CommonJS Module System

The CommonJS (require) Pattern

CommonJS is the traditional module format introduced with Node.js. It loads modules synchronously using the require() function and exports values via module.exports or exports.

Exporting Modules

// math.js - Single export object assignment
const add = (a, b) => a + b;
const subtract = (a, b) => a - b;

module.exports = {
  add,
  subtract
};

// logger.js - Exporting individual properties
exports.logInfo = (msg) => console.log("[INFO]", msg);
exports.logError = (msg) => console.error("[ERR]", msg);

Importing Modules

// app.js
const { add, subtract } = require("./math");
const logger = require("./logger");

console.log(add(10, 5)); // 15
logger.logInfo("System operational");

Module Caching & Scoping

• Modules are cached after the first load. Subsequent calls to require('./file') return the cached export object without re-executing the file.
• Node wraps modules in a wrapper function providing __filename, __dirname, require, module, and exports.

TL;DR

• CommonJS uses require() and module.exports.
• require() runs synchronously.
• Loaded modules are cached in memory after initial resolution.