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State

This pattern allows an object to alter its behavior when its internal state changes. This can be useful when you want to change the behavior of an object based on its current state, and when you want to abstract the details of the state-specific behavior from the object itself.

Here is an example of the state pattern in JavaScript:

class State {

constructor(color) {

this.color = color;

}

handle() {

console.log(`Turning light ${this.color}`);

}

class Context {

constructor() {

this.state = null;

}

setState(state) {

this.state = state;

}

request() {

this.state.handle();

}

const context = new Context();

const redState = new State('red');

const greenState = new State('green');

context.setState(redState);

context.request();

// Output: 'Turning light red'

context.setState(greenState);

context.request();

// Output: 'Turning light green'

In this example, the State class is a class that represents a state of the context object, and the Context class is a class that represents the object whose behavior changes based on its internal state. The State class has a handle() method that performs some action based on the state, and the Context class has a setState() method for setting the current state of the object and a request() method for triggering the action based on the current state.

To use the state pattern, we create instances of the State class to represent the different states of the context object, and then use the setState() and request() methods of the Context class to set the current state and trigger the action based on that state. This allows the context object to alter its behavior based on its internal state, without the client having to know the details of the state-specific behavior.