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Facade Design Pattern

Structural: Facade Pattern

What is the facade design pattern in JavaScript?​

View Answer:
Interview Response: The Facade design pattern is a software design pattern that provides a simple interface to a complex system. A facade is an object that serves as a front-facing interface masking more complex underlying code.

Technical Response: The Façade design pattern creates an interface that protects clients from complex functionality in one or more sub-systems. It's a simple pattern that may appear insignificant, but it's powerful and advantageous. We commonly find it in systems based on a multi-layer architecture.


Code Example:

class ComplexSystem {
    operationA() {
        return 'Doing complex operation A\n';
    }

    operationB() {
        return 'Doing complex operation B\n';
    }

    operationC() {
        return 'Doing complex operation C\n';
    }
}

class Facade {
    constructor(system) {
        this.system = system;
    }

    simpleOperation() {
        let result = '';
        result += this.system.operationA();
        result += this.system.operationB();
        result += this.system.operationC();
        return result;
    }
}

// Using the facade
let system = new ComplexSystem();
let facade = new Facade(system);

console.log(facade.simpleOperation()); // Runs all operations at once and masks the complexity.

In this example, ComplexSystem has multiple methods (operationA, operationB, and operationC) that can be called individually, but it may be more convenient to perform these operations all at once. The Facade class provides a method simpleOperation that does exactly that.

When a client interacts with the Facade instance (by calling simpleOperation), the facade forwards those requests to appropriate methods of the ComplexSystem. The client doesn't need to be aware of the ComplexSystem class, which encapsulates complex functionality. This simplifies the client's interaction with the system.

The Facade pattern belongs to which pattern family?​

View Answer:
Interview Response: The Facade pattern in JavaScript belongs to the Structural pattern family, which focuses on organizing objects and classes to form larger structures and functionalities.

Why use the Facade Design Pattern?​

View Answer:
Interview Response: In JavaScript, you should employ the Facade pattern when you need to provide a simpler and more unified interface to a complex system or set of objects, reducing complexity and improving usability.

Technical Response: The facade pattern makes it easier for a client to interact with a system. As a result, it gets used when an application's underlying code is large and complex, and the client does not need to see it.

It gets used in communicating with methods in a library without understanding what is happening behind the scenes. JavaScript libraries, such as jQuery, are an example.

What are the Façade Pattern's object participants?​

View Answer:
Interview Response: In the Facade pattern in JavaScript, the object participants include a Facade class, which provides a simplified interface, and a set of complex subsystem classes or objects that it hides from clients.

Technical Response: There are two types of objects represented in the Façade Pattern. They consist of the Façade and the Sub Systems (There can be multiple sub-system objects in this pattern). The facade provides a simplified interface to the complex subsystem, while the subsystem classes implement more complex functionality. The client interacts with the facade, not the subsystem.

  • Facade – The Façade understands which sub-systems are in charge of a request and routes client requests to the appropriate sub-system objects.
  • Sub Systems – A sub-system implements and executes specialized sub-system activities, but it has no cohesive knowledge or connection to the Façade itself.

Code Example:

// Subsystem classes
class SubSystemOne {
    methodOne() {
        console.log('SubSystemOne Method');
    }
}

class SubSystemTwo {
    methodTwo() {
        console.log('SubSystemTwo Method');
    }
}

class SubSystemThree {
    methodThree() {
        console.log('SubSystemThree Method');
    }
}

// Facade
class Facade {
    constructor() {
        this.one = new SubSystemOne();
        this.two = new SubSystemTwo();
        this.three = new SubSystemThree();
    }

    wrapOperation() {
        console.log('Wrap Operation Starts');
        this.one.methodOne();
        this.two.methodTwo();
        this.three.methodThree();
        console.log('Wrap Operation Ends\n');
    }
}

// Client code
const facade = new Facade();
facade.wrapOperation();

// This will output:
// Wrap Operation Starts
// SubSystemOne Method
// SubSystemTwo Method
// SubSystemThree Method
// Wrap Operation Ends

In this example, Facade is the facade, SubSystemOne, SubSystemTwo, and SubSystemThree are the subsystem classes. wrapOperation in Facade provides a simplified interface for the operations in the subsystems, and is the method that the client will call.

The subsystem classes may be complex and difficult to use directly, so the facade provides a simple interface to the complex subsystems. The subsystem classes handle tasks required by the facade, but the facade encapsulates this complexity and exposes a simplified interface to the clients. The clients only interact with the facade, not the subsystems.

What are some of the benefits of using the Facade pattern?​

View Answer:
Interview Response: Some benefits of using the Facade pattern in JavaScript include its ability to simplify complex systems for clients, improve code readability and maintainability, and reduce coupling between clients and subsystems.

What are some of the disadvantages of employing the Facade pattern?​

View Answer:
Interview Response: Some disadvantages of using the Facade pattern in JavaScript include the potential for introducing a single point of failure, limiting flexibility, and increasing the complexity of the Facade itself.

Are there any alternatives to using the facade pattern?​

View Answer:
Interview Response: Yes, there are alternative patterns that can simplify complex systems in JavaScript, such as using dependency injection, adapters, or proxies.

How does the Facade Pattern improve code readability?​

View Answer:
Interview Response: It simplifies interaction with subsystems by providing a single, easy-to-use interface.

How does the Facade Pattern affect coupling?​

View Answer:
Interview Response: It decreases coupling by isolating dependencies to a single facade, improving code maintainability.

Can the Facade Pattern increase performance?​

View Answer:
Interview Response: Indirectly, yes. It simplifies interactions and could streamline system use, potentially boosting performance.

Can multiple facades exist for a subsystem?​

View Answer:
Interview Response: Yes, it is possible to have multiple facades for a single subsystem. Each facade might provide a different simplified interface to the subsystem, depending on the context or requirements of the client code.

Code Example:

// Subsystem classes
class SubSystem {
    methodA() {
        console.log('SubSystem Method A');
    }

    methodB() {
        console.log('SubSystem Method B');
    }

    methodC() {
        console.log('SubSystem Method C');
    }
}

// First Facade
class Facade1 {
    constructor(system) {
        this.system = system;
    }

    operationX() {
        console.log('Operation X Starts');
        this.system.methodA();
        this.system.methodB();
        console.log('Operation X Ends\n');
    }
}

// Second Facade
class Facade2 {
    constructor(system) {
        this.system = system;
    }

    operationY() {
        console.log('Operation Y Starts');
        this.system.methodB();
        this.system.methodC();
        console.log('Operation Y Ends\n');
    }
}

// Client code
let system = new SubSystem();

let facade1 = new Facade1(system);
facade1.operationX();

let facade2 = new Facade2(system);
facade2.operationY();

// This will output:
// Operation X Starts
// SubSystem Method A
// SubSystem Method B
// Operation X Ends

// Operation Y Starts
// SubSystem Method B
// SubSystem Method C
// Operation Y Ends

In this example, the SubSystem has three methods, and two different facades (Facade1 and Facade2) provide different simplified interfaces to these methods. Facade1 provides operationX, which uses methodA and methodB of the SubSystem, and Facade2 provides operationY, which uses methodB and methodC.

The client code decides which facade to use based on the operations it wants to perform. The use of multiple facades allows different clients or different parts of the code to interact with the subsystem in different ways, while still abstracting the complexity of the subsystem.

What's the difference between the Facade Pattern and Adapter Pattern?​

View Answer:
Interview Response: The Facade Pattern simplifies an interface, whereas the Adapter Pattern makes incompatible interfaces compatible.

How does the Facade Pattern contribute to encapsulation?​

View Answer:
Interview Response: It encapsulates complex subsystems into a simpler interface, adhering to the principle of information hiding.

What's the role of the Facade Pattern in SOLID principles?​

View Answer:
Interview Response: It directly supports the Interface Segregation Principle, creating smaller, more specific interfaces.

Can you use the Facade Pattern with other design patterns?​

View Answer:
Interview Response: Yes, the Facade Pattern can be combined with other design patterns in JavaScript, like Factory, Singleton, or Decorator Patterns, enhancing encapsulation, instance control, and functionality extension respectively.