Modules

Why modules?

Modules allow splitting a program into multiple reusable files.
➡ This makes code clearer, better organized, and easier to maintain.

Without modules, all code would be in one file, risking variable or function name conflicts.

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Exporting a module

A file can export its functions, variables, or classes to make them available elsewhere.

math.js

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

export const PI = 3.14;

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Importing a module

Another file can then import what it needs.

main.js

import { add, PI } from "./math.js";

console.log(add(2, 3)); // 5
console.log(PI); // 3.14

Use curly braces { } to import named exports.

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Default export

A file can also have one default export.
In that case, you can import it with any name.

utils.js

export default function greet(name) {
  return `Hello ${name}`;
}

// main.js
import greet from "./utils.js";

console.log(greet("Alice")); // Hello Alice

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Named export vs default export

Type	Declaration	Import
Named export	export function add() {}	import { add } from "./file.js"
Default export	export default function greet()	import greet from "./file.js"

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Modules in Node.js

Before ES6 modules, Node.js used CommonJS:

// CommonJS (old)
const fs = require("fs");

// ES6 (modern)
import fs from "fs";

Today, Node.js supports ES6 modules (with the .mjs extension or "type": "module" in package.json).

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Best practices

• Prefer named exports for clarity and flexibility.
• Use a default export only if the module provides a single main feature.
• Organize modules by topic or domain (e.g., math.js, user.js, api.js).
• Always include the .js extension in imports when targeting modern browsers.