You add a <script> tag, the page runs your code. Underneath that one line the browser does a surprising amount of work: it decides when to run the script, fetches every file it depends on, figures out what each file is asking for, keeps every loaded module in memory so it runs exactly once, and evaluates them all in the right order. This is a tour of that machinery.
JavaScript had no built-in module system for most of its life, so the community invented one pattern after another to fill the gap. This is a chronological tour of how we got from global <script> tags to native ES Modules — and why each step happened.
Tree shaking is a term used in the context of web development, particularly with JavaScript and modern front-end build tools, to describe the process of removing unused code from your final bundle. It's like shaking a tree to let the dead leaves (unused code) fall off, so you end up with a lighter, more efficient bundle that only includes the code that's actually used in your application.
This concept became particularly relevant with the rise of module bundlers like Webpack, Rollup, and tools like Parcel, which analyze your import and export statements to determine which parts of a module are used by your application. If a function, class, or variable from a module is never used, the bundler can exclude it from the output bundle, reducing the size of your application and improving load times for your users.