Top 30 Jest Interview Questions and Answers

Learn and master the top 30 Jest framework interview questions along with detailed answers with code snippets.

1. What is Jest?

Jest is a JavaScript testing framework developed by Facebook. It provides an easy-to-use API for writing unit tests, and it also includes features such as test runners, mocking, code coverage, and more.

2. How do you install Jest?

You can install Jest using npm by running the following command:

npm install jest --save-dev

3. How do you write a basic test case in Jest?

Jest uses the `test` function to define test cases. Here’s an example:

test('addition test', () => {
     expect(2 + 2).toBe(4);
});

  4. What is the purpose of `expect` in Jest?

The `expect` function is used to make assertions in Jest. It allows you to test values and conditions in your code. For example:

expect(value).toBe(expected);

5. How do you run Jest tests?

Jest automatically finds and runs test files with the `.test.js` or `.spec.js` extension in your project. You can run Jest using the following command:

npx jest

6. How can you focus on running specific tests in Jest?

You can use the `test.only` function to run only specific test cases. For example:

test.only('focused test', () => {
   // ...
});

 7. How can you run Jest tests in watch mode?

Jest has a watch mode that allows it to re-run tests whenever file changes are detected.

You can run Jest in watch mode using the following command:

npx jest --watch

8. How do you mock a module in Jest?

Jest provides the `jest.mock` function to mock modules. Here’s an example:

jest.mock('./moduleName');

9. How do you mock a function in Jest?

You can use the `jest.fn()` method to create a mock function in Jest. Here’s an example:

const mockFn = jest.fn();
mockFn();
expect(mockFn).toHaveBeenCalled();

10. How can you create a mock implementation for a function in Jest?

You can use the `mockImplementation` method to provide a custom implementation for a mock function. For example:

const mockFn = jest.fn().mockImplementation(() => ‘mocked value’);

expect(mockFn()).toBe(‘mocked value’);

11. How do you test asynchronous code in Jest?

Jest provides various ways to test asynchronous code. One approach is to use the `async/await` syntax. Here’s an example:

test('async test', async () => {
  const result = await asyncFunction();

  expect(result).toBe(expected);

});

12. How do you simulate an event in Jest?

You can use the `fireEvent` function from the `@testing-library/react` package to simulate events. Here’s an example:

import { fireEvent } from '@testing-library/react';

test('button click test', () => {

  const handleClick = jest.fn();

  const { getByRole } = render(<Button onClick={handleClick} />);

  fireEvent.click(getByRole('button'));

  expect(handleClick).toHaveBeenCalledTimes(1);

});

13. How can you test React component snapshots in Jest?

Jest provides a `toMatchSnapshot` matcher that allows you to compare the rendered output of a React component to a stored snapshot. Here’s an example:

test('component snapshot test', () => {
  const component = renderer.create(<MyComponent />);

  const tree = component.toJSON();

  expect(tree).toMatchSnapshot();

});

14. How do you test Redux actions in Jest?

You can test Redux actions by dispatching them and then asserting on the expected changes in the store. Here’s an example:

import configureMockStore from 'redux-mock-store';
import thunk from 'redux-thunk';

import { fetchPosts } from './actions';

const mockStore = configureMockStore([thunk]);

test('fetchPosts action test', () => {

  const store = mockStore({ posts: [] });

  return store.dispatch(fetchPosts()).then(() => {

    const actions = store.getActions();

    expect(actions[0].type).toBe('FETCH_POSTS_SUCCESS');

  });

});

15. How can you test Redux reducers in Jest?

You can test Redux reducers by passing initial state and an action to the reducer, and then asserting on the expected state changes. Here’s an example:

import reducer from './reducer';
import { fetchPostsSuccess } from './actions';

test('fetchPostsSuccess reducer test', () => {

  const initialState = { posts: [] };

  const action = fetchPostsSuccess(['post1', 'post2']);

  const nextState = reducer(initialState, action);

  expect(nextState).toEqual({ posts: ['post1', 'post2'] });

});

16. How do you test API calls in Jest?

You can use the `fetch` function and `async/await` syntax to test API calls in Jest. Here’s an example:

test('API call test', async () => {
const response = await fetch('https://api.example.com/data');

  const data = await response.json();

  expect(data).toEqual({ foo: 'bar' });

});

17. How can you test code coverage in Jest?

Jest can generate code coverage reports using the `–coverage` flag.

Running the command `npx jest –coverage` will generate a coverage report in the `coverage` directory.

18. How do you test error handling in Jest?

You can use the `expect` function with `toThrow` matcher to test error handling. Here’s an example:

test('error handling test', () => {
expect(() => {

    throw new Error('Something went wrong');

  }).toThrow('Something went wrong');

});

19. How can you test React components with external dependencies in Jest?

You can use the `jest.mock` function to mock external dependencies and provide custom implementations. Here’s an example:

jest.mock('axios', () => ({
get: jest.fn(() => Promise.resolve({ data: 'mocked data' })),

}));

test('component with external dependency test', async () => {

  const { getByText } = render(<MyComponent />);

  await waitFor(() => {

    expect(getByText('mocked data')).toBeInTheDocument();

  });

});

20. How do you test timers and asynchronous behavior in Jest?

Jest provides the `jest.useFakeTimers` function to control timers in tests. Here’s an example:

test('timer test', () => {
jest.useFakeTimers();

  const callback = jest.fn();

  setTimeout(callback, 1000);

  jest.runAllTimers();

  expect(callback).toHaveBeenCalled();

});

21. How can you test Redux-saga effects in Jest?

To test Redux-saga effects, you can use the `redux-saga-test-plan` library. It provides a testing DSL specifically designed for testing sagas.

22. How do you test Express.js APIs with Jest?

You can use the `supertest` library along with Jest to test Express.js APIs. Here’s an example:

const request = require('supertest');

const app = require('./app');

test('GET /api/users', async () => {

  const response = await request(app).get('/api/users');

  expect(response.statusCode).toBe(200);

  expect(response.body).toEqual({ users: [] });

});

23. How can you test WebSocket functionality in Jest?

To test WebSocket functionality in Jest, you can use the `ws` library to create a WebSocket server and client.

Then, you can write test cases to verify the WebSocket interactions.

24. How do you test Node.js modules with Jest?

You can use Jest to test Node.js modules by importing them and writing test cases for their functions and methods.

Make sure to mock any external dependencies or use dependency injection when necessary.

25. How can you test code that uses Promises in Jest?

Jest provides the `resolves` and `rejects` matchers to test code that uses Promises. Here’s an example:

test('Promise test', () => {
   return expect(Promise.resolve('value')).resolves.toBe('value');
});

26. How do you test code that involves file operations in Jest?

For testing code that involves file operations, you can use the `fs` module to create temporary files or directories and clean them up after the tests.

27. How can you test code that involves environment variables in Jest?

You can set specific environment variables for your Jest tests using the `testEnvironmentVariables` configuration option in your `package.json` or using a library like `dotenv`.

28. How do you test code that involves browser APIs in Jest?

You can use libraries like `jsdom` and `jsdom-global` to create a browser-like environment in Jest and test code that involves browser APIs.

29. How can you generate code coverage reports in different formats with Jest?

Jest supports generating code coverage reports in different formats such as HTML, text, and Cobertura.

You can specify the desired format using the `–coverageReporters` configuration option in your Jest configuration.

30. How do you configure Jest with custom settings?

You can configure Jest using the `jest.config.js` file or by specifying options in the `package.json` file.

Jest provides a wide range of configuration options, such as test file patterns, module mocking, coverage settings, and more.