Dynamic Generics in Typescript

Issue

In my DB I have different documents:

interface Dog {
    name: string;
}
interface Cat {
    character: string;
}

interface Person {
    wealth: number;
}

interface Data{
    dog:Dog,
    cat:Cat,
    person:Person,
}

And depend on the document name I want to get the document type:

const getStuffFromDb = (type:string):Data.dog|Data.cat|Data.person{
    return apiCall(type);
}

for (const item of ["dog", "cat", "person"]) {
    getStuffFromDb(item);
}

How knowing the name "dog", "cat" or "person", I can infer the type getStuffFromDb returns? What should I put instead of Data.dog|Data.cat|Data.person or how this should look like in the first place?

Data.dog|Data.cat|Data.person is a pseudocode to represent idea, hope it worked.

Solution

The type parameter should be restricted to known keys of Data, then it will be possible to lookup value type of this key:

const getStuffFromDb = <T extends keyof Data>(type: T): Data[T] => {
    return apiCall(type);
}

const foo = getStuffFromDb("cat") // foo is Cat

Playground

Answered By - Aleksey L.