From the ECMAScript2015 spec

4.3.10 undefined value
primitive value used when a variable has not been assigned a value
4.3.12 null value
primitive value that represents the intentional absence of any object value

AC

• Audit usage of | undefined in return types, a function that "returns undefined" should actually be void.
• If the API is supposed to return something or nothing it should always use null to indicate "nothing", ex. getWriteErrorAt
• Investigate eslint rules to enforce this.

Impact Considerations

• null is serialized by JSON where undefined is not
• undefined is (by default) ignored by our BSON library
• undefined when deserialized from BSON is converted to null