> For the complete documentation index, see [llms.txt](https://khaled-hamam.gitbook.io/design-patterns-explained/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://khaled-hamam.gitbook.io/design-patterns-explained/library/behavioral-patterns/state.md).

# State Pattern

Sometimes you find yourself in a [**Finite-State Machine**](https://en.wikipedia.org/wiki/Finite-state_machine) problem, where an object has a few states that can interchange at anytime, and the object behaves differently according to each state.

## Intent

Allow an object to alter its behavior when its internal state changes. The object will appear as if it changed its class.

## When to use

1. When the object's behavior depends on its state and it must change its behavior at run-time depending on the state.
2. When you have a class polluted with massive conditionals that alter how the class behaves according to the current values of the class’s fields.
3. When you have a lot of duplicate code across similar states and transitions of a condition-based state machine.

## Structure

![](https://2149973296-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MKKrm5lFe01Ro4BXbPp%2Fsync%2Fb4ac3e64d86c983277d073cffe25372d74223d60.png?generation=1603463577664228\&alt=media)

* **Context:** defines the interface of interest to the clients (usually it's directly a class that holds the state).
* **State:** defines an interface for encapsulating the behavior associated with a particular state of the Context.
* **ConcreteState:** they are the subclasses that implement the different behaviors.

## Note

Applying this pattern can be overkill if a state machine has only a few states or rarely changes.

## Examples

|                                                                          Source Code                                                                         |   UML   |
| :----------------------------------------------------------------------------------------------------------------------------------------------------------: | :-----: |
| [Example 1](https://github.com/khaled-hamam/ts-design-patterns/tree/369ec4e91fe87ca563f4d94388bceb028fa467bd/library/behavioral-patterns/state/example_1.ts) | // TODO |

You can find the tests [here](https://github.com/khaled-hamam/ts-design-patterns/tree/369ec4e91fe87ca563f4d94388bceb028fa467bd/library/behavioral-patterns/state/index.test.ts).