What does mUUID.v4().toString() mean?

I’m still not clear on what mUUID.v4().toString() means. What I understand is that it generates an Universally Unique ID, but then in converts it into a String. So it’s not a BSON data type, but String data type. Am I correct?

As long as it says in your JS code, it is a JS string.

It becomes a BSON string in its way to the server or when it is processed by the server. The exact details are not really important.

Why do you care about the difference?

You work with JS string in your code and the server do its work as a BSON string.