Return full type T from findOne() and cursor methods instead of Partial<T>

[Copilot]

.findOne() and cursor methods were returning Partial<T> unconditionally, forcing consumers to handle potentially undefined fields even when no projection was used:

interface User {
  name: string;
  email: string;
  age: number;
}

const user = await collection.findOne({ name: 'Alice' });
// Previously: user.name was string | undefined
// Now: user.name is string

Changes

• Updated return types:

  • findOne(): Promise<Partial<T> | null> → Promise<T | null>
  • FindCursor.toArray(): Promise<Partial<T>[]> → Promise<T[]>
  • FindCursor.first(): Promise<Partial<T> | null> → Promise<T | null>

• Added JSDoc remarks explaining that projected fields may be undefined at runtime despite the type being T, accepting this trade-off for better ergonomics in the common case

Behavior with projections

When projections are used, fields are undefined at runtime but typed as T:

const user = await collection.findOne({ name: 'Alice' }, { name: 1 });
// user.email is typed as string but undefined at runtime

This matches MongoDB driver behavior and prioritizes the common non-projection case.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>.findOne returns partial?</issue_title>
<issue_description>Hi, I'm considering using this module for a local project. Is there a specific reason .findOne returns a partial of the provided type?

mongolite/src/collection.ts

Line 391 in 58c29e5

async findOne(filter: Filter<T>, projection?: Projection<T>): Promise<Partial<T> | null> {

Should it not return the full type? (null | T instead of null | Partial)
Also, when providing types to MongoLiteCollection, it seems a bit counter intuitive that the document has to satisfy DocumentWithId (which has an open signature of [key: string]: unknown) when we have a known, specific type.

Thank you for your time! Very useful project, love the mongodb-like API</issue_description>

<agent_instructions>Can you help resolve the issue raised by this user, I would agree, we should return the full type where possible</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes .findOne returns partial? #15

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[semics-tech]

@NewYanko please let me know if this change addresses your comment on #15, I'm no typescript expert so would appreciate your input!