Why can't I have async transformers in Mongoose?

Question

Is there a reason I can't use an asynchronous transformer when defining a Mongoose schema? Here's an example:

const bookSchema = new Schema({
  title: {
    type: String,
    transform: async function(v) {
      return Promise.resolve(v)
    }
  },
});

Answer 1

Turns out this is impossible because transformers are called in toJSON and toObject which are synchronous functions. Instead, make use of middleware (not as clean, but it gets the job done).