In Typescript, it considers `this` to be abstract due to the abstract nature of the base class, leading to the

Question

In Typescript, it considers `this` to be abstract due to the abstract nature of the base class, leading to the

I am working with a base class that is abstract and extends HTMLElement. The base class calls CustomElementRegistry.define. I have encountered an issue where Typescript does not approve of using "this" as the second argument due to the abstract nature of the base class. However, it doesn't make sense because the final class will never be abstract, and it seems that "this" belongs to MyClass in the example. To work around this, I had to resort to using a rather ugly cast. Despite this workaround, the code (in its entirety) functions correctly. Is there a more elegant solution to this problem?

P.S. I apologize for providing an incorrect example earlier, rendering the first three responses irrelevant.

abstract class CustomElement extends HTMLElement {
  constructor() {
    super();
  }
  
  static register(name: string) {
    customElements.define(
     name,
     this, // This argument triggers an error.
     {});
  }
}

class MyElement extends CustomElement {
  constructor() {
    super();
  }
}

const e = new MyElement();
MyElement.register("my-element");

typescript abstract-class

Answer 1

Answer №1

Attempting to register an instance of CustomElement with custom elements is not the correct approach. This logic should be handled at a higher level outside of the class itself.

Imagine what would occur if you were to do the following:

const a = new MyElement();
const b = new MyElement();

In this scenario, both instances would try to register and re-register themselves with the custom element registry each time they are used in markup. This behavior is not ideal.

A better practice would involve setting up your code like this:

customElements.define("custom-element", MyElement, {});
const a = new MyElement();
const b = new MyElement();

Aside from design concerns, achieving what you are attempting within a proper object-oriented paradigm is not possible.

A class does not receive a static reference to itself that can be accessed dynamically. TypeScript is correctly identifying the problem here:

customElements.define(
     "custom-element",
     this, 
// This refers to an *instance* of a class, not to a constructor. 
/// Casting will not solve the issue. You need a reference to the class you intend to register.
     {});

Answer 2

Attempting to register an instance of CustomElement with custom elements is not the correct approach. This logic should be handled at a higher level outside of the class itself.

Imagine what would occur if you were to do the following:

const a = new MyElement();
const b = new MyElement();

In this scenario, both instances would try to register and re-register themselves with the custom element registry each time they are used in markup. This behavior is not ideal.

A better practice would involve setting up your code like this:

customElements.define("custom-element", MyElement, {});
const a = new MyElement();
const b = new MyElement();

Aside from design concerns, achieving what you are attempting within a proper object-oriented paradigm is not possible.

A class does not receive a static reference to itself that can be accessed dynamically. TypeScript is correctly identifying the problem here:

customElements.define(
     "custom-element",
     this, 
// This refers to an *instance* of a class, not to a constructor. 
/// Casting will not solve the issue. You need a reference to the class you intend to register.
     {});

Answer 3

Answer №2

Despite the common belief that "the final class can never be abstract," TypeScript challenges this notion. In TypeScript, you can actually call a static member of an abstract class without any restrictions. For example:

CustomElement.register("xyz"); // allowed

You can even create a subclass of an abstract class with another abstract class and still use the inherited static method:

abstract class Foo extends CustomElement { }
Foo.register("abc"); // allowed

This explains why the compiler error occurs when 'this' is only known to be assignable to 'typeof CustomElement' and not to a concrete subclass.

To address this issue, one approach is to add a 'this' parameter to the register() method. This parameter helps restrict the type of 'this' when the method is called on an object. Here's an example:

abstract class CustomElement extends HTMLElement {
  constructor() { super(); }
  static register(this: new () => HTMLElement, name: string) {
    customElements.define(name, this, {}); // okay
  }
}

With this modification, the compiler understands that 'this' inside register() is of a type that can be assigned to 'new () => HTMLElement', which matches the type expected by customElements.define(). Now, register() compiles without errors.

Additionally, if you try to call register() on an object that is not a concrete constructor, you will receive compiler errors, ensuring type safety:

CustomElement.register("xyz"); // error!
// The 'this' context of type 'typeof CustomElement' is not assignable to method's 'this' 
// of type 'new () => HTMLElement'. Cannot assign an abstract constructor type to a 
// non-abstract constructor type.

abstract class Foo extends CustomElement { }
Foo.register("abc"); // error, same reason

However, intended calls are still permitted when the constructor is concrete:

class MyElement extends CustomElement {
  constructor() { super(); }
}    
MyElement.register("my-element"); // okay

Playground link to code

Answer 4

Despite the common belief that "the final class can never be abstract," TypeScript challenges this notion. In TypeScript, you can actually call a static member of an abstract class without any restrictions. For example:

CustomElement.register("xyz"); // allowed

You can even create a subclass of an abstract class with another abstract class and still use the inherited static method:

abstract class Foo extends CustomElement { }
Foo.register("abc"); // allowed

This explains why the compiler error occurs when 'this' is only known to be assignable to 'typeof CustomElement' and not to a concrete subclass.

To address this issue, one approach is to add a 'this' parameter to the register() method. This parameter helps restrict the type of 'this' when the method is called on an object. Here's an example:

abstract class CustomElement extends HTMLElement {
  constructor() { super(); }
  static register(this: new () => HTMLElement, name: string) {
    customElements.define(name, this, {}); // okay
  }
}

With this modification, the compiler understands that 'this' inside register() is of a type that can be assigned to 'new () => HTMLElement', which matches the type expected by customElements.define(). Now, register() compiles without errors.

Additionally, if you try to call register() on an object that is not a concrete constructor, you will receive compiler errors, ensuring type safety:

CustomElement.register("xyz"); // error!
// The 'this' context of type 'typeof CustomElement' is not assignable to method's 'this' 
// of type 'new () => HTMLElement'. Cannot assign an abstract constructor type to a 
// non-abstract constructor type.

abstract class Foo extends CustomElement { }
Foo.register("abc"); // error, same reason

However, intended calls are still permitted when the constructor is concrete:

class MyElement extends CustomElement {
  constructor() { super(); }
}    
MyElement.register("my-element"); // okay

Playground link to code

In Typescript, it considers `this` to be abstract due to the abstract nature of the base class, leading to the

Answer №1

Answer №2

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