Updating documents on MongoDB prior to version 4.2 was quite limited. It was not possible to set values to a conditional expression, combine fields, or update a field based on the value of another field on the server-side. Tracing a parallel to the SQL update statements, for example, it wasn’t possible to do something like [&]

https://www.percona.com/blog/mongodb-modifying-documents-using-aggregation-pipelines-and-update-expressions/