Issue customizing static method of a subclass from base class

Question

Issue customizing static method of a subclass from base class

Let me illustrate a simplified example of my current code:

enum Type {A, B}

class Base<T extends Type> {
  constructor(public type: T) {}

  static create<T extends Type>(type: T): Base<T> {
    return new Base(type);
  }
}

class A extends Base<Type.A> {
  static override create(): A {
    return super.create(Type.A);
  }
}

However, I am encountering an error:

Class static side 'typeof A' incorrectly extends base class static side 'typeof Base'.
  The types returned by 'create(...)' are incompatible between these types.
    Type 'A' is not assignable to type 'Base<T>'.
      Type 'Type.A' is not assignable to type 'T'.
        'Type.A' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Type'. ts(2417)

Why is this happening? I am confused about how "

'T' could be instantiated with a different subtype of constraint 'Type'

" when I have clearly extended from Base<Type.A>. How can I resolve this issue and create specialized versions of my base class?

typescript inheritance static

Answer 1

Answer №1

There are several issues at play here:

The child class uses value instead of type.

javascript class Base<T extends Type>

requires providing T that extends Type, yet you have provided a value

javascript class A extends Base<Type.A>

In the realm of Generics, only type can be specified, not value/instance. For example:

class Animal {}
class Cat extends Animal {}

class AnimalInfo<T extends Animal> {}

const catInfo = new AnimalInfo<Cat>(); //valid - type is provided

const catInstane = new Cat();
const anotherCatInfo = new AnimalInfo<catInstane>(); // invalid

An allowable albeit less practical scenario could be demonstrated in the following way:

enum Type {
  A,
  B,
}

enum AnotherType {
  C,
}

type CombinedEnum = Type & AnotherType;

class Base<T extends Type> {
  constructor(public type: T) {}

  static create<T extends Type>(type: T): Base<T> {
    return new Base(type);
  }
}

class A extends Base<CombinedEnum> {
}

Static methods cannot be overridden; only instance methods can be.

javascript static override create() ...

is an improper syntax.

super has no relevance in a static method as it pertains to instances rather than static class implementations.

  static method(): Something {
    return super.method();
  }

A correct approach would look like this:


class BaseClassType {
  static method(): Something {
   //...
  }
}

class AnotherClass {
  static method(): Something {
    return BaseClassType.method();
  }
}

Answer 2

There are several issues at play here:

The child class uses value instead of type.

javascript class Base<T extends Type>

requires providing T that extends Type, yet you have provided a value

javascript class A extends Base<Type.A>

In the realm of Generics, only type can be specified, not value/instance. For example:

class Animal {}
class Cat extends Animal {}

class AnimalInfo<T extends Animal> {}

const catInfo = new AnimalInfo<Cat>(); //valid - type is provided

const catInstane = new Cat();
const anotherCatInfo = new AnimalInfo<catInstane>(); // invalid

An allowable albeit less practical scenario could be demonstrated in the following way:

enum Type {
  A,
  B,
}

enum AnotherType {
  C,
}

type CombinedEnum = Type & AnotherType;

class Base<T extends Type> {
  constructor(public type: T) {}

  static create<T extends Type>(type: T): Base<T> {
    return new Base(type);
  }
}

class A extends Base<CombinedEnum> {
}

Static methods cannot be overridden; only instance methods can be.

javascript static override create() ...

is an improper syntax.

super has no relevance in a static method as it pertains to instances rather than static class implementations.

  static method(): Something {
    return super.method();
  }

A correct approach would look like this:


class BaseClassType {
  static method(): Something {
   //...
  }
}

class AnotherClass {
  static method(): Something {
    return BaseClassType.method();
  }
}

Issue customizing static method of a subclass from base class

Answer №1

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