A framework for React
Facebook develops Jest internally to test its own JavaScript codebases — React included. Before Jest, Node testing is fragmented: Mocha + Chai + Sinon, Jasmine, AVA, Tape each with different syntax and assembly. Configuring and running takes a lot of setup.
The release
Jest is open-sourced by Facebook in 2014; versions 14.0 (May 2016) and 16.0 (September 2016) mark the transformation into a mass framework with adoption by Create React App. Main author: Christoph Nakazawa. MIT licence.
test('sum adds numbers', () => {
expect(sum(1, 2)).toBe(3);
});
test('user snapshot', () => {
expect(renderUser()).toMatchSnapshot();
});Features
- Zero config — works out-of-box with ES modules, JSX, TypeScript (via babel-jest)
- Integrated assertion library —
expect()with.toBe(),.toEqual(),.toMatch(),.toThrow()matchers - Built-in mocking —
jest.fn(),jest.mock()for modules - Snapshot testing — serialises output and compares on subsequent runs
- Parallel execution — worker pool for parallel tests
- Code coverage — integrated Istanbul with
--coverage - Watch mode — interactive, re-runs only affected tests
- jsdom environment integrated for DOM tests
Testing React
Around Jest the React testing ecosystem is growing: Enzyme (Airbnb) for component-level testing remains the prevailing choice, with alternatives emerging over time.
Competitors
- Mocha + Chai — historical, more configurable and less opinionated
- AVA — concurrent, minimal
- Jasmine — older
In the Italian context
Jest is becoming the standard choice of Italian React teams: headless e-commerce, fintech and first digital-PA projects are adopting it as default.
References: Jest (Facebook, Open Source 2014). Christoph Nakazawa lead. MIT licence.