Question 1

What is the difference between Jest and other testing frameworks like Mocha or Jasmine?

Accepted Answer

The 30-Second Answer: Jest is an all-in-one testing framework with built-in assertions, mocking, and coverage tools, while Mocha requires separate libraries like Chai for assertions and Sinon for mocking. Jasmine is similar to Jest but lacks snapshot testing and has slower parallel execution, making Jest the modern choice for most projects.

Question 2

What is the difference between it() and test() in Jest?

Accepted Answer

The 30-Second Answer: There is no functional difference - it() and test() are aliases of the exact same function. Use it() for BDD-style specs that read "it should do X" or test() for direct descriptions. Choose one style and stay consistent across your codebase. The 2-Minute Answer (If They Want More): This is one of the most common sources of confusion for developers new to Jest, but the answer is simple: they're completely identical.

Question 3

What is the difference between toEqual() and toStrictEqual()?

Accepted Answer

The 30-Second Answer: toStrictEqual() is a stricter version of toEqual() that checks object types, undefined properties, array sparseness, and class instances, while toEqual() is more lenient and ignores undefined object properties and treats sparse arrays as dense arrays. The 2-Minute Answer (If They Want More): While both matchers perform deep equality checks, toStrictEqual() enforces additional constraints that make tests more rigorous.

Question 4

What is the difference between jest.fn(), jest.mock(), and jest.spyOn()?

Accepted Answer

The 30-Second Answer: jest.fn() creates a standalone mock function, jest.mock() mocks an entire module by replacing all its exports, and jest.spyOn() wraps an existing method to track calls while optionally replacing its implementation. Each serves different mocking scenarios in your tests. The 2-Minute Answer (If They Want More): These three Jest utilities address different mocking needs. jest.

Question 5

What is the difference between mockReturnValueOnce() and mockReturnValue()?

Accepted Answer

The 30-Second Answer: mockReturnValueOnce() applies only to the next call of the mock function, then gets consumed, while mockReturnValue() sets the return value for all subsequent calls. You can chain multiple "Once" calls to create sequences of different return values. The 2-Minute Answer (If They Want More): The key difference is persistence and consumption. When you call mockReturnValue(), you're setting a default return value that persists for all future calls to that mock.

Question 6

What is the difference between mockClear(), mockReset(), and mockRestore()?

Accepted Answer

The 30-Second Answer: mockClear() erases call history but preserves mock implementation, mockReset() erases everything including implementation, and mockRestore() only works on spies to completely restore the original non-mocked function. Each serves progressively deeper levels of cleanup. The 2-Minute Answer (If They Want More): These methods form a hierarchy of mock cleanup operations, each removing more state than the previous.

Question 7

What is the difference between beforeEach, afterEach, beforeAll, and afterAll?

Accepted Answer

The 30-Second Answer: beforeEach and afterEach run before and after every single test, ensuring each test has fresh setup and cleanup. beforeAll and afterAll run once per test suiteâ€”before any tests start and after all tests completeâ€”making them ideal for expensive operations that can be shared across tests. The 2-Minute Answer (If They Want More): The primary difference is frequency of execution.

Question 8

What is the difference between setupFiles and setupFilesAfterEnv?

Accepted Answer

The 30-Second Answer: setupFiles runs before the test framework is initialized and doesn't have access to Jest's globals, making it ideal for polyfills and environment configuration. setupFilesAfterEnv runs after the framework is ready with access to Jest's APIs, making it ideal for custom matchers, global mocks, and test framework configuration. The 2-Minute Answer (If They Want More): The key difference is timing and context.

Question 9

What is the difference between toMatchSnapshot() and toMatchInlineSnapshot()?

Accepted Answer

The 30-Second Answer: toMatchSnapshot() stores snapshots in external .snap files in a __snapshots__ directory, while toMatchInlineSnapshot() writes the snapshot directly into your test file as the second argument. Inline snapshots are better for small snapshots where you want the expected output visible in the test itself, while external snapshots work better for large outputs. The 2-Minute Answer (If They Want More): The key difference is where the snapshot is stored.

Question 10

What is the difference between collectCoverage and --coverage flag?

Accepted Answer

The 30-Second Answer: collectCoverage: true in the Jest config enables coverage collection every time tests run, while the --coverage flag enables it only for that specific test execution. I typically use the config for CI/CD and the flag for on-demand local coverage checks to keep local tests fast. The 2-Minute Answer (If They Want More): The key difference lies in when and how coverage collection is triggered. The collectCoverage option is a configuration setting you place in your jest.config.

Jest Fundamentals(8 questions)

What is Jest and why is it popular for JavaScript testing?

How do you install and configure Jest in a project?

What is the difference between Jest and other testing frameworks like Mocha or Jasmine?

Explain the basic structure of a Jest test file.

What are describe, it, and test blocks in Jest?

What is the difference between it() and test() in Jest?

How does Jest discover and run test files?

What is the jest.config.js file and what are its common configuration options?

Performance and Optimization(10 questions)

How do you speed up Jest test execution?

What is the --runInBand flag and when would you use it?

How does Jest parallelize test execution?

What is the --maxWorkers option?

How do you use --onlyChanged to run tests for changed files?

What is the Jest cache and how do you clear it?

How do you identify slow tests in Jest?

What are the best practices for optimizing large test suites?

How do you use --bail to fail fast?

What is the --findRelatedTests option?

Advanced Features(12 questions)

What are test.each and describe.each for parameterized tests?

How do you skip or focus tests with skip and only?

What is test.todo() and when would you use it?

How do you run tests in a specific order?

What are Jest projects and when would you use multiple projects?

How do you test code that uses environment variables?

What is the moduleNameMapper configuration option?

How do you handle path aliases and module resolution in Jest?

What is the transform option and when do you need custom transformers?

How do you test TypeScript code with Jest?

What is ts-jest and how does it differ from babel-jest?

How do you debug Jest tests?

CI/CD and Integration(8 questions)

How do you run Jest in continuous integration (CI) environments?

What is the --ci flag and what does it do?

How do you generate JUnit XML reports for CI?

How do you integrate Jest with GitHub Actions?

What is the --reporters option for custom output formats?

How do you run Jest in watch mode for development?

What are the different watch mode options?

How do you configure Jest for monorepo setups?

Assertions and Matchers(12 questions)

What are matchers in Jest and how do they work?

Explain the difference between toBe() and toEqual().

What is the difference between toEqual() and toStrictEqual()?

How do you test for null, undefined, and falsy values in Jest?

What are the common numeric matchers (toBeGreaterThan, toBeLessThan, toBeCloseTo)?

How do you test strings with matchers like toMatch() and toContain()?

How do you test arrays and iterables in Jest?

What is the toThrow() matcher and how do you test for exceptions?

How do you use toHaveProperty() for testing object properties?

What is the toMatchObject() matcher and when would you use it?

How do you create custom matchers in Jest?

What is the resolves and rejects matchers for testing promises?

Mocking(16 questions)

What is mocking and why is it important in unit testing?

What is the difference between jest.fn(), jest.mock(), and jest.spyOn()?

How do you create a mock function with jest.fn()?

How do you mock a module with jest.mock()?

What is jest.spyOn() and when would you use it instead of jest.fn()?

How do you mock ES6 modules and named exports?

How do you mock default exports vs named exports?

What are mock implementations and how do you use mockImplementation()?

How do you use mockReturnValue() and mockResolvedValue()?

What is the difference between mockReturnValueOnce() and mockReturnValue()?

How do you clear, reset, and restore mocks?

What is the difference between mockClear(), mockReset(), and mockRestore()?

How do you mock Node.js built-in modules (fs, path, etc.)?

How do you create manual mocks using the mocks directory?

How do you mock timers (setTimeout, setInterval, Date) in Jest?

What is jest.useFakeTimers() and how does it work?

Async Testing(10 questions)

How do you test asynchronous code in Jest?

What are the different ways to handle async tests (callbacks, promises, async/await)?

How do you use the done callback for async tests?

How do you test promises with .resolves and .rejects?

How do you test async/await functions in Jest?

What happens if you forget to return a promise in an async test?

How do you test code that uses setTimeout or setInterval?