How to reference a subclass type in an inheritied static method in TypeScript

reference-subclass-type-in-inheritied-static-method.ts

/**
 * Mason 2019-09-29: This is one way to allow subclasses to inherit a
 * method with a parameter whose type refers to the actual subclass type
 * (and not the parent class's type).
 *
 * One purpose here is to define a base class, that can have various
 * subclasses, and enable all of the subclasses to inherit a class method
 * named configure(), which creates an instance of the subclass and then
 * configures the instance and its various subclass-specific properties.
 *
 * This gets weird in TypeScript, due to polymorphic 'this' not being
 * available for static methods and constructors. This implementation was
 * derived from the comment thread at:
 *
 * https://github.com/microsoft/TypeScript/issues/5863
 *
 * (That thread also shows several of the ways that trying to do this
 * kind of thing can fail to work in TypeScript, which may be surprising.)
 *
 * This implementation uses the "fake this parameter" described here:
 *
 * https://github.com/Microsoft/TypeScript/wiki/What's-new-in-TypeScript#specifying-the-type-of-this-for-functions
 *
 * Combined with `InstanceType<T>` this allows us to capture the type in
 * such a way that the configurator function works in subclasses, and the
 * subclass properties can be auto-completed, etc.
 */
class Configurable {

  static configure<T extends typeof Configurable>(this: T, configurator: ((x: InstanceType<T>) => void)): InstanceType<T> {
    const result = (new this()) as InstanceType<T>;
    configurator(result);
    return result;
  }

}


class Person extends Configurable {

  name = "Alice";
  age  = 0;

  get bio() {
      return `My name is ${this.name}, and I am a ${this.constructor.name}. I am ${this.age} years old.`;
  }
}

class FireFighter extends Person {
  helmetSize?: number;
  hasLicenseToDriveFireEngine = false;
}

class PoliceOfficer extends Person {
  badgeNumber = "0000000";
}

const rand = Person.configure( x => {
  x.age = 117;
  x.name = "Rand";
});

const jane = PoliceOfficer.configure( x => {
  x.name = "Jane";
  x.age = 42;
  x.badgeNumber = "8675309";
});

const configureBiff = (x: FireFighter) => {
  x.hasLicenseToDriveFireEngine = true;
  x.helmetSize = 15;
  x.age = 21;
  x.name = "Biff Wigginberger Jr.";
};

const biff = FireFighter.configure(configureBiff);

console.log(rand.bio);
console.log(jane.bio);
console.log(biff.bio);

// prints:
// My name is Rand, and I am a Person. I am 117 years old.
// My name is Jane, and I am a PoliceOfficer. I am 42 years old.
// My name is Biff Wigginberger Jr., and I am a FireFighter. I am 21 years old.