@jest/core

What is @jest/core?

The @jest/core package is the core engine of Jest, a popular JavaScript testing framework. It provides the functionality necessary to run tests, including the test runner, test watcher, and the ability to configure and initialize tests. It is designed for developers who need to write and run tests for their JavaScript code, offering a powerful and flexible testing environment.

What are @jest/core's main functionalities?

Running tests

This feature allows you to programmatically run your tests using Jest. You can specify options and projects to run tests on, making it flexible for different testing scenarios.

jest.runCLI(options, projects).then(result => { console.log('Tests completed'); });

Configuring Jest

This feature enables you to read and manipulate Jest's configuration programmatically. It's useful for custom setups or when integrating Jest into other tools.

const { readConfig } = require('@jest/core');
readConfig(argc, argv).then(config => { console.log('Jest config:', config); });

Using the TestWatcher

The TestWatcher API allows you to listen to events related to the test lifecycle, such as when a test starts or finishes. This is useful for implementing custom behaviors or integrations.

const { TestWatcher } = require('@jest/core');
const watcher = new TestWatcher({ isWatchMode: true });
watcher.on('test_start', test => { console.log('Test started:', test.name); });

Other packages similar to @jest/core

mocha

Mocha is a feature-rich JavaScript test framework running on Node.js and in the browser, making asynchronous testing simple. It's similar to @jest/core in providing a testing framework, but differs in its interface and default behavior, such as having a different assertion library by default (Chai) and not including a mocking library.

jasmine

Jasmine is a behavior-driven development framework for testing JavaScript code. It does not require a DOM, and it has a clean, obvious syntax so that you can easily write tests. Compared to @jest/core, Jasmine is more focused on the BDD aspect and comes with an assertion library out of the box, whereas Jest provides more utilities for mocking and spying.

ava

AVA is a test runner for Node.js with a concise API, detailed error output, and process isolation for concurrent tests. It's designed to be fast and comes with built-in support for ES6. Unlike @jest/core, AVA emphasizes minimalism and parallel test execution, which can lead to faster test runs for certain projects.

README

@jest/core

Jest is currently working on providing a programmatic API. This is under developemnt, and usage of this package directly is currently not supported.

Changelog

24.6.0

Features

• [expect]: Improve report when matcher fails, part 13 (#8077)
• [@jest/core] Filter API pre-filter setup hook (#8142)
• [jest-snapshot] Improve report when matcher fails, part 14 (#8132)
• [@jest/reporter] Display todo and skip test descriptions when verbose is true (#8038)
• [jest-runner] Support default exports for test environments (#8163)
• [pretty-format] Support React.Suspense (#8180)
• [jest-snapshot] Indent inline snapshots (#8198)
• [jest-config] Support colors in displayName configuration (#8025)

Fixes

• [jest-circus] Fix test retries with beforeAll/beforeEach failures (#8227)
• [expect] Fix circular references in iterable equality (#8160)
• [jest-changed-files] Change method of obtaining git root (#8052)
• [jest-each] Fix test function type (#8145)
• [jest-fake-timers] getTimerCount not taking immediates and ticks into account (#8139)
• [jest-runtime] Allow json file as manual mock (#8159)
• [pretty-format] Print BigInt as a readable number instead of {} (#8138)
• [jest-core] Fix ability to transform dependencies required from globalSetup script (#8143)
• [@jest/reporters] Fix Cannot read property converageData of null (#8168)
• [jest-worker] JEST_WORKER_ID starts at 1 (#8205)
• [jest-config] Use default cwd even if config contains a cwd property (#7923)
• [jest-resolve-dependencies]: Remove internal peer dependencies (#8215)
• [jest-resolve]: Remove internal peer dependencies (#8215)
• [jest-snapshot]: Remove internal peer dependencies (#8215)
• [jest-resolve] Fix requireActual with moduleNameMapper (#8210)
• [jest-haste-map] Fix haste map duplicate detection in watch mode (#8237)

Chore & Maintenance

• [*] Remove flow from code base (#8061)
• [*] Use property initializer syntax in Jest codebase (#8117)
• [*] Move @types/node to the root package.json (#8129)
• [*] Add documentation and tests related to auto-mocking (#8099)
• [*] Add jest-watch-typeahead as a devDependency (#6449)
• [*] upgrade TS to 3.4.0-dev* for incremental builds (#8149)
• [docs] Improve description of optional arguments in ExpectAPI.md (#8126)

Performance

• [jest-haste-map] Optimize haste map data structure for serialization/deserialization (#8171)
• [jest-haste-map] Avoid persisting haste map or processing files when not changed (#8153)
• [jest-core] Improve performance of SearchSource.findMatchingTests by 15% (#8184)
• [jest-resolve] Optimize internal cache lookup performance (#8183)
• [jest-core] Dramatically improve watch mode performance (#8201)
• [jest-transform] Cache regular expression instead of creating anew for every file in ScriptTransformer (#8235)
• [jest-core] Fix memory leak of source map info and minor performance improvements (#8234)
• [jest-console] Fix memory leak by releasing console output reference when printed to stdout (#8233)
• [jest-runtime] Use Map instead of Object for module registry (#8232)