Node Modules

Node.js Modules: Introduction to CommonJS

In Node.js, a Module is essentially a JavaScript file containing code that is designed to perform a specific task. Think of modules as JavaScript libraries. By breaking your code into separate modules, you make your applications easier to manage, debug, and scale.

Node.js uses a module system called CommonJS by default. In this tutorial, you will learn how to use built-in modules, create your own local modules, and properly include them in your projects.

1. What are Built-in Core Modules?

Node.js comes with a set of built-in modules that you can use without having to install anything extra. These modules provide essential functionality like reading files, building web servers, and working with file paths.

To include a built-in module in your file, you use the require() function.

Example: Using the fs (File System) Module

// Require the built-in 'fs' module
const fs = require('fs');

// Use it to write text to a file
fs.writeFileSync('message.txt', 'Hello, Node.js Modules!');
console.log('File written successfully.');

Other highly popular core modules include:

• http: For creating web servers.
• path: For safely working with directory and file paths.
• os: For gathering information about the operating system.

2. Creating Your Own Local Modules

As your application grows, you shouldn't put all your code in a single app.js file. Instead, you can create custom modules and export functions or variables to be used in other files.

Let's create a custom module that handles basic math operations.

1. Create a file named mathUtils.js.
2. Write some functions.
3. Use module.exports to make them available outside the file.

const add = (a, b) => {
  return a + b;
};

const subtract = (a, b) => {
  return a - b;
};
// Export the functions as an object
module.exports = {
  add,
  subtract
};

3. Importing Your Local Modules

Now that we have exported our custom functions from mathUtils.js, we can import them into our main file using the require() function.

When requiring a local module, you must provide the relative path to the file (e.g., ./ for the same directory).

// Require the local module
// Notice the './' which tells Node it's a local file, not a core module
const math = require('./mathUtils');

let sum = math.add(10, 5);
let difference = math.subtract(10, 5);
console.log("Sum:", sum);
console.log("Difference:", difference);

4. Exporting a Single Item

Sometimes, your module only needs to export one single function or class, rather than an entire object of functions. You can assign it directly to module.exports.

function sayHello(name) {
  return `Hello, ${name}!`;
}

// Exporting just the function directly
module.exports = sayHello;

And then in your main file:

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

console.log(greet("Alice")); // Output: Hello, Alice!