Modules: require & exports

Your own modules

As your program grows, putting everything in one file gets messy. Node lets you split code into modules — separate files that each do one job. A file decides what to share with module.exports, and another file pulls that in with require. Think of it like packing tools into a labelled box (module.exports) and then opening that box in another room (require).

First, a file that exports two helper functions so other files can use them:

// math.js
function add(a, b) { return a + b; }
function multiply(a, b) { return a * b; }
module.exports = { add, multiply };

The last line is the key part: module.exports = { add, multiply } hands out an object containing both functions. Anything not on module.exports stays private to this file. Now another file can import and use them:

// app.js
const math = require("./math");
console.log(math.add(2, 3));        // 5
console.log(math.multiply(4, 5));   // 20

Built-in modules

Node ships with useful modules you can require without installing anything — like fs (files), http (servers), path (file paths) and os (info about the computer). Notice these have no ./ in front: that tells Node it is a built-in module, not one of your own files.

const os = require("os");
console.log("Platform:", os.platform());
console.log("Free memory:", os.freemem());