Javascript Design Patterns interview questions for experienced/Javascript Design Patterns Interview Questions and Answers for Freshers & Experienced

What Is Behavioural Design Pattern?

The most of these design patterns are specifically concerned with communication between objects or classes.
We have 11th types of Design Patterns i.e.
1. Chain of responsibility
2. Command
3. Interpreter
4. Iterator
5. Mediator
6. Memento
7. Observer
8. State
9. Strategy
10. Template method
11. Visitor

Posted Date:- 2021-10-09 07:45:26

What Is Creational Design Pattern?

The creational patterns are ones that create objects for you, rather than having you instantiate objects directly. This pattern gives us more flexibility in deciding which objects need to be created for a given case.

We have five types of Design Patterns i.e.
1. Singleton
2. Factory
3. Abstract Factory
4. Prototype
5. Builder

Posted Date:- 2021-10-09 07:44:45

What Is MVC Pattern?

The Model View Controller (MVC) is a pattern used in software engineering. It is not a design pattern. It is a pattern only. The MVC is most popular pattern now.

These patterns separate the applications structure into three parts that are model, view and controller.

1. Model – The model is responsible for the data access layer and it responds to the requested view and controllers.
2. View – The view is the presentation layer of the application and also called users interface.
3. Controller – The controller is the business logic layer for the application and the controller is responsible for responding to user input and performs interactions on the data model objects and also controls to the models and view and controller work between the view and model.

Posted Date:- 2021-10-09 07:40:43

What are the types of participants of the prototype design pattern you will get?

The following are some of the types of prototype design pattern you will get and they are:

* Prototype registry: this type of registry, services have all the prototypes accessible using all the simple parameters.

* Clients: clients are the one who will be responsible for using the prototype design pattern.

Posted Date:- 2021-10-09 07:39:06

What are the types of the proxy design you will get in the design pattern?

The following are some of the types of the proxy design you will get in the design pattern they are:

Remote Proxy
Virtual proxy
Protection Proxy
Smart proxy

Posted Date:- 2021-10-09 07:37:55

What are the most important Software design patterns?

The following are some of the most important software design patterns:

* Singleton
* Factory method
* Strategy
* Observer
* Builder
* Adapter
* State

Posted Date:- 2021-10-09 07:36:32

What are the languages used in the design pattern?

The following are the languages used in the design pattern:


Posted Date:- 2021-10-09 07:35:00

Mention what is the difference between VO and JDO?

The difference between VO and JDO is that the JDO is a persistent technology that competes against entity beans. It allows to create POJO (plain old java objects) and persevere them to the database.

Posted Date:- 2021-10-09 07:28:38

Explain the Intercepting Filter Design Pattern and also mention its benefits?

The intercepting filter design pattern is used to intercept and manipulate a request and response before and after the request processing. Filters perform the authentication/ authorization/ logging or tracking of request and then forward the requests to corresponding handlers. Let's have a look at some basic entities of Intercepting design pattern.


It performs a certain task before or after the execution of request by request handler.

Filter Chain

It contains multiple filters and helps to execute them in defined order on target.


The target object is the request handler

Filter Manager

It manages the filters and Filter Chain.


The client object is one who sends a request to the Target object.

Benefits of Intercepting Filter Design Pattern

* Filter pattern provides central control with loosely coupled handlers.
* It expands reusability.
* The new filter can be added at any time without affecting the client's code.
* Filters can be selected dynamically during program execution.

Posted Date:- 2021-10-09 07:27:53

What are the MVC patterns?

This pattern is one of the most-used patterns from J2EE Design pattern category. It is quite similar to the concept of Model-View-Controller. The abbreviation MVC is taken from the Model-view-controller concept.

Models are objects, used as blueprints for all of the objects that will be used in the application.

Views contain the presentational aspect of the data and information located in the models.

Controllers control both model and view as they serve as a connection between the two objects. The controller plays the role of an interface between View and Model and also intercepts all the incoming requests.

Posted Date:- 2021-10-09 07:26:49

What's the difference between the Dependency Injection and Service Locator patterns?

The service locator is used to create class dependencies. The Class is still responsible for creating its dependencies no matter whether if it is using service locator or not.

Service locators are also used to hide dependencies. We can't say by looking at an object whether it connects with a database or not when it obtains connections from a locator.

With Dependency injection, the class which contains its dependencies neither knows nor cares where they came from.

One significant difference is that Dependency injection is much easier to unit test because we can pass in it mock implementations of its dependent objects. We could combine the two objects and apply the service locator.

Posted Date:- 2021-10-09 07:25:59

How is Bridge pattern is different from the Adapter pattern?

The motive of the Adapter pattern is to make interfaces of one or more classes to look similar.

The Bridge pattern is designed to isolate a class's interface from its implementation so we can vary or substitute the implementation without changing the client code.

Posted Date:- 2021-10-09 07:25:28

Explain some different type of proxies?

There are many cases where the proxy pattern is beneficial. Let's have a look at some different proxies.

Protection proxy

It controls access to the real subject based on some condition.

Virtual proxies

Virtual proxies are used to instantiate the expensive object. The proxy manages the lifetime of the real subject in the implementation.

It decides the need for the instance creation and when to reuse it. Virtual proxies optimize performance.

Caching proxies

Caching proxies are used to cache expensive calls to the real subject. There are many caching strategies that the proxy can use.

Some of them are read-through, write-through, cache-aside, and time-based. The caching proxies are used for enhancing performance.

Remote proxies

Remote proxies are used in distributed object communication. The remote proxy causes execution on the remote object by invoking a local object method.

Smart proxies

Smart proxies are used to implement log calls and reference counting to the object.

Posted Date:- 2021-10-09 07:24:59

What is the proxy pattern, and what does it do?

The term Proxy stands for an object representing another object. The proxy pattern provides a substitute or placeholder for another purpose to control access to it.

According to Gangs of four, a Proxy Pattern "provides control for accessing the original object."

We can perform many security operations like hiding the information of the original object, on-demand loading, etc.

It is also called as placeholder or surrogates.

Posted Date:- 2021-10-09 07:24:25

Is it possible to create a clone of a singleton object?

Yes, it is possible to create a clone of a singleton object.

Posted Date:- 2021-10-09 07:23:47

Can you write Thread-safe Singleton in Java?

There are many ways to write a Thread-safe singleton in Java.

* Thread-safe Singleton can be written by writing singleton using double-checked locking.

* Another way is, by using static Singleton instance initialized during class loading.

* By using Java enum to create a thread-safe singleton, this is the most straightforward way.

Posted Date:- 2021-10-09 07:23:03

Mention advantage of Builder design pattern in Java?

Advantages of builder design patterns are as follows.

* It facilitates with a clear separation between the construction and representation of an object.
* It provides improved control over the construction process.
* The constructor parameter is reduced and is provided in highly readable method calls.
* In design Pattern, the object is always instantiated in a complete state.
* In the Builder design pattern, Immutable objects can be quickly built in the object building process.

Posted Date:- 2021-10-09 07:22:08

What are Some Design Patterns which are used in the JDK library?

Some of the design patterns which are used in the JDK library are as follows.

* The decorator pattern is used by Wrapper classes.
* Singleton pattern is used by Calendar classes (Runtime).
* The Wrapper classes use factory pattern like Integer.valueOf.
* Event handling frameworks use observer pattern like swing, awt

Posted Date:- 2021-10-09 07:17:40

Can you describe the uses of the composite pattern?

It is used in the following cases:

When we want to represent a partial or full hierarchy of objects.
In case we need to add the responsibilities dynamically to the individual object without affecting other objects.

Posted Date:- 2021-10-09 07:15:11

What are the advantages of Composite design Pattern in Java?

Composite design pattern allows clients to operate collectively on objects that may or may not represent a hierarchy of objects.

Advantage of composite design patterns is as follows.

It describes the class hierarchy that contains primitive and complex objects.
It makes it easy to add new kinds of the component.
It facilitates with the flexibility of structure with a manageable class or interface.

Posted Date:- 2021-10-09 07:14:43

Difference between Strategy and State design Pattern in Java?

This question is a commonly asked Java design pattern interview question as both Strategy and State pattern has the same structure. The UML class diagram of both patterns looks precisely the same, but their intent is different.

The state design pattern is used to manage and define the state of an object, while the Strategy pattern is used to describe a set of an interchangeable algorithm.

Posted Date:- 2021-10-09 07:13:59

What is the decorator pattern in Java explain it with an example?

The decorator pattern is one of the popular Java design patterns. It is common because of its heavy usage in (package). The Decorator Pattern uses composition in place of inheritance to extend the functionality of an object at runtime.

BufferedReader and BufferedWriter are some excellent examples of decorator pattern in Java.

Posted Date:- 2021-10-09 07:13:34

Illustrate the uses of Adapter Patterns?

It is used in the following cases:

When an object requires to utilize an existing class with an incompatible interface.
In case we want to create a reusable class that collaborates with classes which don't have compatible interfaces.

Posted Date:- 2021-10-09 07:13:10

What are the Adapter patterns?

Adapter pattern converts the interface of a class into another interface based on the requirement.

In other words, it let you convert the interface according to requirement while using the class service with a different interface.

It is also known as Wrapper.

Posted Date:- 2021-10-09 07:12:44

Explain the Singleton pattern?

Singleton pattern in Java is a pattern which allows a single instance within an application. One good example of the singleton pattern is java.lang.Runtime.

Singleton Pattern states that define a class that has only one instance and provides a global point of access to it.

In other words, it is the responsibility of the class that only a single instance should be created, and all other classes can use a single object.

Posted Date:- 2021-10-09 07:12:05

Explain Structural Patterns in Java?

Structural patterns are used to provide solutions and efficient standards regarding class compositions and object structures. They depend on the concept of inheritance and interfaces to allow multiple objects or classes to work together and form a single working whole.

Structural design patterns are responsible for how classes and objects can be composed to form larger structures.

Posted Date:- 2021-10-09 07:11:06

What Is Factory Pattern?

It is the most used design pattern in Java.
These design patterns belong to the Creational Pattern as this pattern provides one of the best ways to create an object.
In the Factory pattern, we don't expose the creation logic to the client and refer the created object using a standard interface.
Factory Pattern allows the sub-classes to choose the type of objects to create.
The Factory Pattern is also known as Virtual Constructor.

Posted Date:- 2021-10-09 07:10:22

What Is Gang of Four (GOF)?

In 1994, four authors Erich Gamma, Ralph Johnson, Richard Hel, and John Vlissides published a book titled Design Patterns Elements of Reusable Object-Oriented Software. This book introduced the concept of Design Pattern in Software development.

These four authors are known as Gang of Four GOF.

Posted Date:- 2021-10-09 07:09:36

Mention what is the difference between VO and JDO?

The difference between JDO and VO is that the JDO is a persistent technology that compete against entity beans in enterprise application development. It enables you to create POJO (plain old java objects) and persist them to the database.

Posted Date:- 2021-10-09 07:09:11

Mention what is the limitation of using singleton pattern?

The singleton pattern ensures that a class has only one instance and to provide a global point of access to it. But at the same time this becomes its limitation as most classes in an application you will need to create multiple instances.

Posted Date:- 2021-10-09 07:08:51

Mention what is the difference between “throw” and “throws”?

Keyword “Throw” is used to explicitly throw as an exception, while “Throws” is utilized to handle checked exceptions for re-intimating the compiler that exceptions are being handled. The throws need to be used in the method’s definition and also while invoking the method that raises checked exceptions.

Posted Date:- 2021-10-09 07:08:27

Explain how can you prevent creating another instance of singleton using clone() method?

The preferred way to prevent creating another instance of a singleton is by not implementing Cloneable interface and if you do just throw an exception from clone() method “ not to create a clone of singleton class”.

Posted Date:- 2021-10-09 07:08:09

Mention in how many ways can you write singleton class in Java?

One can write singleton class in Java in four ways

Singleton with public static final field initialized during class loading
Singleton generated by static nested class, also referred as singleton holder pattern
Singleton by synchronizing get instance () method
From Java 5 on-wards using Enums

Posted Date:- 2021-10-09 07:07:54

Explain in singleton pattern whether it is better to make the whole getinstance() method synchronized or just critical section is enough? Which one is preferable?

Synchronization of whole getinstance() method is costly and is only needed during the initialization on singleton instance, to stop creating another instance of Singleton. Therefore it is better to only synchronize critical section and not the whole method.

Posted Date:- 2021-10-09 07:07:29

Mention when to use a Factory Pattern?

Factory pattern can be used,

When a class does not know which class of objects needs to create
When class specifies its sub-classes to specify which objects to create
In programming language, you can use factory pattern where you have to create an object of any one of sub-classes depending on the given data

Posted Date:- 2021-10-09 07:06:08

Mention when can you use the Intercepting pattern?

Intercepting pattern is used when you have to do some pre-processing or post processing with request or response of the application.

Posted Date:- 2021-10-09 07:05:51

Mention which pattern is useful when one has to pass data with multiple attributes in one shot from client to server?

Transfer Object Pattern is useful when one has to pass data with multiple attributes in one shot from client to the server.

Posted Date:- 2021-10-09 06:57:36

Mention why access to the non-static variable is not allowed from static method in Java?

You cannot access non-static data from static context because non-static va

Posted Date:- 2021-10-09 06:57:22

Mention how one should describe a design pattern?

To describe a design pattern, following things need to be taken care of

Pattern name and classification
Problem and solution
Consequences : Variation and language dependent alternatives should also be addressed
Know Uses: Identify the uses in the real systems and its efficiency

Posted Date:- 2021-10-09 06:53:55

Is it possible to write thread safe singleton in Java?

To write thread safe singleton in Java there are multiple ways for example by using static singleton instance initialized during class loading, by writing singleton using double checked locking. Java Enum is the simplest way to create thread safe singleton.

Posted Date:- 2021-10-09 06:53:25

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