# Jest

URL: https://softwaredictionary.org/terms/jest
Category: Testing & Quality
Last updated: 2026-10-03
Pronunciation: JEST

In short: Jest is a popular JavaScript testing framework that bundles a test runner, assertions, mocking, snapshots and code coverage in one package with little setup.

## What is Jest?

Jest was created at Facebook, now Meta, and has been part of the OpenJS Foundation since 2022. Before it, JavaScript projects usually combined several tools: one to run tests, one for assertions, another for mocks and another for coverage. Jest bundles all of these, so `npm install --save-dev jest` and a test file are enough to start.

Tests are written with `test` (or `it`) and grouped with `describe`. Inside, `expect` checks results with matchers such as `toBe`, `toEqual` or `toThrow`. Jest finds files ending in `.test.js` or `.spec.js` on its own, runs them in parallel worker processes, and in watch mode re-runs only the tests affected by the files you changed.

Mocking is built in: `jest.fn()` creates fake functions that record how they were called, and `jest.mock()` replaces whole modules, such as an API client, so unit tests stay fast and don't depend on the network. Snapshot tests save the output of a component or function and fail if it later changes unexpectedly, and `--coverage` reports which lines the tests ran.

A common misconception is that Jest tests run in a real browser. By default they run in Node.js, optionally with jsdom, a simulated browser environment; for real browsers, end-to-end tools such as Playwright are used. In projects built with Vite, many teams now choose Vitest, which offers a Jest-compatible API.

## Key takeaways

- Jest is an all-in-one JavaScript testing framework.
- It includes a test runner, assertions, mocking, snapshots and coverage.
- Tests use describe, test and expect with matchers such as toEqual.
- jest.fn() and jest.mock() replace real dependencies in unit tests.
- Tests run in Node.js or jsdom, not in a real browser.

## Example: A unit test with a mocked function

```javascript
// cart.test.js
import { total } from "./cart";

describe("total", () => {
  test("adds up item prices", () => {
    expect(total([{ price: 10 }, { price: 5 }])).toBe(15);
  });

  test("applies a discount from a fake service", () => {
    const getDiscount = jest.fn().mockReturnValue(0.1); // a mock
    expect(total([{ price: 100 }], getDiscount)).toBe(90);
    expect(getDiscount).toHaveBeenCalledTimes(1);
  });
});
```

## Frequently asked questions

**What is the difference between Jest and Vitest?**

Both are JavaScript test runners with very similar APIs. Vitest is built on Vite and reuses its configuration, so it fits Vite projects and starts quickly; Jest is older, framework-independent and very widely used.

**Is Jest only for React?**

No. Jest became popular with React, but it tests any JavaScript or TypeScript code, including Node.js services, utilities and other frameworks.

**What is snapshot testing in Jest?**

A snapshot test saves the output of a function or component to a file the first time it runs, then fails later if the output changes, so unintended changes are noticed during review.

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