Is it possible to restrict typescript object to contain only properties defined by its class?

Question

Here is my code

async getAll(): Promise {
    return await dbQuery(); // dbQuery returns User[]
}

class User {
    id: number;
    na         


        

Answer 1

Typescript uses structural typing instead of nominal typing to determine type equality. This means that a type definition is really just the "shape" of a object of that type. It also means that any types which shares a subset of another type's "shape" is implicitly a subclass of that type.

In your example, because a User has all of the properties of GetAllUserData, User is implicitly a subtype of GetAllUserData.

To solve this problem, you can add a dummy property specifically to make your two classes different from one another. This type of property is called a discriminator. (Search for discriminated union here).

Your code might look like this. The name of the discriminator property is not important. Doing this will produce a type check error like you want.

async function getAll(): Promise {
  return await dbQuery(); // dbQuery returns User[]
}

class User {
  discriminator: 'User';
  id: number;
  name: string;
}

class GetAllUserData {
  discriminator: 'GetAllUserData';
  id: number;
}