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typescriptgenericstypescript-genericstype-constraints
Victor Mukherjee
Victor Mukherjee

Reputation: 11025

Typescript generic type constraint from array of types

I have an array of types declared as

const APIS = [NewsApi, CommentsApi, FileApi];

I want to have a method that accepts one of these types from that APIS array, like the following:

myMethod<T extends anyof APIS>(apiType: T): T {
    const api = new apiType(); //pseudo code, does not work off course
    // some common configuration code
    return api;
}

. I cannot do myMethod<T extends NewsApi|CommentsApi>(): T, as the APIS array is auto generated. Is this possible with typescript?

Update:

Aleksey's solution works great for passing in the input. However, the return type cannot be inferred from the usage.

const myApi = myMethod(NewsApi);

In above case, myApi is not of type NewsApi, but NewsApi|CommentsApi|FileApi. Is it possible to set the return type explicitly to instance of input api type?

Upvotes: 0

Views: 1424

Answers (1)

Aleksey L.
Aleksey L.

Reputation: 37938

If you can change definition of the array to readonly tuple:

const APIS = [NewsApi, CommentsApi, FileApi] as const;

then you can get union of possible array values using lookup type:

type ApiConstructors = typeof APIS[number];

function myMethod<T extends InstanceType<ApiConstructors>>(apiType: new () => T): T {
  return new apiType();
}

const api = myMethod(NewsApi); // api is of type NewsApi

InstanceType utility used to get instance type of a constructor function.

Playground

Upvotes: 3

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