Adding methods to es6 child class

I would like to make a new class that derives from Array that has its own methods in addition to the regular array methods.

export class ObjectCollection<T extends Iidentifier> extends Array<T> {
  constructor(collection: T[] = []) {
    super(...collection);
  }
  // used for getting instance from a collection of objects
  public getInstance(id: Id) {
    return this.find(item => item.Id.toString() === id.toString()) || new 
    Array<T>();
  }
}

I have also have child classes that inherit from that class.

export class TargetCategories extends ObjectCollection<TargetCategory> {
  constructor(categories: TargetCategory[] = []) {
    super(categories);
  }
  public getCategoryType(id: Iidentifier): string {
    return this[id].Category.length < 2 ? "dummy" : "saved";
  }
}

When I instantiate a TargetCategories object, I'm only able to use methods that are in array though. I'm not able to call things like getCategoryType and getInstance.