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Delightful JavaScript Testing.

Version published
Install size
16.5 MB

Package description

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);


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.


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

  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'

    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





  • [expect] [BREAKING] Differentiate between MatcherContext MatcherUtils and MatcherState types (#13141)
  • [jest-circus] Add support for test.failing.each (#13142)
  • [jest-config] [BREAKING] Make snapshotFormat default to escapeString: false and printBasicPrototype: false (#13036)
  • [jest-config] [BREAKING] Remove undocumented collectCoverageOnlyFrom option (#13156)
  • [jest-environment-jsdom] [BREAKING] Upgrade to jsdom@20 (#13037, #13058)
  • [@jest/globals] Add jest.Mocked, jest.MockedClass, jest.MockedFunction and jest.MockedObject utility types (#12727)
  • [jest-mock] [BREAKING] Refactor Mocked* utility types. MaybeMockedDeep and MaybeMocked became Mocked and MockedShallow respectively; only deep mocked variants of MockedClass, MockedFunction and MockedObject are exported (#13123, #13124)
  • [jest-mock] [BREAKING] Change the default jest.mocked helper’s behavior to deep mocked (#13125)
  • [jest-snapshot] [BREAKING] Let babel find config when updating inline snapshots (#13150)
  • [@jest/test-result, @jest/types] [BREAKING] Replace Bytes and Milliseconds types with number (#13155)
  • [jest-worker] Adds workerIdleMemoryLimit option which is used as a check for worker memory leaks >= Node 16.11.0 and recycles child workers as required (#13056, #13105, #13106, #13107)
  • [pretty-format] [BREAKING] Remove ConvertAnsi plugin in favour of jest-serializer-ansi-escapes (#13040)
  • [pretty-format] Allow to opt out from sorting object keys with compareKeys: null (#12443)


  • [jest-config] Fix testing multiple projects with TypeScript config files (#13099)
  • [@jest/expect-utils] Fix deep equality of ImmutableJS Record (#13055)
  • [jest-haste-map] Increase the maximum possible file size that jest-haste-map can handle (#13094)
  • [jest-runtime] Properly support CJS re-exports from dual packages (#13170)
  • [jest-snapshot] Make prettierPath optional in SnapshotState (#13149)
  • [jest-snapshot] Fix parsing error from inline snapshot files with JSX (#12760)
  • [jest-worker] When a process runs out of memory worker exits correctly and doesn't spin indefinitely (#13054)

Chore & Maintenance

  • [*] [BREAKING] Drop support for Node v12 and v17 (#13033)
  • [docs] Fix webpack name (#13049)
  • [docs] Explicit how to set n for --bail (#13128)
  • [docs] Update Enzyme URL (#13166)
  • [jest-leak-detector] Remove support for weak-napi (#13035)
  • [jest-snapshot] [BREAKING] Require rootDir as argument to SnapshotState (#13150)




🃏 Delightful JavaScript Testing

  • 👩🏻‍💻 Developer Ready: Complete and ready to set-up JavaScript testing solution. Works out of the box for any React project.

  • 🏃🏽 Instant Feedback: Failed tests run first. Fast interactive mode can switch between running all tests or only test files related to changed files.

  • 📸 Snapshot Testing: Jest can capture snapshots of React trees or other serializable values to simplify UI testing.

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Last updated on 25 Aug 2022

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