CommonJS

🔄 CommonJS (CJS)

CommonJS is the module system Node.js has used from the start, to split code into files and reuse it.

• Export with module.exports

• Import with require()

• Loads modules synchronously

👉 Features

• One file = one module

• Private scope by default

• Built into Node.js — no setup needed

• Modules are cached after the first load, so requiring the same file twice doesn't re-run it

🧪 Sample Code

math.js

function add(a, b) {
  return a + b;
}
module.exports = add;

app.js

const add = require('./math');
console.log(add(2, 3)); // 5

🛠 When to Use CommonJS

• ⚡ Quick backend scripts — faster to set up, simpler syntax

• 🗂️ Legacy Node.js projects that already use require() throughout

💡 Good to Know

• Requiring a module dynamically (conditionally, inside an if) works fine in CJS — that's harder in ES Modules.

• .js is the default file extension; no package.json config required.