Type of subdocument not inferred in toJSON()

[richardsimko]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

8.2.3

Node.js version

latest

MongoDB server version

latest

Typescript version (if applicable)

5.4.3

Description

Following the example for subdocuments in TypeScript (https://mongoosejs.com/docs/typescript/subdocuments.html) the type of toJSON() isn't inferred on the subdocument.

Steps to Reproduce

The setup code is an exact copy from the docs, in the end are my tests.

import { Schema, model } from 'mongoose';

import type { Model, Types } from 'mongoose';
// Subdocument definition
interface Names {
  _id: Types.ObjectId;
  firstName: string;
}
// Document definition
interface User {
  names: Names[];
}

// TMethodsAndOverrides
type UserDocumentProps = {
  names: Types.DocumentArray<Names>;
};
type UserModelType = Model<User, {}, UserDocumentProps>;

const userSchema = new Schema<User, UserModelType>(
  {
    names: [new Schema<Names>({ firstName: String })],
  },
  { timestamps: true },
);

// Create model
const UserModel = model<User, UserModelType>('User', userSchema);


const doc = await UserModel.findOne({});
const jsonDoc = doc?.toJSON();
// Type string
jsonDoc?.names[0]?.firstName;
const jsonNames = doc?.names[0]?.toJSON();
// Type any
jsonNames?.firstName;

Expected Behavior

Since jsonDoc?.names[0]?.firstName is correctly inferred I would expect doc?.names[0]?.toJSON().firstName to also be correct.

[IslandRhythms]

import { Schema, model, connect, connection } from 'mongoose';

import type { Model, Types } from 'mongoose';
// Subdocument definition
interface Names {
  _id: Types.ObjectId;
  firstName: string;
}
// Document definition
interface User {
  names: Names[];
}

// TMethodsAndOverrides
type UserDocumentProps = {
  names: Types.DocumentArray<Names>;
};
type UserModelType = Model<User, {}, UserDocumentProps>;

const userSchema = new Schema<User, UserModelType>(
  {
    names: [new Schema<Names>({ firstName: String })],
  },
  { timestamps: true },
);

// Create model
const UserModel = model<User, UserModelType>('User', userSchema);


async function run() {
  await connect('mongodb://localhost:27017');
  await connection.dropDatabase();
  await UserModel.create({
    names: [{ firstName: 'John'}]
  })
  const doc = await UserModel.findOne({});
  const jsonDoc = doc?.toJSON();
  // Type string
  console.log(jsonDoc?.names[0]?.firstName);
  const jsonNames = doc?.names[0]?.toJSON();
  // Type any
  console.log(jsonNames?.firstName);
  const name: string = jsonNames?.firstName;
  console.log('done');
}

run();

[richardsimko]

I don't follow, of course you can assign jsonNames?.firstName to name, it has type any?

If you tweak your run() function like this:

async function run() {
  await connect('mongodb://localhost:27017');
  await connection.dropDatabase();
  await UserModel.create({
    names: [{ firstName: 'John' }],
  });
  const doc = await UserModel.findOne({});
  const jsonDoc = doc?.toJSON();
  // Type 'string | undefined' is not assignable to type 'boolean'
  const test: boolean = jsonDoc?.names[0]?.firstName;
  const jsonNames = doc?.names[0]?.toJSON();
  // No error
  const name: boolean = jsonNames?.firstName;
  console.log('done');
}

You'll see there is no error even though I'm trying to assign what should be a string to a boolean. I would expect both of those assignments to have the same error.