How do I set up node project and install jest?

# Step 1: Environment Setup 1. Ensure Node.js (v18+) is installed: [nodejs.org](https://nodejs.org). 2. Create project: `mkdir jest-learn && cd jest-learn && npm init -y` 3. Install Jest: `npm install --save-dev jest` 4. Add to `package.json` scripts: ```json "scripts": { "test": "jest" } ``` Verify with `npm test` (should show no tests).

How do I write and run your first test?

# Step 2: First Test 1. Create `sum.js`: ```js function sum(a, b) { return a + b; } module.exports = sum; ``` 2. Create `sum.test.js`: ```js const sum = require('./sum'); test('adds 1 + 2 to equal 3', () => { expect(sum(1, 2)).toBe(3); }); ``` 3. Run `npm test`. Expected: `PASS ./sum.test.js ✓ adds 1 + 2 to equal 3`.

How do I explore matchers and assertions?

# Step 3: Matchers Mastery Enhance tests with matchers: - `toEqual` for objects: `expect({a:1}).toEqual({a:1});` - `toBeTruthy()`, `toBeFalsy()` - `toMatch(/regex/)` for strings - Numbers: `toBeGreaterThan(3)`, `toBeCloseTo(0.1)` Add to `sum.test.js`: ```js test('null is falsy', () => { expect(null).toBeFalsy(); }); ``` Run `npm test`. See [matchers docs](https://jestjs.io/docs/using-matchers). 50

How do I test async code and mocks?

# Step 4: Async & Mocks 1. Mock async `fetchUser.js`: ```js // fetchUser.js async function fetchUser(id) { // simulate return {id, name: 'User'}; } module.exports = fetchUser; ``` 2. Test `user.test.js`: ```js const fetchUser = require('./fetchUser'); test('fetches user', async () => { await expect(fetchUser(1)).resolves.toEqual({id:1, name:'User'}); }); ``` 3. Mock: `jest.mock('./fetchUser'); fetchUser.mockResolvedValue({name:'Mock'});` Run tests.

How do I snapshots, coverage & cli mastery?

# Step 5: Advanced Features 1. Snapshots: `expect(obj).toMatchSnapshot();` → `npm test --updateSnapshot` 2. Coverage: `npm test -- --coverage` 3. Watch: `npm test -- --watch` 4. CLI: `--bail`, `--verbose`, `--onlyChanged` Add snapshot test: ```js test('object snapshot', () => { expect({a:1}).toMatchSnapshot(); }); ``` Explore full CLI. 48

Master Jest for JavaScript Testing in 5 Steps

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Step-by-Step Guide

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Step 1: Environment Setup

Ensure Node.js (v18+) is installed: nodejs.org.
Create project: mkdir jest-learn && cd jest-learn && npm init -y
Install Jest: npm install --save-dev jest
Add to package.json scripts:

"scripts": {
  "test": "jest"
}

Verify with npm test (should show no tests).

Why this step matters:

-Establishes a reliable testing environment
-Prepares for scalable project testing workflows

20-30 minutes

Node.js v18+, npm, VS Code, Terminal

$0

Definition of Done

•Node project initialized
•Jest installed as dev dependency
•'npm test' runs without errors

Common Mistakes to Avoid

❌ Installing Jest globally

→Use `npm install --save-dev jest` for local project isolation

❌ Skipping package.json script

→Add '"test": "jest"' to run tests easily

❌ Using outdated Node version

→Upgrade to Node.js v18+ from nodejs.org

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🌐 Helpful Websites

Jest Official Getting Started

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Master Jest for JavaScript Testing in 5 Steps