Obtaining the specific property types from generic interfaces still involves an unnecessary reliance on generic types

In the scenario where I define an interface with a generic like this:

   interface I1<S> {
      a: string;
      b: genericType<S>
}

When attempting to access the type of property a using I1['a'], TypeScript raises the following error:

TS2314: Generic type 'I1<S>' requires 1 type argument(s).

Is it not correct since the extracted property type is not actually dependent on <S>? Perhaps there is a misunderstanding in my understanding of how TypeScript functions or this should indeed be acceptable.

Playground Link

typescripttypescript-generics

Answer №1

The type of property a is not dependent on S, however, you must include the type parameter as part of the lookup. Here are some possible solutions:

1.) Set S to unknown

type T1 = I1<unknown>["a"] // string

2.) Specify a default for S in the interface

interface I2<S = unknown> {
  a: string;
  b: S
}

type T2 = I2["a"] // string

3.) Keep it generic

type T3<S> = I1<S>["a"]  // T3<S> = string
// may not be very useful in this specific scenario

Check out this example

Answer №2

You must define the interface as follows:

interface SomeI<T> {
      x: string;
      y: T;
}

And specify the type of T when creating the object:

In the example below, I have assigned a string type to the generic type parameter T for the "y" property, but you can choose any type you prefer.

const myIObject: SomeI<string> = { x: "x", y: "y" };
const myIObject2: SomeI<number> = { x: "x", y: 10 };

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