Which operation is primarily used to insert a new document if no document matches the filter criteria?

The operation that is primarily designed to insert a new document if no document matches the filter criteria is the upsert operation. An upsert operation is a combination of an update and an insert; it updates a document if a match is found, and if no match is found based on the filter criteria, it inserts a new document with the specified values.

Using updateOne with the upsert option set to true allows you to achieve this functionality. When invoking the updateOne method, setting the upsert parameter to true instructs MongoDB to create a new document if the filter criteria don't match any existing documents. Thus, the upsert capability is crucial for situations where you want to ensure that a document is created in the event that it does not already exist in the database.

The other operations mentioned, such as insertOne, replaceOne, and updateOne without the upsert option, do not directly provide this insert-if-not-found functionality. For instance, insertOne only adds a new document, while replaceOne and updateOne are focused on updating existing documents without an inherent mechanism to check the presence of a matched document and insert a new one if needed, unless specifically configured to do so with upsert. Therefore, the upsert operation is essential for