Modules

What you'll learn

How to share code between files with modules, and how to import Node's built-in modules.

Why it matters

Real programs are many files. Modules keep each file small and focused, and let you reuse code instead of copying it.

Explanation

A module is just a file. It chooses what to share with export, and other files bring it in with import.

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

// main.js
import { add } from "./math.js";
console.log(add(2, 3));

For Node to treat your files as modern ES modules, either name them .mjs or put "type": "module" in package.json.

Node also ships built-in modules that you import with the node: prefix: node:fs (files), node:path (file paths), node:os (system info).

Example

Two files, math.mjs and main.mjs:

// math.mjs
export function add(a, b) {
  return a + b;
}
export const PI = 3.14159;

// main.mjs
import { add, PI } from "./math.mjs";
import path from "node:path";

console.log(add(2, 3));
console.log(PI);
console.log(path.basename("/home/asha/notes.txt"));

Run with node main.mjs.

Step-by-step walkthrough

1. math.mjs exports a function and a constant. Nothing else in the file is visible outside.
2. main.mjs imports exactly those two names with curly braces.
3. The path to your own file starts with ./. Built-ins use the node: prefix.
4. path.basename returns the last part of a path: notes.txt.

Try it yourself

Add a multiply export to math.mjs and print multiply(4, 5). Press the button to see the output of the original main.mjs.

Common mistakes

• Forgetting ./ on your own files: import "math.mjs" looks for a package.
• Missing the file extension in ES module imports.
• Mixing require and import in the same file.
• Not enabling ES modules with .mjs or "type": "module".

Quick quiz

Summary

• A module is a file; export shares, import uses.
• Built-in modules are imported with the node: prefix.
• Use .mjs or "type": "module" for ES modules.

Next: npm and installing packages.