Top Jest JS Interview Questions and Answers for Developers in 2023

Jest is one of the most popular testing frameworks for JavaScript. More than 3,898,000 public GitHub repositories use it. More and more developers are using Jest to test React, Node, Angular, and other JavaScript apps. This has increased the need for engineers who know how to write and maintain Jest tests.

In this article, we’ll explore some of the most common and important Jest interview questions that employers ask candidates. Mastering these Jest concepts will help you stand out in interviews and demonstrate your ability to build reliable, well-tested applications.

Beginner Jest Interview Questions

Let’s start with some basic Jest interview questions that employers often ask in early rounds:

What is Jest?

Jest is an open-source JavaScript testing framework maintained by Facebook. For testing JavaScript apps, it was made to be quick, reliable, and simple to use. Some of the powerful features that Jest has are mocking modules, parallel test execution, snapshot testing, and code coverage reports that are built right in. It works with other JavaScript frameworks, such as Angular and Vue, and works well with React.

Why do we need Jest?

We need Jest to automate the process of testing our JavaScript code and ensuring it works as expected Jest allows us to

• Write test suites with descriptive names to organize our tests
• Create test cases to verify distinct behaviors and scenarios
• Use assertions like toEqual to test values and results
• Generate code coverage reports to identify untested areas
• Refactor code confidently knowing tests will catch regressions

We can ship bug-free JavaScript code and make changes more quickly by testing it thoroughly with Jest.

What is a test suite in Jest?

A test suite is a collection of related test cases testing a specific feature or unit of code. In Jest we create a test suite using the describe function which takes a string and callback function containing the test cases

//Sum test suitedescribe('Sum function', () => {  // Individual test cases  test('correctly adds two numbers', () => {    // Test case  });  test('returns error if args not numbers', () => {    // Test case   });});

The test cases use the test function to test distinct behaviors.

What is a test case in Jest?

A test case is a single test that checks a specific scenario, use case, or behavior of the code being tested. Jest test cases contain one or more assertions like toBe, toEqual, or toThrow to verify the expected result. A descriptive name for the test case identifies the specific behavior being tested:

test('adds positive numbers', () => {  expect(sum(2, 3)).toBe(5); });

Test cases act as documentation and specification for how code should behave.

How do you install Jest?

Jest can be installed in a project using npm by running:

npm install --save-dev jest

This installs Jest and saves it in your project’s package.json file as a dev dependency.

After installing, create test files with the .test.js suffix and run tests using npm test. Make sure to install any required dependencies like react-test-renderer for testing React components.

What is a snapshot test in Jest?

A snapshot test captures the rendered output of a component and saves it as a snapshot file. On subsequent test runs, it compares the rendered output to the previous snapshot. If the output matches, the test passes. Any changes cause the test to fail, signalling potential regressions.

Snapshot tests ensure UI components render as expected:

test('renders correctly', () => {  const tree = renderer.create(<MyComponent />).toJSON();  expect(tree).toMatchSnapshot();}); 

Snapshots complement other test types like unit tests to prevent unintended changes.

What is code coverage in Jest?

Code coverage refers to the amount of code in percentage covered by your tests. Jest can generate coverage reports to highlight untested parts of your codebase.

Enable coverage reports by using the --coverage flag when running Jest. The coverage report is output to a coverage folder detailing the lines, functions, branches, and statements covered by tests.

Code coverage identifies areas that need better testing to improve quality and confidence.

How do you run a single test case in Jest?

The --testNamePattern flag can be used to run a single test case by passing a regex pattern matching the test name:

npm test -- --testNamePattern="add numbers" 

This runs only tests with names matching “add numbers”. You can also pass a more generic regex to run multiple related tests.

Single test execution is useful when debugging a failing test or focusing on a specific behavior during development.

This covers some fundamental Jest concepts frequently assessed in initial screening rounds. Let’s now look at some more advanced questions.

Intermediate Jest Interview Questions

Here are some common Jest questions asked in later rounds to evaluate your deeper knowledge:

How does Jest enable snapshot testing for React components?

Jest snapshot testing is enabled by rendering React components using the react-test-renderer package. This renders components to pure JavaScript objects without depending on the DOM.

Jest then serializes the rendered output to a file using toJSON(). On subsequent test runs, it re-renders and compares to the saved snapshot using toMatchSnapshot(). Any changes trigger test failures.

This approach lets Jest efficiently test React component rendering without using a browser.

What techniques does Jest provide for mocking modules and functions?

Jest provides built-in support for mocking modules and functions using jest.mock(moduleName) and jest.fn() respectively.

jest.mock(moduleName) replaces the ES module with a mock that allows selectively overriding parts of the module for testing purposes.

jest.fn() creates mock functions with additional tracking and assertions like how many times it was called.

Mocking is useful for simulating dependencies and isolating code under test from external factors. It helps test only the intended logic.

How can you generate code coverage reports using Jest?

Pass the --coverage flag when running Jest tests:

jest --coverage

This will output a detailed code coverage report under the coverage/ folder showing the number and percentage of lines, statements, functions, and branches covered by tests.

Set coverage thresholds that will fail builds if overall coverage or file coverage drops below a threshold.

Code coverage identifies untested parts of the codebase needing better test coverage.

What techniques help make Jest testing efficient and performant?

Jest uses various techniques to optimize test performance:

• Parallel test execution: Tests are run concurrently across multiple child processes to maximize CPU usage.

• Cacheable results: Hashes for test files are generated to cache results and skip unnecessary reruns.

• Watch mode: Only affected tests are rerun when changes are made.

• Interactive CLI: Failed tests can be debugged in the CLI using enter for watch mode.

• Fail fast: Tests are aborted on the first failure to avoid slow runs.

These techniques make Jest testing very fast and efficient compared to other frameworks.

How can you match complex nested object structures using Jest matchers?

Jest provides the expect.objectContaining() matcher to partially match complex objects and nested structures.

It matches received objects with expected objects, ignoring unmatched keys:

const expected = {  foo: {    bar: 5,  },};expect(received).toEqual(expect.objectContaining(expected)); 

This asserts received contains the expected object structure – great for testing complex payloads.

Testing React with Jest Interview Questions

Since Jest was created at Facebook for testing React, interviewers often ask React testing questions:

How do you test React components with Jest and React Testing Library?

Render components using render from React Testing Library.

Interact with components using utility functions like fireEvent.

Make assertions using matchers like getByText and queryByText to test rendered UI elements.

Avoid implementation details – test only component behavior visible to users.

This approach helps make tests resilient to implementation changes.

What techniques help make UI testing with Jest and React Testing Library more maintainable?

Avoid relying on implementation details like component names, CSS classes, and internal state. These often change causing fragile tests.

Query elements semantically using utils like getByRole and getByLabelText matching actual content.

Use data-testid attributes for elements without semantic queries.

Follow best practices like avoiding global test setup and clearing state between tests.

This focuses tests on component behavior visible to users, avoiding fragile tests when implementations change