CODEWITHSUNDEEP
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Consti P
Consti P

Reputation: 455

Declare object with same keys as other

I want to declare a mapping of one object to another.

The first object can have any string keys and values of a generic type.

I want to map this object with the same keys, and the types of the values can be anything (but nice would be if I could extract them from the generic). Specifically these are properties of a class and the first is passed in the constructor.

class C {
  ob1: {
    [key: string]: Wrapper<any>
  };
  ob2; // should have the same keys as ob1

  constructor(o?: { [key: string]: Wrapper<any> }) {
    this.ob1 = o;
    // map ob2 from o
  }
}

Upvotes: 0

Views: 160

Answers (1)

jcalz
jcalz

Reputation: 327624

I think the way to do this is to make C itself a generic class with a type parameter T corresponding to either the type of ob1 or ob2, and then use mapped types to express "same keys but different/related values". For example:

type MappedWrapper<T> = { [K in keyof T]: Wrapper<T[K]> }
class C<T> {
  ob1: MappedWrapper<T>
  ob2: T;
  constructor(o: MappedWrapper<T>) { // made param required here
    this.ob1 = o;
    this.ob2 = mapUnwrap(o); // some function that unwraps properties
  }
}

Here we say that T is the type of ob2, while ob1 has the type MappedWrapper<T>, a mapped type where each property of ob1 is mapped to a Wrapper-version.

Depending on the implementation and declaration of Wrapper and related types, such as:

type Wrapper<T> = { x: T };
function wrap<T>(x: T): Wrapper<T> {
  return { x };
}
function unwrap<T>(x: Wrapper<T>): T {
  return x.x;
}
function mapUnwrap<T>(x: MappedWrapper<T>): T {
  return Object.fromEntries(Object.entries(x).map(([k, v]) => [k, unwrap(v)])) as any as T;
}

you can verify that this works as expected:

const c = new C({ a: wrap(123), b: wrap("hello"), c: wrap(true) });
/* const c: C<{ a: number; b: string; c: boolean;}> */

c.ob1.a.x.toFixed(); // no error
c.ob2.b.toUpperCase(); // no error

Playground link to code

Upvotes: 1

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