How to create a generic conditional type in Typescript?

Question

typescript

Hello!

The problem is: Let's say I want to create a Creature type with the properties:
- type,
- subtype,
- params

And in case I declare type = 'People', the subtype must be of type: 'adult' | 'child'.
If subtype = 'child', params should be {age: number; school: string;}
If subtype = 'adult', params must be of type {height: number, weight: number, age: number}

Thus, I want to do type = 'animal', subtype = 'fish' | 'cat'. And depending on the subtype, assign the type to the params property.

What for? To then declare, for example, the creatures: Creature[] array, in which to cram objects of the same type, with different types property.
How can this be implemented in TS'e, except for the usual enumeration with such a wand: "|".
Also, I will be grateful for suggestions on how to get around this, but at the same time, so that some kind of dependent type check is saved.

Thanks in advance!

Answer 1

If type and subtype are strings, then you can do this:

type CreatureBasis = {
    People: {
        child: {
            age: number;
            school: string;
        };
        adult: {
            age: number;
            height: number;
            weight: number;
        };
    };
    Animal: {
        fish: {
            waterBody: string;
        };
        cat: {
            catchMouses: boolean;
        };
    };
};
type Creature = {
    [K0 in keyof CreatureBasis]: {
        [K1 in keyof CreatureBasis[K0]]: {
            type: K0;
            subtype: K1;
            params: CreatureBasis[K0][K1];
        };
    }[keyof CreatureBasis[K0]];
}[keyof CreatureBasis];

https://www.typescriptlang.org/play?#code/C4TwDgpg...
TypeScript was created to type any game that can be found in the dynamic world of JavaScript. As a result, it has a Turing complete type system, from which there are 2 consequences:
1. with due diligence, you can type any Wishlist
2. some especially fancy types can hang a typechecker