import and export

                export const x = 1; / export default fn; / import x, { y } from "./file.js";
              

Purpose

export makes a binding from one module (file) available to others; import brings it in. Each module has its own scope — nothing is shared between files unless explicitly exported and imported.

Syntax

// math.js
export function add(a, b) {
  return a + b;
}
export const PI_ROUNDED = 3.14;
export default function multiply(a, b) {
  return a * b;
}

// app.js
import multiply, { add, PI_ROUNDED } from './math.js';
import { add as addNumbers } from './math.js'; // renaming on import
import * as math from './math.js'; // everything, as one namespace object

Behavior notes

• A module can have any number of named exports but at most one default export.
• Named exports must be imported by their exact exported name (or explicitly renamed with as); a default export can be imported under any local name, since there’s only one.
• Browsers load a module with <script type="module" src="app.js">; module scripts defer automatically and always run in strict mode.
• Node treats a file as a module either via a .mjs extension, or a "type": "module" field in package.json applying to .js files in that package.
• Import paths for local files must be relative (./math.js) or absolute, including the file extension in most current tooling — unlike some older bundler configurations that allowed omitting it.

Common mistakes

• Forgetting the file extension in a relative import path where the runtime/tooling requires it.
• Mixing up named and default export syntax when importing — import { multiply } for a default export throws, since default exports aren’t inside the named-export braces.

Runtime notes

ES modules are core ECMAScript, standard in every current browser and Node version. Older CommonJS (require/module.exports) syntax still appears in many existing Node codebases — covered in the Intermediate tier’s Node module.