CODEWITHSUNDEEP
typescripttypescript-genericstypescript3.0
CShark
CShark

Reputation: 1622

In TypeScript, how can i get a generic type parameter to be inferred in a method that has multiple type parameters?

I'm trying to get the type of T3 to be inferenced properly in the example below, but it is just being shown as unknown. I can't understand how it can properly infer the base type T2 when calling Testing.testIt, but not the type parameter for the class it extends (T3). Here is the running example. I was trying to figure out if the infer keyword would be useful here, but I can't see how that would fit into this.

export interface Type<T> extends Function {
    new (...args: any[]): T;
}

class GenericClass<T> {
}

class ChildClass extends GenericClass<string> {

}

class Testing {
    static testIt<T3, T2 extends GenericClass<T3>>(testClass: Type<T2>): T3 {
      console.log('testIt called');
      return '' as any;
    }

    static testIt2(val: string): void {
      console.log(val);
    }
}

const result = Testing.testIt(ChildClass);
Testing.testIt2(result);

Upvotes: 1

Views: 54

Answers (1)

Jeff Mercado
Jeff Mercado

Reputation: 134841

You'll want to define a helper to extract the type:

type ExtractGenericClassType<T> = T extends GenericClass<infer TInner> ? TInner : never;

class Testing {
    static testIt<T extends GenericClass<any>>(testClass: Type<T>): ExtractGenericClassType<T> {
      ...
    }
}

Moreover, you need to ensure that your generic class actually uses the generic type parameter, otherwise this will not work.

Upvotes: 2

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