It seems that when promoteValues: false is specified for an operation the error codes are also returned as BSON objects:

Without promoteValues: false:

coll.aggregate([{}]).toArray().catch(err => err.code)
40323

With promoteValues: false:

coll.aggregate([{}], {promoteValues: false}).toArray().catch(err => err.code)
Int32 { _bsontype: 'Int32', value: 40323 }

Couldn't find anything about this in our docs or spec, but I would expect the error code to stay a number regardless of how the method is invoked.

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Related note captured from NODE-3123: The behavior of the driver related to the promoteValues is currently not very well documented, and should be improved (see NODE-3222).