Undefined

The Undefined type is a BSON data type that represents a value that is not defined. It is primarily used to indicate that a field does not have a value or that a variable has not been assigned a value. However, the Undefined type is rarely used in practice, as it has been deprecated in favor of using null to represent the absence of a value. The use of null is more common and recommended for indicating missing or non-existent data in documents.

Visit the following resources to learn more:

• @official@BSON Types
• @official@Query for Null or Missing Fields
• @article@Migrate Undefined Data and Queries
• @article@Master Null Handling in MongoDB