jest?

Answer 44

`expect` is the assertion function used to test values in Jest.

Question 45

How do you skip a test in Jest?

Answer 45

Use `test.skip()` or prefix the test name with `x` (e.g., `xtest()` or `describe.skip()`)

Question 46

How do you debug Jest tests?

Answer 46

Use `--inspect-brk` with Node, or add `debugger` and run `node --inspect-brk node_modules/.bin/jest`.

Question 47

What is the benefit of combining Mocha with Chai and Sinon?

Answer 47

Chai provides expressive assertions, while Sinon adds powerful mocking and spying capabilities to Mocha.