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jest

Delightful JavaScript Testing.

30.0.0
Source
npmnpm
Version published
Weekly downloads
29M
-17.44%
Maintainers
5
Weekly downloads
Ā 
Created

What is jest?

Jest is a delightful JavaScript Testing Framework with a focus on simplicity. It works with projects using: Babel, TypeScript, Node, React, Angular, Vue, and more. It's designed to ensure correctness of any JavaScript codebase. It allows you to write tests with an approachable, familiar, and feature-rich API that gives you results quickly.

What are jest's main functionalities?

Unit Testing

Jest allows you to write unit tests for your JavaScript functions. In this example, a simple test is written to check if the addition of 1 and 2 equals 3.

test('adds 1 + 2 to equal 3', () => {
  expect(1 + 2).toBe(3);
});

Mocking

Jest provides a powerful mocking library that lets you mock objects, functions, and modules. In this example, an HTTP request is mocked to test the behavior of a function that fetches user data.

jest.mock('../api');

test('should fetch users', () => {
  const users = [{name: 'Bob'}];
  const resp = {data: users};
  axios.get.mockResolvedValue(resp);

  return Users.all().then(data => expect(data).toEqual(users));
});

Snapshot Testing

Snapshot testing enables you to test your UI or any serializable value to ensure it does not change unexpectedly. This example demonstrates testing an object against a snapshot, with some fields expected to match specific types rather than values.

test('will check the matchers and pass', () => {
  const user = {
    createdAt: new Date(),
    id: Math.floor(Math.random() * 20),
    name: 'LeBron James'
  };

  expect(user).toMatchSnapshot({
    createdAt: expect.any(Date),
    id: expect.any(Number)
  });
});

Asynchronous Testing

Jest supports testing of asynchronous code. In this example, an asynchronous function `fetchData` is tested to ensure it resolves with the expected value 'peanut butter'.

test('the data is peanut butter', async () => {
  await expect(fetchData()).resolves.toBe('peanut butter');
});

Other packages similar to jest

    mocha

    Mocha is a feature-rich JavaScript test framework running on Node.js and in the browser, making asynchronous testing simple. Mocha is often compared to Jest because of its flexible and configurable nature, but it requires more setup and configuration for features like mocking and snapshot testing, which Jest provides out of the box.

    jasmine

    Jasmine is a behavior-driven development framework for testing JavaScript code. It does not rely on browsers, DOM, or any JavaScript framework. Similar to Jest, it comes with an easy syntax that makes writing tests simple. However, Jest offers more modern features like snapshot testing and built-in support for test coverage.

    ava

    AVA is a test runner for Node.js with a concise API, detailed error output, and process isolation that lets you develop with confidence. AVA is designed to handle asynchronous testing more efficiently than Jest but lacks some of Jest's features like snapshot testing and built-in mocking.

    chai

    Chai is a BDD / TDD assertion library for node and the browser that can be delightfully paired with any JavaScript testing framework. It's often used alongside other testing frameworks like Mocha or Jest to provide more expressive assertions. Unlike Jest, Chai is purely an assertion library and does not provide test runners or mocking utilities.

Keywords

ava
babel
coverage
easy
expect
facebook

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Package last updated on 10 Jun 2025

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U.S. Patent No. 12,346,443 & 12,314,394. Other pending.