在 TypeScript 中，如何在具有多个类型参数的方法中获取泛型类型参数？

Question

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);

Answer 1

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.