Using modules on the client was infeasible because an application can have hundreds of dependencies and sub-dependencies; having to download them all when someone visits the page is going to increase the time-to-first-render (TTFR), drastically impacting the user experience (UX). Therefore, JavaScript modules, as we know them today, began their ascent on servers with Node.js modules.
In Node.js, a single file corresponds to a single module:
$ tree.├── greeter.js└── main.js0 directories, 2 files
For instance, both of the preceding files—greeter.js and main.js—are each their own module.
