Capstone Project: Advanced Unit Testing with Jest

Welcome to your final project! 🎉

In this capstone, you will apply everything you learned about unit testing in JavaScript using Jest.
The project contains several small but realistic modules — your job is to write comprehensive tests for them.

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📂 Project Structure

advanced-unit-testing/
│── package.json # Project setup with Jest
│── jest.config.js # Jest configuration
│── README.md # Instructions (this file)
│
├── src/utils # Source code (functions/classes to test)
│ ├── math.js
│ ├── stringUtils.js
│ ├── userService.js
│ ├── asyncService.js
│ └── eventEmitter.js
│
└── tests/ # Your test files (currently empty)
├── math.test.js
├── stringUtils.test.js
├── userService.test.js
├── asyncService.test.js
└── eventEmitter.test.js

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🧑‍💻 Your Mission

Your goal is to write Jest tests inside the tests/ folder.
Each source file in src/ has a corresponding empty test file.

The challenge is to test:

• Pure functions (no dependencies, just logic).
• Async functions (Promises, async/await).
• Class methods with dependencies (mocking).
• Event-driven behavior (event emitters).

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📌 Task Breakdown

1. math.js → Basic Functions

Functions: add, divide, factorial

👉 What to test:

• ✅ Normal cases (e.g., add(2, 3) → 5)
• ✅ Edge cases (e.g., divide by zero should throw an error)
• ✅ Recursion & boundaries (e.g., factorial(0) → 1, negative numbers should throw)

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2. stringUtils.js → String Manipulation

Functions: capitalize, sanitize, reverse

👉 What to test:

• ✅ Capitalization of lowercase & mixed case words
• ✅ sanitize removes HTML tags (but leaves plain text)
• ✅ reverse works with normal & special characters

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3. userService.js → Service with Dependency

Class: UserService (depends on a database object)

👉 What to test:

• ✅ createUser creates a valid user object
• ✅ Throws error if input is missing
• ✅ getUser calls database and returns result
• ✅ Use mocks (Jest fn() or jest.mock) instead of a real database

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4. asyncService.js → Async Testing

Function: fetchData(url)

👉 What to test:

• ✅ Resolves with correct response for a valid URL
• ✅ Rejects with an error for invalid URL
• ✅ Works with async/await and .then/.catch

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5. eventEmitter.js → Event-Driven Behavior

Class: MyEmitter (extends EventEmitter)

👉 What to test:

• ✅ Event listeners (message event should receive the message)
• ✅ Error handling (error event emits errors)
• ✅ Multiple listeners get called correctly

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🚀 Getting Started

Install dependencies:

   npm install

Run all tests:

npm run test 

Run only one file:

npm run test math.test.js

🚀 Good Luck