When a class decorator is returned as a higher-order function, it is unable to access static values

Check out this showcase:

function Decorator(SampleClass: Sample) {
  console.log('Inside the decorator function');

  return function (args) {
    console.log('Inside the high order function of the decorator: ', args);
    let sample = new SampleClass();
  }
}

@Decorator
class Sample {

  a = 1;
  static b = 2;

  constructor(args) {
    console.log('Inside the constructor of Sample')
    console.log('Instance values of Sample: ', this.a);
    console.log('Static value of Sample: ', Sample.b);
  }
}

new Sample(123);

After compiling and running the code, it displays:

Inside the decorator function
Inside the high order function of the decorator:  123
Inside the constructor of Sample
Instance values of Sample:  1
Static value of Sample:  undefined

I have noticed that I can access the static value b in the high order function using SampleClass.b. However, accessing it within an instance of Sample seems to be challenging.

Answer №1

When utilizing decorators, the key concept to grasp is that by returning a value from the decorator, you essentially replace the decorated class entirely. This means that although you can successfully implement a new Foo using the returned function, it won't inherit any properties from the original Foo, rendering them unavailable on the replaced version.

To resolve this issue, one straightforward solution is to create and return an anonymous class that inherits from the class supplied to the decorator.

function Bar(FooClass: typeof Foo) {
    console.log('inside Bar decorator');
    return class extends FooClass {
        constructor(args: any) {
            super(args);
            console.log('within Bar higher-order function: ', args);
        }
    };
}

@Bar
class Foo {

    a = 1;
    static b = 2;
    constructor(args: any) {
        console.log('within Foo constructor')
        console.log('Instance of Foo with a: ', this.a);
        console.log('Static property b of Foo: ', Foo.b);
    }
}

new Foo(123);

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