Custom postgres JSON_AGG function help

class JsonAgg<
  T extends SelectedFields,
  AllowNull extends boolean = false,
> extends SQL<never> {
  private filterProp: SQL | null = null;
  private orderByProp: SQL | null = null;
  private allowNull: AllowNull;

  constructor(select: T, allowNull = false as AllowNull) {
    const builtJson = jsonBuild(select);
    super([sql`COALESCE(json_agg(${builtJson}`]);
    this.allowNull = allowNull;
  }

  orderBy(orderBy: SQL | undefined) {
    this.orderByProp = orderBy ?? null;
    return this;
  }

  filter(filterProp: SQL | undefined) {
    this.filterProp = filterProp ?? null;
    return this;
  }

  //@ts-expect-error this function is hard to override
  override as(
    alias: string,
  ): SQL.Aliased<
    AllowNull extends true
      ? SelectResultFields<T>[] | null
      : SelectResultFields<T>[]
  > {
    if (this.orderByProp) {
      this.append(sql`ORDER BY ${this.orderByProp}`);
    }
    this.append(sql`)`); 
    if (this.filterProp) {
      this.append(sql`FILTER (WHERE ${this.filterProp})`);
    }
    this.append(sql`,${this.allowNull ? sql.raw("null") : sql.raw("'[]'")})`);
    if (!alias) throw new Error("deprecated");
    return super.as(alias);
  }
}