Mongoose is a library used for Mongo to be able to run asynchronous functions.

Query middleware is supported for the following Model and Query functions. In query middleware functions,this refers to the query.

• count
• deleteMany
• deleteOne
• find
• findOne
• findOneAndDelete
• findOneAndRemove
• findOneAndUpdate
• remove
• update
• updateOne
• updateMany

Pre

Pre middleware functions are executed one after another, when each middleware calls next.

let schema = new Schema(..);
schema.pre('save', function(next) {
  // here is where we have it do the stuff we need
  next();
});

https://mongoosejs.com/docs/middleware.html