Why does TypeScript not complain about properties not included in the state when casting?

Question

Why does TypeScript behave the way it does in my case?

If I directly type an object, it complains about properties which are not defined in the interface. But if I cast the object it allows do add any random properties not defined in the interface.

Best explained with an example:

interface House {
  windows: number;
  garden: boolean;
}

const house1: House = {
  windows: 5,
  garden: true,
  garage: true // not allowed
};

const whatever = {
  house2: <House> {
    windows: 3,
    garden: true,
    garage: true // why is it here allowed?
  }
};

Related: stackoverflow.com/questions/32167593/…

k0pernikus
– k0pernikus

2019-06-27 15:25:22 +00:00
Commented Jun 27, 2019 at 15:25 — k0pernikus
– k0pernikus, Commented Jun 27, 2019 at 15:25

jcalz · Accepted Answer · 2023-11-09 16:12:55Z

3

It works because it is a type assertion. Basically telling the compiler what type it is but not guarding it e.g.

const house1 = <House> {
  windows: 5,
  garden: true,
  garage: true // allowed
};

Basically you tell the ts-compiler to not perform special checking or restructuring of data.

You would type guard it with the appropriate type for the properties e.g.

const whatever: { house2: House } = {
    house2: {
        windows: 3,
        garden: true,
        garage: true // not allowed
    }
};

edited Nov 9, 2023 at 16:12

jcalz

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answered Jun 27, 2019 at 12:26

Murat Karagöz

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9 Comments

k0pernikus Over a year ago

Could you please make the distinction between "type casting" and "type assertions" more clear?

Murat Karagöz Over a year ago

@k0pernikus There is no "type casting", there is not much difference between both terms. Typescript called it "type assertion" since it sounds more like the language framework. Other than that, "casting" implies that there might be a run time support in other languages, but in Typescript it is purely at compile time.

mnewmedia Over a year ago

Ok, and is there a way to cast it, that it’s not only telling the compiler what type it is, but also guards it?

Murat Karagöz Over a year ago

@mnewmedia Added.

mnewmedia Over a year ago

Ok, got it. But since in real-life app the house2 is wrapped by an object(whatever) which is in an array. And sometimes it’s a different type than a House. So the way it behaves makes it now, much more complicated than it could be.

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