Declaring optional dependencies

Angular introduced the @Optional decorator, which allows us to deal with dependencies that don't have a registered provider associated with them. Let's suppose that a dependency of a provider is not available in any of the target injectors responsible for its instantiation. If we use the @Optional decorator, during the instantiation of the dependent provider a value of the missing dependency will be passed as null.

Now, let's take a look at the following example:

abstract class SortingAlgorithm {
  abstract sort(collection: BaseCollection): Collection;
}

class BaseCollection {
  getDefaultSort(): SortingAlgorithm {
    // get some generic sorting algorithm...
    return null;
  }
}

class Collection extends BaseCollection {
  public sort: SortingAlgorithm;

  constructor(sort: SortingAlgorithm) {
    super();
    this.sort = sort || this.getDefaultSort...