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
24.9.0
[expect] Highlight substring differences when matcher fails, part 1 (#8448)[expect] Highlight substring differences when matcher fails, part 2 (#8528)[expect] Improve report when mock-spy matcher fails, part 1 (#8640)[expect] Improve report when mock-spy matcher fails, part 2 (#8649)[expect] Improve report when mock-spy matcher fails, part 3 (#8697)[expect] Improve report when mock-spy matcher fails, part 4 (#8710)[expect] Throw matcher error when received cannot be jasmine spy (#8747)[expect] Improve report when negative CalledWith assertion fails (#8755)[expect] Improve report when positive CalledWith assertion fails (#8771)[expect] Display equal values for ReturnedWith similar to CalledWith (#8791)[expect, jest-snapshot] Change color from green for some args in matcher hints (#8812)[jest-snapshot] Highlight substring differences when matcher fails, part 3 (#8569)[jest-core] Improve report when snapshots are obsolete (#8665)[jest-cli] Improve chai support (with detailed output, to match jest exceptions) (#8454)[*] Manage the global timeout with --testTimeout command line argument. (#8456)[pretty-format] Render custom displayName of memoized components (#8546)[jest-validate] Allow maxWorkers as part of the jest.config.js (#8565)[jest-runtime] Allow passing configuration objects to transformers (#7288)[@jest/core, @jest/test-sequencer] Support async sort in custom testSequencer (#8642)[jest-runtime, @jest/fake-timers] Add jest.advanceTimersToNextTimer (#8713)[@jest-transform] Extract transforming require logic within jest-core into @jest-transform (#8756)[jest-matcher-utils] Add color options to matcherHint (#8795)[jest-circus/jest-jasmine2] Give clearer output for Node assert errors (#8792)[jest-runner] Export all types in the type signature of jest-runner (#8825)[jest-cli] Detect side-effect only imports when running --onlyChanged or --changedSince (#8670)[jest-cli] Allow --maxWorkers to work with % input again (#8565)[babel-plugin-jest-hoist] Expand list of whitelisted globals in global mocks (#8429)[jest-core] Make watch plugin initialization errors look nice (#8422)[jest-snapshot] Prevent inline snapshots from drifting when inline snapshots are updated (#8492)[jest-haste-map] Don't throw on missing mapper in Node crawler (#8558)[jest-core] Fix incorrect passWithNoTests warning (#8595)[jest-snapshots] Fix test retries that contain snapshots (#8629)[jest-mock] Fix incorrect assignments when restoring mocks in instances where they originally didn't exist (#8631)[expect] Fix stack overflow when matching objects with circular references (#8687)[jest-haste-map] Workaround a node >=12.5.0 bug that causes the process not to exit after tests have completed and cancerous memory growth (#8787)[docs] Replace FlowType with TypeScript in CONTRIBUTING.MD code conventions[jest-leak-detector] remove code repeat (#8438)[docs] Add example to jest.requireActual (#8482)[docs] Add example to jest.mock for mocking ES6 modules with the factory parameter (#8550)[docs] Add information about using jest.doMock with ES6 imports (#8573)[docs] Fix variable name in custom-matcher-api code example (#8582)[docs] Fix example used in custom environment docs (#8617)[docs] Updated react tutorial to refer to new package of react-testing-library (@testing-library/react) (#8753)[docs] Updated imports of react-testing-library to @testing-library/react in website (#8757)[jest-core] Add getVersion (moved from jest-cli) (#8706)[docs] Fix MockFunctions example that was using toContain instead of toContainEqual (#8765)[*] Make sure copyright header comment includes license (#8783)[*] Check copyright and license as one joined substring (#8815)[docs] Fix WatchPlugins jestHooks.shouldRunTestSuite example that receives an object (#8784)[*] Enforce LF line endings (#8809)[pretty-format] Delete obsolete link and simplify structure in README (#8824)[docs] Fix broken transform link on webpack page (#9155)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.
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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