Forcing excess-property checking on variable passed to TypeScript function

Question

Is there a way to force excess-property checking to happen, not just for an inlined object literal but one that derives from a variable?

For example, suppose I have an interface and a function

interface Animal {
    speciesName: string
    legCount: number,
}

function serializeBasicAnimalData(a: Animal) {
    // something
}

If I call 

serializeBasicAnimalData({
    legCount: 65,
    speciesName: "weird 65-legged animal",
    specialPowers: "Devours plastic"
})

I would get an error -- which for my situation, is what I want.  I only want the function to take in a generic animal description without extra specifics.

On the other hand, if I first create a variable for it, I don't get the error:

var weirdAnimal = {
    legCount: 65,
    speciesName: "weird 65-legged animal",
    specialPowers: "Devours plastic"
};
serializeBasicAnimalData(weirdAnimal);

So my question is: Is there a way to somehow force TypeScript to apply "excess property checking" to the function parameter irrespective of whether it's an inlined object or an object that has previously been assigned to a variable?

 

Answer 1

Hope this helps, this will cause it to fail. The underlying cause here is Typescripts reliance on structural typing which is alot better than the alternative which is Nominal typing but still has its problems.

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
}

var weirdAnimal = {
    legCount: 65,
    speciesName: "weird 65-legged animal",
    specialPowers: "Devours plastic"
};
serializeBasicAnimalData(weirdAnimal); // now correctly fails

Answer 2

I needed this to enforce an object shape in Redux.

I have combined the answer in this great article and Shannon's answer in this thread here. I think this gives a tiny bit more concise way to implement this:

export type StrictPropertyCheck<T, TExpected, TError> = T extends TExpected
  ? Exclude<keyof T, keyof TExpected> extends never
    ? T
    : TError
  : TExpected

Here ^^ I put the T extends TExpected in the StrictPropertyCheck, that is all the difference actually but I thought linking the article above would help others landing on this thread.

Usage for redux action creator:

export type Credentials = {
  email: string
  password: string
}

export type StrictCreds<T> = T &
  StrictPropertyCheck<
    T,
    Credentials,
    'ERROR: THERE ARE EXCESS PROPERTIES IN CREDENTIALS OBJECT'
  >

export type AuthActionType =
  | {
      type: AuthAction.LOGIN
      payload: StrictCreds<Credentials>
    }
  | { type: AuthAction.LOGOUT 

export const login = <T>(credentials: StrictCreds<T>): AuthActionType => {
  return {
    type: AuthAction.LOGIN,
    payload: credentials as Credentials,
  }
}