# Jest — Delightful JavaScript Testing Framework

> Jest is a delightful JavaScript testing framework with a focus on simplicity. Zero-config for most JS/TS projects, snapshot testing, mocking, code coverage, and parallel test execution. Created by Facebook and used to test React, Instagram, and many large codebases.

## Install

Save the content below to `.claude/skills/` or append to your `CLAUDE.md`:

## Quick Use

```bash
npm i -D jest @types/jest ts-jest
# Or for TypeScript projects
npx ts-jest config:init
```

```ts
// sum.ts
export function sum(a: number, b: number): number {
  return a + b;
}

// sum.test.ts
import { sum } from "./sum";

describe("sum", () => {
  test("adds 1 + 2 to equal 3", () => {
    expect(sum(1, 2)).toBe(3);
  });

  test("handles negative numbers", () => {
    expect(sum(-1, -2)).toBe(-3);
  });
});
```

```bash
npx jest                     # Run all tests
npx jest --watch             # Watch mode
npx jest --coverage          # With coverage report
```

## Intro

Jest is a delightful JavaScript testing framework created by Facebook (now Meta). Zero-config for most React and TypeScript projects. Features include snapshot testing, built-in mocking, code coverage, parallel execution, and an intuitive API. Used to test React, Instagram, and many Meta properties.

- **Repo**: https://github.com/jestjs/jest
- **Stars**: 45K+
- **Language**: TypeScript
- **License**: MIT

## What Jest Does

- **Zero-config** — works out of the box for most JS/TS projects
- **Snapshot testing** — capture and compare UI output
- **Mocking** — `jest.fn()`, `jest.mock()`, `jest.spyOn()`
- **Code coverage** — built-in Istanbul coverage
- **Parallel execution** — tests run in isolated workers
- **Watch mode** — re-run changed tests
- **Matchers** — rich assertion library (`toBe`, `toEqual`, `toThrow`, `toMatchSnapshot`)
- **Timer mocks** — fake timers for setTimeout/setInterval
- **Async testing** — async/await, promises, callbacks

## Comparison

| Runner | Speed | TS | ESM | Watch |
|---|---|---|---|---|
| Jest | Medium | Via ts-jest | Experimental | Yes |
| Vitest | Fast | Native | Native | Yes |
| Mocha | Medium | Via ts-node | Yes | Yes |
| Node test runner | Fast | Via tsx | Yes | Yes |

## FAQ

**Q: Jest vs Vitest?**
A: Vitest is faster (Vite-powered), native ESM, and TS works without ts-jest. Vitest is recommended for new Vite projects. Jest has a larger ecosystem and more legacy projects.

**Q: How to configure TypeScript?**
A: Use the `ts-jest` transformer or `@swc/jest` (faster). Configure the `transform` field in `jest.config.ts`.

## Sources

- Docs: https://jestjs.io
- GitHub: https://github.com/jestjs/jest
- License: MIT

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Source: https://tokrepo.com/en/workflows/jest-delightful-javascript-testing-framework-4240433c
Author: AI Open Source