Mastering TypeScript 3 - Third Edition

The State pattern

The GoF describe two design patterns named the State pattern, and the Mediator pattern. The State pattern uses a set of concrete classes that are derived from a base class to describe a particular state. As an example, consider creating an enum to describe the states that a door can be in. At first glance, a door could be either Open or Closed. In this case, a simple if...else control flow will likely take care of any logic we wish to apply.

Consider, however, what happens to our control flow and logic if we needed a Locked and Unlocked state, or if it were a sliding door for a SlightlyAjar, HalfOpen, AlmostFullyOpen, and FullyOpen state. The State design pattern allows us to easily define these states, and adjust our logic ...