Is there an alternative tool you can suggest, to allow us securely run arbitrary JS? I was looking at Apple's JavaScriptCore to run JS and and if it happens that I need any level of access to the system(i.e. files) simply handle that in Swift and pass the file to the JS. Would that be a secure approach?
If you want to isolate your program, you should use an OS-level sandbox like bubblewrap or a lightweight VM like Firecracker. I’m not familiar with Apple’s JavaScriptCore, but if it doesn’t provide any access to the system (and instead relies on passing arguments from Swift code), it might also be a viable approach.
Yes, my understanding is that it's the pure language interpreter without anything about filesystems or web browser. You need to create an interface in Swift/Objective-C or C to put in and get data out of the execution context.