Browser still has to parse+run all of that JS though. Because JS-in-browsers is antimodular in that all previous JS on a given page influences all subsequently loaded JS, even if using just script tags with a URL. In a way this is very similar to the pains experienced by the C++ community for many years now.
C++ doesn't have a "load this module" statement. I does have an "include all the contents of X file here", and file X has to deal with possible duplication and cyclic dependencies, usually with something like
#ifndef _SOME_FILE_H
#define _SOME_FILE_H
// code goes here
#endif
Also any "module" can define any global variable, so name clashes (esp. with C libraries) are possible. Most languages with proper modules the "global" variables are module-wide.
An equally nasty issue happens with JS, where many libraries must be loaded in the proper order and not duplicated. There are a bunch of module systems but none is part of the spec. The good thing is that module systems are easy to make thanks to closures.
The short answer is that "it's complicated". The most accessible long answer I'm aware of for non-C++'ers is at [1] (video). If you prefer text, the standardese version is at [2]. Unfortunately I cannot find a concise text version for laymen except perhaps [3].
Browser still has to parse+run all of that JS though. Because JS-in-browsers is antimodular in that all previous JS on a given page influences all subsequently loaded JS, even if using just script tags with a URL. In a way this is very similar to the pains experienced by the C++ community for many years now.