📦 Day 2: Node.js Modules — Core Modules, Custom Modules, require and exports

👨‍🏫 What You’ll Learn Today:

• What is a module in Node.js?

• Core vs Custom Modules

• How to use require and exports

• Real-life examples

• Practical tasks for hands-on learning

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✅ What is a Module?

A module in Node.js is like a small building block of code that performs a specific task.
You can think of it as a separate file that contains reusable logic.

📦 Instead of writing all code in one file, we divide it into modules and import them wherever needed.

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📂 Types of Node.js Modules

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🔧 Core Modules – Built-in Features

Node.js gives us many useful modules out of the box.

Let’s explore 3 common ones:

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1. fs (File System)

Used to read/write files.

const fs = require('fs');

fs.writeFileSync('hello.txt', 'Hello from Node.js!');

✅ Output: Creates a file named hello.txt with content inside.

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2. path

Used to work with file paths.

const path = require('path');

const filePath = path.join(__dirname, 'hello.txt');
console.log(filePath);

✅ Output: Full path of hello.txt

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3. os

Gives info about your computer system.

const os = require('os');

console.log("OS type:", os.type());
console.log("Free memory:", os.freemem());

✅ Output: Your system type and available memory.

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🧩 Custom Modules – Your Own Code

You can create your own modules in separate files and use them anywhere using require().

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📝 Example: Create a Calculator Module

➤ Step 1: Create a file calculator.js

// calculator.js

function add(a, b) {
  return a + b;
}

function subtract(a, b) {
  return a - b;
}

// Export functions to be used in other files
module.exports = {
  add,
  subtract
};

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➤ Step 2: Create a file app.js to use this module

// app.js

const calc = require('./calculator');

console.log("Addition:", calc.add(5, 3));
console.log("Subtraction:", calc.subtract(10, 4));

🟢 Output:

Addition: 8
Subtraction: 6

💡 require('./calculator') is used to import your custom module.

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📚 module.exports and require() in Detail

👉 You can export a single function, object, or even class.

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🎯 Real-Life Use Case: Utilities Module

Suppose you're building a blog backend, and you need a function to format the current date.

➤ utils.js

function getCurrentDate() {
  const date = new Date();
  return date.toLocaleDateString();
}

module.exports = { getCurrentDate };

➤ blog.js

const utils = require('./utils');

console.log("Post Date:", utils.getCurrentDate());

✅ This makes your code reusable, clean, and modular.

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💡 Why Professionals Use Modules

• Code Reusability

• Better Structure in Large Projects

• Easier Maintenance

• Clean Separation of Concerns

✅ In a real backend project, your code will be split into modules like:

• routes

• controllers

• models

• utilities

• services

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🔄 Practice Tasks

1. Create a module math.js with multiply and divide functions

2. Use it in another file and print results

3. Create a custom module info.js that exports your name, role, and institute

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🔚 Summary

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🎓 You're now ready to build modular, scalable backend apps!

Would you like me to continue with Day 3: File System module (fs) – reading and writing files in the same detailed format?