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Decorator

Decorator in TypeScript

Decorator is a Conceptual pattern that allows adding new behaviors to objects dynamically by placing them inside special wrapper objects.

Using decorators you can wrap objects countless number of times since both target objects and decorators follow the same interface. The resulting object will get a stacking behavior of all wrappers.

Learn more about Decorator

Usage of the pattern in TypeScript

Complexity:

Popularity:

Usage examples: The Decorator is pretty standard in TypeScript code, especially in code related to streams.

Identification: Decorator can be recognized by creational methods or constructor that accept objects of the same class or interface as a current class.

Conceptual Example

This example illustrates the structure of the Decorator design pattern. It focuses on answering these questions:

  • What classes does it consists of?
  • What roles do these classes play?
  • In what way the elements of the pattern are related?

index.ts: Conceptual Example

/**
 * The base Component interface defines operations that can be altered by
 * decorators.
 */
interface Component {
    operation(): string;
}

/**
 * Concrete Components provide default implementations of the operations. There
 * might be several variations of these classes.
 */
class ConcreteComponent implements Component {
    public operation(): string {
        return 'ConcreteComponent';
    }
}

/**
 * The base Decorator class follows the same interface as the other components.
 * The primary purpose of this class is to define the wrapping interface for all
 * concrete decorators. The default implementation of the wrapping code might
 * include a field for storing a wrapped component and the means to initialize
 * it.
 */
class Decorator implements Component {
    protected component: Component;

    constructor(component: Component) {
        this.component = component;
    }

    /**
     * The Decorator delegates all work to the wrapped component.
     */
    public operation(): string {
        return this.component.operation();
    }
}

/**
 * Concrete Decorators call the wrapped object and alter its result in some way.
 */
class ConcreteDecoratorA extends Decorator {
    /**
     * Decorators may call parent implementation of the operation, instead of
     * calling the wrapped object directly. This approach simplifies extension
     * of decorator classes.
     */
    public operation(): string {
        return `ConcreteDecoratorA(${super.operation()})`;
    }
}

/**
 * Decorators can execute their behavior either before or after the call to a
 * wrapped object.
 */
class ConcreteDecoratorB extends Decorator {
    public operation(): string {
        return `ConcreteDecoratorB(${super.operation()})`;
    }
}

/**
 * The client code works with all objects using the Component interface. This
 * way it can stay independent of the concrete classes of components it works
 * with.
 */
function clientCode(component: Component) {
    // ...

    console.log(`RESULT: ${component.operation()}`);

    // ...
}

/**
 * This way the client code can support both simple components...
 */
const simple = new ConcreteComponent();
console.log('Client: I\'ve got a simple component:');
clientCode(simple);
console.log('');

/**
 * ...as well as decorated ones.
 *
 * Note how decorators can wrap not only simple components but the other
 * decorators as well.
 */
const decorator1 = new ConcreteDecoratorA(simple);
const decorator2 = new ConcreteDecoratorB(decorator1);
console.log('Client: Now I\'ve got a decorated component:');
clientCode(decorator2);

Output.txt: Execution result

Client: I've got a simple component:
RESULT: ConcreteComponent

Client: Now I've got a decorated component:
RESULT: ConcreteDecoratorB(ConcreteDecoratorA(ConcreteComponent))

Decorator in Other Languages

Decorator in Java Decorator in C# Decorator in PHP Decorator in Python Decorator in Ruby Decorator in Swift