Modules

As a program grows past one file, ES modules keep it organised: each file owns a focused job (cohesion) and exposes a small set of names for others to use (its interface). JavaScript's module system is import / export.

Named exports

Mark anything you want to share with export, and import it by name:

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

// app.js
import { add, PI } from "./math.js";
add(2, 3);   // 5

The names in the { } must match the exported names. You can rename on import with as: import { add as sum } from "./math.js".

Default exports

A module can have one default export — useful when a file is "about" one main thing:

// logger.js
export default function log(msg) {
  console.log("[app]", msg);
}

// app.js
import log from "./logger.js";   // no braces; you choose the name

Use named exports for a module that provides several things; a default export for a module whose whole purpose is one value (a component, a class, a main function).

Why modules matter

Modules are how the abstraction and architecture lessons play out in real code: each file hides its internals and offers a clear surface, so you can change one without breaking the others, and a reader can understand a file from its exports alone.

Where to go next

Last in Language basics: TypeScript basics — adding a type layer that catches mistakes before you run.