The Top 15 Jest.js Interview Questions You Need to Know in 2023

Jestjs has quickly become one of the most popular JavaScript testing frameworks, providing an intuitive, zero-configuration testing experience As companies continue to adopt Jest for testing their JavaScript codebases, knowledge of Jest is becoming an increasingly sought-after skill for developers.

In technical interviews, expect to encounter a fair share of Jest.js questions that evaluate your understanding of the framework and ability to test JavaScript code using Jest I’ve interviewed with many companies where Jest questions featured prominently, covering basics like setup and configuration as well as advanced usage with mocks, async code, and integration with React

To help you prepare and succeed in your next Jest.js interview, I’ve compiled this list of 15 common and important Jest interview questions. These questions test your knowledge on aspects like Jest architecture, testing methodologies, mocking, snapshot testing, and integration with React, among other topics.

I’ve provided detailed explanations and code samples for each question to make sure you understand the key Jest concepts assessed in interviews. Let’s get started!

1. How do you configure Jest for testing a React application? Walk me through the setup.

To configure Jest to test a React codebase, first install Jest and React Testing Library as dev dependencies:

npm install --save-dev jest @testing-library/react

Then create a jest.config.js file to configure Jest:

module.exports = {  roots: ['<rootDir>/src'],  testMatch: ['**/__tests__/**/*.{js,jsx,ts,tsx}', '**/?(*.)(spec|test).{js,jsx,ts,tsx}'],  transform: {    '^.+\.(js|jsx|mjs|cjs|ts|tsx)$': '<rootDir>/node_modules/babel-jest'   },};

This sets the root directory for tests, test file patterns, and Babel for transformation

Finally, add a test script in package.json:

"scripts": {  "test": "jest" }

Now Jest can be run just using npm test.

2. How is Jest different from other JavaScript testing frameworks? What advantages does it offer?

Unlike other JavaScript testing frameworks, Jest comes batteries included – it provides everything needed for testing like assertions, mocks, test runners, and matchers out-of-the-box without requiring additional libraries.

Some key advantages Jest offers:

• Zero configuration testing: Jest requires no complex setup for basic testing. Just install and start writing tests.

• Fast interactive mode: Jest automatically picks up changes and runs tests in the background. No need to manually re-run tests.

• Isolated tests: Jest tests run independently in their own environment, avoiding interference between tests.

• Snapshot testing: Jest can capture UI component renders and compare to previous snapshots to detect changes.

• Mocking support: Jest has built-in mocking support so external dependencies can be easily mocked.

• Code coverage: Jest can generate code coverage reports with no extra libraries needed.

Overall, Jest simplifies testing JavaScript codebases with its batteries-included approach.

3. Explain the purpose of describe(), test(), expect() in Jest.

Jest provides several global methods and matchers that help structure and write tests:

• describe(): Used to group related tests into test suites. Often represents a module or feature being tested.

• test(): Defines an individual test case in a test suite. Takes the test name and implementation function as arguments.

• expect(): Expectation matchers that wrap test values and assertions. Commonly chained with .toBe(), .toEqual() etc to test expected vs actual result.

For example:

describe('Math utils', () => {  test('can add two numbers', () => {    expect(add(1, 2)).toBe(3);  });});

describe() groups related tests, test() defines each test case, expect() wraps the assertion.

4. How does Jest isolate tests from each other? Why is this useful?

Jest isolates tests by executing each test file in a separate process. This prevents tests from affecting each other’s global state such as modifying global variables or mocks.

Isolated test execution is useful for:

• Avoiding conflicts between tests that may change global state.

• Tests can run parallelly without affecting others as they run in separate processes.

• Consistent test results as tests are independent of external state changes.

• Tests can use same variable/mock names without colliding.

Overall, isolation makes tests more reliable and prevents flaky test failures due to external state changes.

5. What is mock function in Jest? When would you use it?

A mock function in Jest allows you to replace the actual implementation of a function with a dummy function that lets you control return values and track function calls.

Use cases include:

• Mocking random or time-based functions to make them deterministic for testing.

• Isolating unit tests by mocking integrations to external services.

• Simulating error conditions by throwing errors from mock functions.

• Avoiding repetitive initialization for setup/teardown in tests.

For example, to mock a function that makes an external API call:

jest.mock('./api'); api.getUser.mockResolvedValue({  name: 'John'});

Mocking allows controlling function behavior easily for testing purposes.

6. How do you test async code (e.g. fetch API calls) with Jest?

Jest supports multiple approaches to test async code:

• Async/await: Use async/await syntax in test and await on Promises.

• Return Promise: Return a Promise from test and Jest will wait for resolution.

• Done callback: Jest will wait for done() callback to execute assertions.

• Resolvers: Use .resolves or .rejects matchers to assert promise resolution.

For example, testing fetch API call:

// Async/awaittest('fetches users', async () => {  const data = await fetch('/users');  expect(data.length).toBe(3);  });// Return Promisetest('fetches users', () => {  return fetch('/users')    .then(data => {      expect(data.length).toBe(3);   });});

So Jest provides multiple approaches to test async code without callbacks.

7. How are snapshots useful in testing React components?

Snapshots in Jest capture the rendered markup of React components. On subsequent test runs, current snapshots are compared to previously committed snapshots. Any changes to the rendered markup will result in the test failing.

Benefits of snapshot testing React components:

• Detect unintended markup changes from component code changes.

• Provide protection against accidental UI regressions.

• Snapshots are straightforward to write and maintain.

• Serve as documentation of component render state.

• Complement unit testing for comprehensive coverage.

For example, snapshot testing a <Button> component:

test('matches snapshot', () => {  const button = renderer.create(<Button>Click</Button>);   expect(button).toMatchSnapshot();});

This saves the rendered button markup to snapshots for comparison.

8. How do you update Jest snapshots when markup changes intentionally?

When markup changes in components are intentional, Jest snapshots need to be updated to avoid failing tests. There are two approaches to update snapshots:

1. Re-generate snapshots: Delete old snapshots and re-run tests so new snapshots are generated.

2. Review and commit updates: Open snapshot file, review changes, and commit updated snapshots.

Jest provides a handy --updateSnapshot (or -u) CLI flag to use the first approach and re-generate snapshots automatically by re-running tests.

The second approach allows manually reviewing snapshots before committing updates. This helps avoid inadvertently committing wrong snapshots.

I prefer reviewing snapshots before committing updates. But --updateSnapshot flag can be handy if many snapshots changed.

9. You have a utility function that randomly returns different values. How would you test it with Jest?

To test a utility function that returns random values, I would:

1. Use Jest’s mock functionality to override the random implementation with a predictable fake.

2. Reset the mock before each test to isolate randomness to each test case.

For example:

import utils from './utils';jest.mock('uuid', () => {  return {    random: () => 0.5 // Stub random to always return 0.5  }})beforeEach(() => {  uuid.random.mockReset();