Force extra property checks if object is explicitly casted or at-least give us a StrictPropertyCheck/Exact type

[nojvek]

TypeScript Version: 3.1.1

Search Terms: extra property checks, explicit cast, exact type

Code

interface Switch {
   on: boolean;
}
const a = <Switch>{on: false, blah: 123}; // no error, ¯\_(ツ)_/¯ 
const b: Switch = {on: false, blah: 123}; // error, may only specify known propery and blah is extra

Expected behavior:
Explicit cast should force extra property checks like the object literal does. I feel like TS team thinks this is intentional and may be a big breaking change.

Alternatively please give us an exact type if possible e.g StrictPropertyCheck<T> so we can do something like const b = <StrictPropertyCheck<SwitchState>>{...}

This stackoverflow thread has some magic vodoo for forcing extra property checks for functions that take arguments. https://stackoverflow.com/questions/54775790/forcing-excess-property-checking-on-variable-passed-to-typescript-function

type StrictPropertyCheck<T, TExpected, TError> = Exclude<keyof T, keyof TExpected> extends never ? {} : TError;

interface Animal {
    speciesName: string
    legCount: number,
}

function serializeBasicAnimalData<T extends Animal>(a: T & StrictPropertyCheck<T, Animal, "Only allowed properties of Animal">) {
    // something
}

Would be nice if we could have a similar mapped type that worked for objects.

Actual behavior:
No error

Playground Link:
https://typescript-play.js.org/#code/JYOwLgpgTgZghgYwgAgJIGED2BbADpkCcAHgGUw5IA+ZAbwChknmBnCyALmXMogG5GzJggIxgAcy4MhM5ABMI8AK4AbMD0gB+Lhv7IA9PrTIA7nHDzFcVevYowmZACMUbKMARgVAT2QIAFhAIANYQcshw4nCgbNx2yLhQmLjQYMAQLIKyTJB4KrzayAAUbLw6dgCUyAC8NObeArIAvlnMSrhyZcWlkACqHV0ACnBQaXAqZHZUVbXIAG6YwHICLfQI+SwsyFh4BERgk7w0wHkQ2PtbGDj4hCS6NNLMuEpOKh7IPRDlvAKtIiBuJSeTBQIpVR6yMD+YAsAB0nxqyAA8k4AFZBMCwuCbCQgIq0JoRLa6AA0yChMNh-zE4lhCmUal0FUazFWQmer3e4ggYD8ogkYLorSEUB5SigICF2Rk9OsjLsUkJ2LivBJwtZrTZbQGkBKdn6nU4yGGo2A40O1HB6qYKPRnixOPEeIpcM+ZM+Bt4zM19FW9FAkFgiBQpBMwDAAV0UuYBC4TkwmBUEHMK3oaw2xLDEf8Oxu+2QEAAHpAQHItrm9ncs5GptGmBy3ghkNzedSBVbpaKwOLJRDpZYGbYumRq-57rQDEYAOqYVThFzIEAeFDAGDk6FbEwg0LhLP+We8ou4RvhvzY3kOTDW-ux5DwFQsCBq-vZV5wfxcACMACYAMxkwxF0caAkigMkAHqAB0AH0ikAEYYKmg-RwKYABaSw4HCEQFGvIQmmfbIWjwtNfTTf5YjgRER3DGsjloW970fMk3w-ZAf1-Jo+EnICCygUCIJg+DEOQ5A1gIWInB0Uco2qOgGPGJjnHyVj2M47iQJBMlsDgXwCB8D4UgQVdfGCEBMBMSVEmSaBfHMedlOQGEC2LKA4CAA

Related Issues:
#9312, #28464

[RyanCavanaugh]

Sounds like #7481, though it's hard to tell because the example doesn't require a type assertion.

[nojvek]

I'm not sure I understand #7481

As per the typescript play example I would like someway of telling TS to give me stricter property checks that flow through form interface to concrete implementation.

i.e

interface IComponent<State> {
      state: State;
      config: {
          defaultState?: State; // some way to say I want this strict e.g StrictPropertyCheck<State>
          template?: (state: State) => any;
      }
      update: (stateUpdate: Partial<State>) => void;
}

class Component<State> implements IComponent<State> {
    public state: State;

    constructor() {
        this.state = Object.assign({} as State, this.config.defaultState);
    }

    public get config() {
        return {
            defaultState: {} as State,
        }
    }

    update(stateUpdate: Partial<State>) {
        Object.assign(this.state, stateUpdate);
    }
}


interface SwitchState {
    on: boolean;
}

class SwitchComponent extends Component<SwitchState> {
    public get config() {
        return {
            defaultState: {
                on: false,
                blah: 123, // no error, ¯\_(ツ)_/¯  - dead code - I want TS to throw error for extra properties
            },
        }
    }
}

[RyanCavanaugh]

There's already a way to do that, though - put a return type annotation on config().

#7481 covers the pros and cons of various options here

[nojvek]

Oh cool, that worked. The question is why is the type not inferred from the base interface. Why do I have to tell TS again ?

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.