Exploring the concepts of TypeScript interface inheritance and the powerful capabilities of generics in type inference

I'm facing a dilemma with TypeScript interfaces, generics, classes... not exactly sure which one is causing the issue. My mind is in overdrive trying to find a solution but I can't seem to simplify things. Here's what I'm grappling with : Let's say I have the following interfaces :

interface Animal {
legs: number;
}
interface Cat extends Animal { 
meouw : string;
}
interface Dog extends Animal { 
waf : stringl 
}

I want to achieve something like this :

interface GenericAnimal  { specimen : <T extends Animal> } ;
let someAnimals : GenericAnimal[] = [
{ specimen : {legs : 3, meouw : 'mrrrr'} } , 
{ specimen : {legs : 1, waf : 'hrrr' }
];

The goal is for the GenericAnimal interface to only accept 'specimen'-s that extend the Animal interface. However, when initializing a GenericAnimal instance, I should be able to access the properties of the extending interfaces through Intellisense. I've ruled out using GenericAnimal<T> because my someAnimals array would need to accommodate various 'animals' (let's say there are more than 100). Using a union type may also not be the best solution. Any suggestions? Also, is there a way to infer the type of each item in the array after destructuring it (or iterating through its members)? Thanks.

typescriptoopinterfacetypescript-generics

Answer №1

If discriminating unions are not your preference, you can opt for using typeguards:

interface Animal {
    legs: number;
}
function isAnimal(potentialAnimal: any): potentialAnimal is Animal {
    return typeof potentialAnimal === "object" && "legs" in potentialAnimal && typeof potentialAnimal.legs === "number";
}

interface Cat extends Animal {
    meouw: string;
}
function isCat(potentialCat: any): potentialCat is Cat {
    return isAnimal(potentialCat) && "meouw" in potentialCat && typeof (potentialCat as Cat).meouw === "string"
}

interface Dog extends Animal {
    waf: string;
}
function isDog(potentialDog: any): potentialDog is Dog {
    return isAnimal(potentialDog) && "waf" in potentialDog && typeof (potentialDog as Dog).waf === "string"
}



interface GenericAnimal<T extends Animal = Animal> {
    specimen: T
              & Record<string, any> // Needed to stop extraneous prop check
};
let someAnimals: GenericAnimal[] = [
    {
        specimen: {
            legs: 3,
            meouw: 'mrrrr'
        }
    },
    {
        specimen: {
            legs: 1,
            waf: 'hrrr'
        }
    }
];

someAnimals.map((genericAnimal: GenericAnimal): Animal => {
    return genericAnimal.specimen;
}).forEach(animal =>{
    if(isCat(animal)){
        console.log(animal.meouw);
    } else if(isDog(animal)) {
        console.log(animal.waf);
    }
});


if(isCat(someAnimals[0].specimen)) {
    console.log(someAnimals[0].specimen.meouw);
}

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