Workaround for accessing class type arguments in static method in Typescript

Question

The following error

Static members cannot reference class type parameters.

results from the following piece of code

Answer 1

You could do something like this:

function Resource<T>() {
  abstract class Resource {
    /* static methods */
    public static list: T[] = [];
    public static async fetch(): Promise<T[]> {
      return null!;
    }
    /*  instance methods */
    public save(): Promise<T> {
      return null!
    }
  }
  return Resource;
}

In the above Resource is a generic function that returns a locally declared class. The returned class is not generic, so its static properties and methods have concrete types for T. You can extend it like this:

class Model extends Resource<Model>() {
  // overloading should also work
  public static async fetch(): Promise<Model[]> {
    return super.fetch();
  }
}

Everything has the types you expect:

 Model.list; // Model[]
 Model.fetch(); // Promise<Model[]>
 new Model().save(); // Promise<Model>

So that might work for you.

The only caveats I can see right now:

• There's a bit of duplication in class X extends Resource<X>() which is less than perfect, but I don't think you can get contextual typing to allow the second X to be inferred.

• Locally-declared types tend not to be exportable or used as declarations, so you might need to be careful there or come up with workarounds (e.g., export some structurally-identical or structurally-close-enough type and declare that Resource is that type?).

Anyway hope that helps. Good luck!