allowDeclareFields example and commentary

allowDeclareFields.ts

// The setup: I have an Entity class hierarchy for server/common/client, and each entity has
//   a reference to an entity_manager, which should be templated to the appropriate type so
//   that an entity getting other entities from the manager always get the correct type of
//   entity back.

// The common code:
interface EntityManager<Entity extends EntityCommon = EntityCommon> {
  entities: Partial<Record<number, Entity>>;
}

class EntityCommon {
  manager: EntityManager;
  constructor(manager: EntityManager) {
    this.manager = manager;
  }
}

// Then, how do we declare our server entity such that `manager` is the correct type?

// Under current settings, I used a "definite assignment assertion operator" to avoid the error about
//   "not assigned definitively in the constructor":
class EntityServer extends EntityCommon {
  manager!: EntityManager<EntityServer>;

  constructor(manager: EntityManager<EntityServer>) {
    super(manager);
  }
}

// But, seems the more correct approach is to enable allowDeclareFields, and just
//   declare the type of the field:
class EntityServer extends EntityCommon {
  declare manager: EntityManager<EntityServer>;

  constructor(manager: EntityManager<EntityServer>) {
    super(manager);
  }
}


// However, code generation changes with this option enabled... the "current"
//   code above generates this beautiful code:
function EntityCommon(manager) {
	this.manager = manager;
}
function EntityServer(manager) {
	EntityCommon.call(this, manager);
}

// With allowDeclareFields on, this code would now, unfortunately, generate this,
//   which is both longer, and functions differently (however, it more closely
//   matches how ES class inheritance works, indicating the "current" code is
//   just "wrong").
function EntityCommon(manager) {
	this.manager = undefined;
	this.manager = manager;
}
function EntityServer(manager) {
	EntityCommon.call(this, manager);
	this.manager = undefined; // !!! Overwriting the desired value!
}

// Adjusting to use `declare`, the child class's field gets stripped entirely
//   by typescript, and then generates this correct, if wordy, code:
function EntityCommon(manager) {
	this.manager = undefined;
	this.manager = manager;
}
function EntityServer(manager) {
	EntityCommon.call(this, manager);
}