mongo.undefined: The mongo.undefined class
In jonkatz2/rmongodb: R-MongoDB driver

Objects of class "mongo.undefined" are used to represent undefined values in BSON documents.

mongo.undefined objects are strings (a character vector) with a single value of "UNDEFINED"

mongo.undefined objects have "mongo.undefined" as their class so that
mongo.bson.buffer.append() may detect them and append the appropriate BSON undefined value to a buffer.

These mongo.undefined values may also be present in a list and will be handled properly by mongo.bson.buffer.append.list() and mongo.bson.from.list().

mongo.undefined.create,
mongo.bson.buffer.append,
mongo.bson.buffer.append.list,
mongo.bson.buffer,
mongo.bson.

buf <- mongo.bson.buffer.create()
undef <- mongo.undefined.create()
mongo.bson.buffer.append(buf, "Undef", undef)
l <- list(u1 = undef, One = 1)
mongo.bson.buffer.append.list(buf, "listWundef", l)
b <- mongo.bson.from.buffer(buf)

# the above will create a mongo.bson object of the following form:
# { "Undef": UNDEFINED, "listWundef" : { "u1" : UNDEFINED, "One" : 1 } }