Modules: Splitting Code into Files

Why modules exist

A real backend is thousands of lines. Nobody sane keeps that in one file. Node lets you split code into modules: files that export some values and import what they need from others.

The classic Node style is called CommonJS. A file exports by assigning to module.exports, and imports with require:

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

module.exports = { add };

// app.js
const mathUtils = require("./mathUtils");

console.log(mathUtils.add(2, 3)); // 5

The ./ means "a file next to me". Without it, require("fs") reaches for a built-in module, or one installed with npm, Node's package manager: a command-line tool plus a public registry of shared code, which you will use to install Express in unit 5.

The newer syntax: ES modules

Modern JavaScript added its own module syntax, ESM, which Node also supports (in .mjs files or with "type": "module" in package.json):

// mathUtils.mjs
export function add(a, b) {
  return a + b;
}

// app.mjs
import { add } from "./mathUtils.mjs";
console.log(add(2, 3));

Either way, the shape is the same: a module is an object of things it chose to share. The exercises here run in one file, so you will practice that shape directly.