{
 userId: OjbectId,
 updatedAt: Date
 }

If you know how to write in SQL, the same principle applies to MongoDB. This is a very common grouping aggregation query. The answer is below, but I still hope you can try to search for the answer online and exercise your ability to solve problems independently.

db.collection.aggregate([
 {$group: {_id: "$userId", maxUpdatedAt: {$max: "$updatedAt"}}}
 ]);

UseaggregationThe$maxOperator can easily get the answer.