ES Modules: import & export

Variables: const, let & var (The Box Metaphor)

Learn how variables act as labeled containers for data using const, let, and var to manage lexical scope and reassignment safely in modern applications.

1. Modular Architecture

ES6 Modules (import / export) allow you to split JavaScript code into separate, reusable files with strict scope boundaries.

2. Why It Matters

Modules prevent global scope pollution, clarify explicit file dependencies, and enable modern bundlers (Vite, Next.js) to tree-shake unused code.

3. Named vs Default Exports

• Named Exports: Export multiple items per file by name. Must be imported using exact matching curly braces { func }.
• Default Export: Export one primary value per file. Can be imported with any local name without curly braces.

// mathUtils.js (Module File)
export const add = (a, b) => a + b;       // Named export
export const subtract = (a, b) => a - b;  // Named export

export default function multiply(a, b) {  // Default export
  return a * b;
}

// app.js (Main File)
import multiply, { add, subtract } from "./mathUtils.js";

console.log(add(2, 3));      // 5
console.log(multiply(2, 3)); // 6

4. Re-exporting Utilities

Modules can aggregate utilities into clean entry points:

// index.js
export { add, subtract } from "./mathUtils.js";

5. Common Mistakes

• Mixing Up Curly Braces: Importing a named export without curly braces (import add from './mathUtils.js') attempts to load the default export, resulting in undefined or import errors!
• Omitting Relative Paths in Node/Browser: Importing local files requires ./ or ../ relative paths (import { add } from "./mathUtils.js"). Omiting ./ causes Node to look inside node_modules.

6. Next Steps

Use named exports for utility libraries. Next, let's explore JavaScript Proxy Objects!