Why does array.map convert class instance to object?

I have an array called Users[] which holds class instances of User. I create this array by mapping over query results like so:

function GetUserByUserData(data : UserData[] | ReadonlyArray<UserData>) : User[]
{
    return data.map((userData) => {
        return new User(UserData);
    });
}

Note: UserData is an object of type {id: number; [key: string]: any;}.
Then, I use a method to create rows based on some class properties like so:

//User class
export default class User
{
    private _data: UserData;
    get data()
    {
        return this._data;
    }
}

let rows = Users.map((user) => makeRowFromUser(user));

function makeRowFromUser(user: User)
{
    console.log(user); //logs {_data}
    console.log(user.data); //undefined
    console.log(user._data); //logs data normally
}

I realize since _data is private I shouldn’t be able to access it using user._data.

Why does this happen? Does array.map destructure the Users[] array? Does the user argument get destructured before being passed to the function? And since this is unintended behavior on my part, is there a way to prevent this from happening so I can pass the User class instance as a class instance?