jest
Advanced tools
Package description
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.
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');
});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 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 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 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.
Changelog
29.0.0
[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)[*] [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)Readme
🃏 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.
Read More: https://jestjs.io/
FAQs
Delightful JavaScript Testing.
The npm package jest receives a total of 18,702,331 weekly downloads. As such, jest popularity was classified as popular.
We found that jest demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 3 open source maintainers collaborating on the project.
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