If you've ever opened a JavaScript file and found 3,000 lines of tangled functions, global variables clashing with each other, and no clear structure — you already understand why modules exist, even if you haven't used them yet. Modules are one of the most important concepts in modern JavaScript. They solve a fundamental problem: how do you split your code into organized, reusable, and maintainable pieces without everything falling apart? In this guide, we'll start from the problem modules solve, then walk through how to export, import, and structure your code like a professional developer. No bundler configuration. No framework abstractions. Just vanilla JavaScript and the module system built into the language itself. Table of Contents The Problem: Why Your Code Falls Apart Without Modules What Are JavaScript Modules?…