How do I set up node.js environment and install jest?

# Step 1: Environment Setup Ensure Node.js (version 18+) is installed from [nodejs.org](https://nodejs.org). Create a new project: ```bash mkdir jest-learning cd jest-learning npm init -y ``` Install Jest as dev dependency: ```bash npm install --save-dev jest ``` Add test script to `package.json`: ```json "scripts": { "test": "jest" } ``` Verify installation: ```bash npx jest --version ``` This outputs Jest version (e.g., 30.2.0). 50 52

How do I write and run your first basic test?

# Step 2: First Test Create `sum.js`: ```javascript function sum(a, b) { return a + b; } module.exports = sum; ``` Create `sum.test.js`: ```javascript const sum = require('./sum'); test('adds 1 + 2 to equal 3', () => { expect(sum(1, 2)).toBe(3); }); ``` Run: ```bash npm test ``` Expected: PASS ./sum.test.js ✓ adds 1 + 2 to equal 3 Debug failing tests by fixing `sum.js` if needed (e.g., add return). 50 52

How do I explore jest matchers and assertions?

# Step 3: Matchers Mastery Expand `sum.test.js`: ```javascript test('adds correctly', () => { expect(sum(1, 2)).toBe(3); expect(sum).toBeDefined(); expect([1,2]).toEqual([1,2]); expect(null).toBeNull(); }); ``` Run `npm test` to verify. Key matchers from docs: - `toBe`: exact equality - `toEqual`: deep equality - `toBeTruthy()`, `toContain`, etc. Practice: Add tests for edge cases like sum(0,0), negatives. 49

How do I test asynchronous code with promises and await?

# Step 4: Async Testing Create `fetchData.js`: ```javascript function fetchData() { return Promise.resolve('peanut butter'); } module.exports = fetchData; ``` Test in `fetchData.test.js`: ```javascript const fetchData = require('./fetchData'); test('async resolves', async () => { const data = await fetchData(); expect(data).toBe('peanut butter'); }); test('alternative', () => { expect(fetchData()).resolves.toBe('peanut butter'); }); ``` Handle rejects similarly with `.rejects`. 48

How do I implement mock functions and modules?

# Step 5: Mocking Essentials Mock in test: ```javascript const mockFn = jest.fn(); mockFn.mockReturnValue(42); expect(mockFn()).toBe(42); expect(mockFn).toHaveBeenCalled(); ``` Module mock: ```javascript jest.mock('./math', () => ({ doMath: jest.fn(() => 5) })); ``` Spy: `const spy = jest.spyOn(obj, 'method');` Run coverage: `npm test -- --coverage` 47

Learn Effective JavaScript Testing with Jest in 5 Steps

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Instant Toolkit

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Step-by-Step Guide

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Step 1: Environment Setup

Ensure Node.js (version 18+) is installed from nodejs.org.

Create a new project:

mkdir jest-learning
cd jest-learning
npm init -y

Install Jest as dev dependency:

npm install --save-dev jest

Add test script to package.json:

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

Verify installation:

npx jest --version

This outputs Jest version (e.g., 30.2.0).

Why this step matters:

-Builds a reproducible testing environment essential for consistent results
-Enables immediate hands-on practice with real project setup

30-60 minutes

Node.js, npm, VS Code, Terminal

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Definition of Done

•Node.js and npm verified with `node --version` and `npm --version`
•Jest installed and version command succeeds
•Test script added to package.json

Common Mistakes to Avoid

❌ Installing Jest without --save-dev

→Always use `npm install --save-dev jest` for dev dependencies

❌ Forgetting to add 'test' script in package.json

→Manually add '"test": "jest"' under scripts

❌ Using global install for project work

→Use local install via npx or npm scripts

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🌐 Helpful Websites

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Learn Effective JavaScript Testing with Jest in 5 Steps