What causes the inaccessibility of methods added via decoration?

Let's say I have a Directive decorator that adds a static method called factory to its target:

function Directive<T extends { new (...args: any[]): {} }>(constructor: T) {
  return class extends constructor {
    static factory(...args): ng.IDirectiveFactory {
      const c: any = () => {
        return constructor.apply(this, args);
      };
      c.prototype = constructor.prototype;
      return new c(...args);
    }
  };
}

I also define the type using an interface:

interface BaseDirective extends ng.IDirective {
  factory(): ng.IDirectiveFactory;
}

Now, when I declare my class like this:

@Directive
class FocusedDirective implements BaseDirective {....

Why do I receive a message saying

Class 'FocusedDirective' incorrectly implements interface 'BaseDirective'.
  Property 'factory' is missing in type 'FocusedDirective'.

Shouldn't @Directive automatically add this property for me?

Answer №1

When utilizing decorators, it is important to note that they cannot modify the class type directly. Instead, you can call your decorator as a function and save the result in a new class that will contain the desired method. This way, you can use the new class in place of the original:

const ExtendedDirectiveWithDirective = Directive(ExtendedDirective);

To streamline this process further, you have the option of using class expressions without an intermediate class:

const ExtendedDirective = Directive(class implements BaseDirective{

});

Answer №2

You are facing two distinct challenges. The first issue lies in the discrepancy between `factory` being a static method in your implementation, while it is defined as a regular method in your interface:

interface BaseDirective  {
    factory(): ng.IDirectiveFactory;
}

This misalignment can cause problems for you. To simplify the implementation process, I suggest converting the static method to a regular one.

function Directive<T extends { new(...args: any[]): {} }>(constructor: T) {
    return class extends constructor {
        factory(...args: any[]): ng.IDirectiveFactory {
            const c: any = () => {
                return constructor.apply(this, args);
            };
            c.prototype = constructor.prototype;
            return new c(...args);
        }
    };
}

The second challenge pertains to decorators not altering the class signature as you might expect. This limitation has been highlighted as an often-requested feature, with complexities hindering its implementation. An important consideration is whether the class implementation should reference the mutated type after applying the decorator. In your scenario, would the content within `{...}` have knowledge of `factory()` or not? Although most people anticipate it would, the decorator remains pending at this stage.

To circumvent this issue, it is advisable to forego using decorator syntax and instead utilize the decorator function as a conventional function to create a new class. The revised syntax appears as follows:

class FocusedDirective extends Directive(class {

    // implementation without dependency on factory
    prop: string = "hey";
    foo() {
        this.prop; // permissible
        // this.factory(); // impermissible
    }

}) implements BaseDirective {

    // implementation reliant on factory
    bar() {
        this.prop; // permissible
        this.foo(); // permissible
        this.factory(); // permissible
    }

}

This approach also addresses the concern of whether the implementation acknowledges the decorator, as evidenced by the separation of functionality inside and outside the decorator function.


Returning to the static/instance predicament, enforcing constraints on the static aspect of a class necessitates direct action with the class constructor itself. Consider the following approach:

interface BaseDirective {
  // define any relevant instance operations here
}

interface BaseDirectiveConstructor {
    new(...args: any[]): BaseDirective;
    factory(): ng.IDirectiveFactory;
}

class FocusedDirective extends Directive(class {
   // implementation excluding BaseDirectiveConstructor
}) implements BaseDirective {
    // implementation encompassing BaseDirectiveConstructor
    bar() {
        FocusedDirective.factory(); // permissible
    }
}

function ensureBaseDirectiveConstructor<T extends BaseDirectiveConstructor>(t: T): void {}
ensureBaseDirectiveConstructor(FocusedDirective);

The `ensureBaseDirectiveConstructor()` function confirms that the `FocusedDirective` class constructor features a static `factory()` method aligning with the expected type. Any deviations from this standard will prompt an error notification.


I trust this guidance proves beneficial. Best of luck with your endeavors.

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