How does jest.fn() differ from jest.mock()?

jest.fn() creates a mock function whose calls you can watch. jest.mock('module') replaces a whole module with a mock within the test.

What is a snapshot test and when does it hurt?

A test compares output against a saved snapshot. It is quick to write, but nobody reads big snapshots and they just get updated. So keep them small and targeted.

How do you test async code?

Return a promise or use async / await and expect(...).resolves or rejects. Fake timers handle timers.

Each test file runs in its own context, so one test's global state does not affect another and parallel runs are safe.

Jest: what it is, when to use it, interview questions

What it is

Facebook (Meta) created Jest in 2014, and today the project belongs to the OpenJS Foundation. It bundles everything for unit tests: running, expect assertions, mocks, timers, snapshot tests and coverage reports.

Tests run in parallel in isolated workers, and watch mode reruns only changed tests. jsdom provides a DOM, and for React you add React Testing Library.

Jest was the standard in the JS world for a long time. New Vite projects more often pick Vitest, but in many existing codebases (including React Native) Jest remains the default.

Use it when

Unit and integration tests of JS / TS code.
React Native or an existing project where Jest is already set up.
You need snapshot tests and built-in coverage without extra packages.

Avoid it when

A new Vite project with ES modules: Vitest is simpler to configure.
End-to-end browser tests: use Playwright or Cypress.

Key concepts

describe / it / expect: Test groups, individual tests and result assertions.
Mocks: jest.fn() and jest.mock() replace functions and modules.
Snapshot: Saves output and compares against it in later runs.
Fake timers: Control of setTimeout and Date in tests.
Coverage: A report of which lines and branches ran.

Example

A simple test and a jest.fn() spy.

Interview questions

1. How does jest.fn() differ from jest.mock()?
jest.fn() creates a mock function whose calls you can watch. jest.mock('module') replaces a whole module with a mock within the test.
2. What is a snapshot test and when does it hurt?
A test compares output against a saved snapshot. It is quick to write, but nobody reads big snapshots and they just get updated. So keep them small and targeted.
3. How do you test async code?
Return a promise or use async / await and expect(...).resolves or rejects. Fake timers handle timers.
4. Why isolate tests?
Each test file runs in its own context, so one test's global state does not affect another and parallel runs are safe.

Interview questions from the bank

How do unit, integration, and e2e tests differ from each other?JuniorTesting
How do a stub, mock, spy, and fake differ?JuniorTesting
Does high code coverage guarantee the quality of tests?JuniorTesting
What is TDD, and what are its advantages and drawbacks?JuniorTesting
What makes a good test structure, and what are Arrange, Act, Assert?JuniorTesting
How do you test asynchronous code correctly in Jest or Vitest?JuniorTesting
Why use parameterized tests?JuniorTesting
What is a regression test, and how do you add one when fixing a bug?JuniorTesting

All questions: Testing

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Jest vs Vitest

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Jest

What it is

Use it when

Avoid it when

Key concepts

Example

Interview questions

1. How does jest.fn() differ from jest.mock()?

2. What is a snapshot test and when does it hurt?

3. How do you test async code?

4. Why isolate tests?

Interview questions from the bank

Comparisons with this technology

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