InferSchemaType: Allow null for optional fields

[JavaScriptBach]

Prerequisites

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

Mongoose version

6.7.5

Node.js version

16.17.1

MongoDB server version

4.4

Typescript version (if applicable)

No response

Description

Currently optional fields in schemas accept either the field type or undefined. Can we change this to also accept null? AFAIK null should be treated the same as undefined in this case, and it makes the types more usable. In our codebase, we generally treat null and undefined the same in our types, unless we're forced to do otherwise due to explicit behavior difference such as https://mongoosejs.com/docs/migrating_to_6.html#removed-omitundefined.

i.e. we are using

type Maybe<T> = T | null | undefined

Steps to Reproduce

If I define the following schema:

const ExampleSchema = new Schema({
  example: String
});
type ExampleSchemaFields = InferSchemaType<typeof ExampleSchema>;

It infers the following type:

// type ExampleSchemaFields = {
//   example?: string | undefined;
// }

Can we change this to example?: string | null | undefined?

Expected Behavior

No response

[JavaScriptBach]

@vkarpov15 I'm happy to help implement this change if you can give me a pointer of where to edit. I tried looking in https://github.com/Automattic/mongoose/blob/master/types/inferschematype.d.ts but couldn't find how we type optional fields.

[tx46]

Yes, this is causing bugs for us. null is entirely different from undefined (trivially, null can be understood as intentionally set to nothing)