In browsers + Node, what is the scope of a module?

Question

I'm wondering what the scope of a module is in both browsers and Node. I'm specifically trying to understand whether a module-level variable is created once per app, or many times. Is a new instance of the module created on each import, or is it the exact same module shared across all imports? Many thanks!

Answer 1

There are several JS module flavours - ESM, CommonJS, AMD. Their common trait is that they are evaluated once on first import, at least under normal circumstances. Doing the opposite would make them inefficient for sharing data.

Exporting class instance is a common way to share one instance across the application without making a class a singleton.

The ways that can make a module be evaluated multiple times (intentionally or not) include having many copies of a module, different filename case on import in case-insensitive system, the modification of Node module cache.