We could if we had a library to do it.

How are you going to save a JS value without JS?

I don't need a Js value, I want the version string here.

In C++ you need a type before you can read from it. A JS C++ library that doesn't parse all possible JSON strings is not a JS library.
Also we do have a library for reading JS values, we probably don't need to add a second one if we can just add one more line at the right place.

I'm currently trying to just create a new scriptinterface, but the g_ScriptRuntime isn't initialized when InitVFS is called yet too, so it's kind of a dead end.
Only caching the the JS Interface which is only called from the JS GUI might be an alternative, but I didn't give in yet.

Sorry I feel like there's a misunderstanding here. I don't need a "JS Library", I just need something that parses JSON into something usable by C++, so that I can use the mod version and put it in my C++ thingy.

library for reading JS values

Are you referring to Spidermonkey? I feel like initialising a spider monkey runtime just for parsing "mod.json" to C++ is extremely overkill.

We could initialise Spidermonkey at the very startup, and use it instead, but that feels like a bit of an anti-pattern.

My approach is to parse mod.json in C++ without relying on the JS runtime.

JSON = JavaScript Object Notation and it can hold any natively serializable JS object.

I feel like initialising a spider monkey runtime just for parsing "mod.json" to C++ is extremely overkill.

Importing a new library seems more lightweight?
I didn't suggest to create a new Runtime, only a new ScriptInterface

We could initialise Spidermonkey at the very startup, and use it instead, but that feels like a bit of an anti-pattern.

Not sure what you mean with anti-pattern here. Initializing one thing before the other before using it seems right.
Making the mod mounting code that currently doesn't depend on SM depend on SM seems like an anti-pattern maybe ("dependency hell") maybe.
But the greater problem that prevents us from doing so is that there is a code comment stating that VFS should be initialized before anything else, which we probably don't want to play with. But that isn't the more attractive of the proposed alternatives to explore anyway. I'll post some new brokenwipdiff or better later hopefully.

My approach is to parse mod.json in C++ without relying on the JS runtime.

I don't feel understood. As I said, think about the data structures that are required to hold any JS object that can be expressed in JSON.
It will require you to use a JS data type.
So you either have to propose adding a new library that can hold JS objects, use an existing library that can hold JS objects or you write your own mod.json parser that parses each character individually.