README.md

State Pattern

The State pattern allows an object to change its behavior when its internal state changes. It is particularly useful for implementing state machines or managing state-dependent behavior.

Example

// State interface
class State {
  handle(context) {
    throw new Error("This method must be overridden!");
  }
}

// Concrete States
class ConcreteStateA extends State {
  handle(context) {
    console.log("State A handling request.");
    context.setState(new ConcreteStateB());
  }
}

class ConcreteStateB extends State {
  handle(context) {
    console.log("State B handling request.");
    context.setState(new ConcreteStateA());
  }
}

// Context
class Context {
  constructor(state) {
    this.setState(state);
  }

  setState(state) {
    console.log(`State changed to ${state.constructor.name}`);
    this.state = state;
  }

  request() {
    this.state.handle(this);
  }
}

Explaining the code

1. State Interface

   • The State class defines an interface for encapsulating the behavior associated with a particular state of the Context.

2. Concrete States

   • ConcreteStateA and ConcreteStateB are concrete implementations of the State interface.
   • Each concrete state implements the handle method, which performs actions specific to that state and

3. Context

   • The Context class maintains an instance of a State subclass that defines the current state.
   • The setState method changes the current state and logs the state transition.
   • The request method delegates the request to the current state's handle method.

Usage

const context = new Context(new ConcreteStateA());
context.request(); // State A handling request. State changed to ConcreteStateB
context.request(); // State B handling request. State changed to ConcreteStateA

• A Context object is created with an initial state (ConcreteStateA).
• Calling reques on the context triggers the current state's handle method, which performs an action and transitions to the next state.

Summary

This code defines a State interface and two concrete states (ConcreteStateA and ConcreteStateB). The Context class maintains the current state and delegates state-specific behavior to the current state object. The state transitions are managed within the handle methods of the concrete states, allowing the context to change its behavior dynamically.