Node.js — CommonJS

What is CommonJS?

CommonJS is the original module system for Node.js. Every file in Node is treated as a separate module, and CommonJS provides require() to import and module.exports to export.

Exporting with module.exports

// math.js
function add(a, b) {
  return a + b;
}

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

module.exports = { add, subtract };

Importing with require()

// app.js
const math = require('./math');

console.log(math.add(3, 4));       // 7
console.log(math.subtract(10, 3)); // 7

Destructuring Imports

const { add, subtract } = require('./math');

console.log(add(5, 5)); // 10

Exporting a Single Value

// config.js
const config = {
  port: 3000,
  db: 'localhost',
  secret: 'my-key'
};

module.exports = config;

// app.js
const config = require('./config');
console.log(config.port); // 3000

Exporting a Function

// greet.js
function greet(name) {
  return `Hello, ${name}!`;
}

module.exports = greet;

const greet = require('./greet');
console.log(greet('Alice')); // Hello, Alice!

How require() Works

1. Resolves the file path
2. Checks if the file is cached (modules are cached after first load)
3. Loads and executes the file
4. Returns module.exports

// Built-in modules don't need a path
const fs = require('fs');
const path = require('path');
const http = require('http');

// Third-party modules use the package name
const express = require('express');

Module Caching

// require() caches the result — the file runs only once
const a = require('./math');
const b = require('./math');
console.log(a === b); // true — same object

Circular Dependencies

When two files import each other, Node resolves what it can:

// a.js
const b = require('./b');
module.exports = { name: 'a' };

// b.js
const a = require('./a'); // Partial export (name might be undefined)
module.exports = { name: 'b' };

Circular dependencies work but can cause subtle bugs — restructure your code to avoid them.

CommonJS vs ES Modules

Feature	CommonJS	ES Modules
Syntax	require() / module.exports	import / export
Loading	Synchronous	Asynchronous
Where	Node.js native	Browser + Node.js
File extension	.js	.mjs or "type": "module"

Best Practices

• Export a clear API — don't export internal helpers
• Use named exports (module.exports = { fn1, fn2 }) over attaching to exports
• Keep one module per file
• Avoid circular dependencies
• Use __dirname and __filename for path resolution

Practice

1. Create a utils.js module with capitalize() and reverse() functions
2. Import and use them in a separate app.js file
3. Create a config.js that exports database settings
4. Build a small file reader module using fs and export a readJSON() function