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Command

This pattner allows you to encapsulate a request or action as an object, decoupling the sender of the request from the receiver of the request. This can be useful when you want to parametrize objects with operations, queue or log requests, or support undo/redo.

Here is an example of the command pattern in JavaScript:

class Command {

execute() {}

undo() {}

}

class AddCommand extends Command {

constructor(receiver, value) {

super();

this.receiver = receiver;

this.value = value;

}

execute() {

this.receiver.add(this.value);

}

undo() {

this.receiver.subtract(this.value);

}

class SubtractCommand extends Command {

constructor(receiver, value) {

super();

this.receiver = receiver;

this.value = value;

}

execute() {

this.receiver.subtract(this.value);

}

undo() {

this.receiver.add(this.value);

}

class Calculator {

constructor() {

this.currentValue = 0;

}

add(value) {

this.currentValue += value;

}

subtract(value) {

this.currentValue -= value;

}

const calculator = new Calculator();

const addCommand = new AddCommand(calculator, 10);

const subtractCommand = new SubtractCommand(calculator, 5);

addCommand.execute();

console.log(calculator.currentValue); // Output: 10

subtractCommand.execute();

console.log(calculator.currentValue); // Output: 5

subtractCommand.undo();

console.log(calculator.currentValue); // Output: 10

In this example, the Command class is an abstract class that defines the structure for a command, and the AddCommand and SubtractCommand classes are concrete implementations of the Command class that add or subtract a value from a calculator. The Calculator class represents the receiver of the command, and has methods for adding and subtracting values.

To use the command pattern, we first create instances of the concrete commands and pass them the receiver and any necessary arguments.

Then, when we want to execute a command, we call the execute() method on the command object. If we want to undo the command, we can call the undo() method, which will reverse the effects of the command.