CommonJS Modules

Every Node.js application is made up of modules — files that export functionality and import it from other files. Before ES Modules arrived in Node.js, CommonJS was the only module system. You’ll encounter it constantly in the Node.js ecosystem.

What a module is

A module is any .js or .ts file. By default, everything in a file is private — variables, functions, classes. You explicitly choose what to expose.

module.exports

To make something available to other files, assign it to module.exports:

math.js

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

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

module.exports = { add, subtract }

require()

To use exports from another file, call require() with the file path:

index.js

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

console.log(add(2, 3))       // 5
console.log(subtract(10, 4)) // 6

The ./ prefix means “relative to the current file.” Without it, Node.js looks in node_modules.

Module resolution order

When you require('something'), Node.js checks in this order:

1. Built-in modules (fs, path, http) — no file needed
2. Relative paths (./math, ../utils) — looks for the exact file
3. node_modules — looks for a package with that name

const fs = require('fs')        // built-in
const math = require('./math')  // relative file
const express = require('express') // from node_modules

The module wrapper

Node.js wraps every module in a function before executing it:

(function(exports, require, module, __filename, __dirname) {
  // your module code runs here
})

This is why __filename (the current file’s path) and __dirname (the current file’s directory) are available in any CommonJS module — they’re injected as arguments. It’s also why require is available without importing it.

Default exports

You can export a single value as the entire module.exports:

greet.js

module.exports = function greet(name) {
  return `Hello, ${name}!`
}

index.js

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

When you’ll see CommonJS

Many npm packages still ship CommonJS. You’ll see it in:

• Older Node.js tutorials and codebases
• Some npm packages (check their package.json for "main" field)
• Configuration files like eslint.config.cjs

The Bulletin API uses ES Modules (next lesson), but understanding CJS helps when you hit require in the wild.

Exercise

1. Create src/utils/math.ts that exports add, subtract, and multiply functions.
2. Create src/test.ts that requires the math module and logs results of each function.
3. Run with tsx src/test.ts and verify the output.

Recap

• module.exports makes values available to other files.
• require('./path') imports those values.
• Module resolution: built-ins → relative paths → node_modules.
• __filename and __dirname are injected by Node.js’s module wrapper.
• CommonJS is still common in the ecosystem even though ES Modules are now preferred.