Determine the appropriate generic type based on a sub class

I'm currently working on deducing the type of an attribute within a generic class.

For instance:

abstract class A<T> {
  attr: T;
}

class B extends A<number> {
  attr = 1;
}

type Custom = {
  value: string;
};
class C extends A<Custom> {
  value = "1";
}

const a: A<any> = new B();
const b: A<any> = new C();

const instances: A<any>[] = [a, b];

instances.forEach((instance) => {
  // In this case, I am trying to correctly define the type of attr
  const attr = instance.attr;
});

Is there a way to achieve this? It's likely that the issue stems from utilizing 'any' to specify the types for a and b.

Answer №1

Avoid using unnecessary type annotations as they may widen the type instead of narrowing it. TypeScript already has the ability to infer the narrowed type without explicit annotations:

abstract class A<T> {
  attr: T;
}

class B extends A<number> {
  attr = 1;
}

type Custom = {
  value: string;
};
class C extends A<Custom> {
  value = "1";
}

const a = new B();
const b = new C();

const instances = [a, b];

instances.forEach((instance) => {
  const attr = instance.attr; // The type is now inferred as `number | Custom`
});

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