Partial<this> not working as expected

[felixfbecker]

I refactored the typings of sequelize to use Partial<this> to type .update(), but it is not working as expected.

TypeScript Version: 2.3.4

Code

http://www.typescriptlang.org/play/index.html#src=%0D%0Aclass%20User%20%7B%0D%0A%20%20%20%20activated%3A%20boolean%3B%0D%0A%20%20%20%20async%20activate()%3A%20Promise%3Cvoid%3E%20%7B%0D%0A%20%20%20%20%20%20%20%20await%20this.update(%7B%20activated%3A%20true%20%7D)%3B%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20async%20update(keys%3A%20Partial%3Cthis%3E)%3A%20Promise%3Cvoid%3E%20%7B%0D%0A%20%20%20%20%20%20%20%20%2F%2F%20Do%20SQL%20UPDATE%20query%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A

// A *self-contained* demonstration of the problem follows...

class User {
    activated: boolean;
    async activate(): Promise<void> {
        await this.update({ activated: true });
    }
    async update(keys: Partial<this>): Promise<void> {
        // Do SQL UPDATE query
    }
}

Expected behavior:
Should compile, as { activated: true } should satisfy Partial<User> and in turn Partial<this>

Actual behavior:

Argument of type '{ activated: true; }' is not assignable to parameter of type 'Partial<this>'.

[drew-r]

I'm suffering with this as well.

class Foo {
  constructor() {}
  x: number;
  bar() {
    // compile error - Type '{ x: number; }' is not assignable to type 'Partial<this>'.
    const y: Partial<this> = { x: 10 };
  }
}

[mhegazy]

Duplicate of #19388.

this is a generic type. it represents the type of the derived class at instantiation time. The compiler can not guarantee that type of a property on this has not been overridden by a derived class.

Seems that what you want here is Partial<Base> and not Partial<this>

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.

[felixfbecker]

@mhegazy if I used Partial<Base>, how would this work?

class Model {
  update(values: Partial<Model>): void {}
}

class User {
  firstName: string
}

const user = new User()
user.update({ firstName: 'Felix' }) // Property firstName does not exist on type Model

[mhegazy]

in this case you need Partial<this>, but setting a default in the function body is unsafe.

[felixfbecker]

Sorry, what do you mean with setting a default in the function body is unsafe?

[mhegazy]

    async activate(): Promise<void> {
        await this.update({ activated: true });  // u do not know what type `activated` will be an a more derived type
    }

[felixfbecker]

Maybe I am misunderstanding you, but any subclass should follow Liskov's substitution principle, so if the superclass declared activated: boolean, the subclass shouldn't change that type?

[mhegazy]

they are substituable for read, but not write.. the sub class are guaranteed to have a type boolean, but they can have a more specialized type, e.g. true. Please see #19388 for explanation.

[felixfbecker]

Hmm, is there any language feature that would solve this use case?

[mhegazy]

readonly/strict variance, see #18770 for more links