Modify the data type of an object member based on its original type

I am seeking to convert the data type of each member in an object based on the specific member variable.

Here is an example:

class A {
    fct = (): string => 'blabla';
}

class B {
    fct = (): number => 1;
}

class C {
    fct = (): { obj: string } => ({
        obj: 'la'
    });
}

export const Contracts = {
    a: A,
    b: B,
    c: C
};

type myContracts = typeof Contracts;

type MySuperType<T> = {
    [P in keyof T]: string;
};

type a = MySuperType<myContracts>;

The snippet provided yields:

type a = {
    a: string;
    b: string;
    c: string;
}

Desired output:

type a = {
    a: string;
    b: number;
    c: { obj: string };
}

Answer №1

Create a custom type called MySuperType that enforces the requirement for objects to have values of a specific class, where instances of this class must contain a property named fct which is a function. Obtain the individual instances by using the InstanceType utility and then extract the return type of the functions using the ReturnType utility.

type FunctionClass = {
  new(): { fct: (...args: unknown[]) => unknown}
}
type MySuperType<T extends Record<string, FunctionClass>> = {
    [P in keyof T]: ReturnType<InstanceType<T[P]>['fct']>;
};

type example = MySuperType<myContracts>;

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