Virtuals in TypeScript

Virtuals are computed properties: you can access virtuals on hydrated Mongoose documents, but virtuals are not stored in MongoDB. Mongoose supports auto typed virtuals so you don't need to define additional typescript interface anymore but you are still able to do so.

Automatically Inferred Types:

To make mongoose able to infer virtuals type, You have to define them in schema constructor as following:

import { Schema, Model, model } from 'mongoose';

const schema = new Schema(
  {
    firstName: String,
    lastName: String,
  },
  {
    virtuals:{
      fullName:{
        get(){
          return `${this.firstName} ${this.lastName}`;
        }
        // virtual setter and options can be defined here as well.
      }
    }
  }
);

Set virtuals type manually:

You shouldn't define virtuals in your TypeScript document interface. Instead, you should define a separate interface for your virtuals, and pass this interface to Model and Schema.

For example, suppose you have a UserDoc interface, and you want to define a fullName virtual. Below is how you can define a separate UserVirtuals interface for fullName.

import { Schema, Model, model } from 'mongoose';

interface UserDoc {
  firstName: string;
  lastName: string;
}

interface UserVirtuals {
  fullName: string;
}

type UserModel = Model<UserDoc, {}, UserVirtuals>; // <-- add virtuals here...

const schema = new Schema<UserDoc, UserModel, UserVirtuals>({ // <-- and here
  firstName: String,
  lastName: String
});

schema.virtual('fullName').get(function() {
  return `${this.firstName} ${this.lastName}`;
});