From https://github.com/mongodb/mongo/blob/r3.3.9/src/mongo/bson/oid.h#L238

We have 3 modes, "JS", "10gen", and "Strict".

• "JS" is Javascript JSON compatible
• "10gen" is Shell parseable
• "Strict" is strict RFC compliant JSON

"JS" is distinguished from "Strict" only by having a Date constructor.

However, as documented in http://docs.mongodb.org/manual/reference/mongodb-extended-json/ and confirmed by removing the option and compiling, the "JS" mode is not used anywhere in the server. The correct answer may not be to take this out now, but this should make it clear that backwards compatibility doesn't need to be preserved for any of this dead code if this system is ever refactored.