Jest

What is Jest?

Jest is a JavaScript testing framework that ensures correctness of JavaScript codebases by letting developers write tests through an approachable, familiar and feature-rich API. Jest is designed to work out of the box with zero configuration on most JavaScript projects, and it supports projects using Babel, TypeScript, Node, React, Angular, Vue and more.

Key Features

• Zero config: Jest aims to work out of the box, config free, on most JavaScript projects, so teams can start writing tests without building a configuration file first.
• Snapshots: Jest makes tests that keep track of large objects with ease, and snapshots live either alongside tests or embedded inline.
• Isolated, parallel tests: Tests are parallelized by running them in their own processes to maximize performance, and Jest ensures tests have unique global state so parallel runs stay reliable.
• Fast and safe test runs: Jest runs previously failed tests first and re-organizes runs based on how long test files take, which shortens feedback cycles.
• Code coverage: Adding the --coverage flag generates code coverage with no additional setup, and Jest can collect coverage from entire projects, including untested files.
• Easy mocking: Jest uses a custom resolver for imports in tests, making it simple to mock any object outside a test's scope, with a rich Mock Functions API for spying on function calls using readable test syntax.
• Great exceptions: When tests fail, Jest provides rich context explaining why, which helps developers diagnose failures faster.
• Complete API toolkit: From it to expect, Jest places the entire testing toolkit in one place, backed by thorough documentation and active maintenance.

Use Cases

• Frontend framework teams: Developers building with React, Angular or Vue can use Jest to test components and application logic without assembling a separate testing stack.
• TypeScript and Babel projects: Teams already compiling with Babel or TypeScript can adopt Jest for testing since it works with both toolchains.
• Node.js backend development: Node projects can use Jest as a config-free test runner with parallel, isolated test processes.
• Large-object assertions: Teams that need to verify large objects or serialized output can use snapshot tests kept alongside test files or inline.
• Coverage-driven workflows: Projects that need visibility into untested code can run Jest with the --coverage flag to collect coverage across the entire project, including files without tests.

Who It Is For

Jest is aimed at JavaScript developers and engineering teams working across frameworks such as React, Angular, Vue, Node, Babel and TypeScript who need a testing framework that requires little configuration and can be extended to match their requirements. It solves the problem of assembling and maintaining a test setup by providing an approachable, feature-rich API that gives results quickly, with documentation and maintenance handled by the Jest Core Team and contributors.

Pricing

Jest is an open-source project maintained by the Jest Core Team and contributors, with development supported through Open Collective, where more than 600 donors sponsor Jest for $3 or more per month. The source material does not state any paid tiers or pricing for using Jest itself.

Frequently Asked Questions

What is Jest used for?

Jest is a JavaScript testing framework used to ensure the correctness of JavaScript codebases. It lets developers write tests with an approachable, familiar and feature-rich API, and it works with projects using Babel, TypeScript, Node, React, Angular, Vue and more.

Does Jest require configuration?

No. Jest aims to work out of the box, config free, on most JavaScript projects. It requires little configuration and can be extended to match a project's requirements.

Which frameworks and languages does Jest support?

Jest works with projects using Babel, TypeScript, Node, React, Angular, Vue and more. It is a JavaScript testing framework designed for any JavaScript codebase.

How does Jest handle code coverage?

Jest generates code coverage when the --coverage flag is added, with no additional setup needed. Jest can collect coverage information from entire projects, including untested files.

Who uses Jest?

Jest reports 100m+ downloads in the last month and use on over 15,000,000 public repos on GitHub, and it is used extensively at a range of companies listed on the project site. It is maintained by the Jest Core Team with contributions from the open-source community.