20 JavaScript Object-Oriented Programming Interview Questions and Answers

If you’re interviewing for a position that involves JavaScript programming, you may be asked questions about object-oriented programming (OOP). OOP is a programming paradigm that uses objects and their relationships to each other to design and write programs. It’s a popular approach to programming, and knowing how to answer questions about it can help you impress the interviewer and land the job. In this article, we’ll discuss some common JavaScript OOP interview questions and how to answer them.

JavaScript Object-Oriented Programming Interview Questions and Answers

Here are 20 commonly asked JavaScript Object-Oriented Programming interview questions and answers to prepare you for your interview:

1. What is object-oriented programming?

Object-oriented programming is a programming paradigm that is based on the concept of objects. These objects can contain data, as well as code that is executed in order to manipulate that data. Objects can also be passed around as arguments to functions, or be returned from functions, allowing for a great deal of flexibility and code reuse.

2. Can you give me a brief overview of the different features of OOP?

The main features of OOP are encapsulation, inheritance, and polymorphism. Encapsulation is the process of hiding data and methods inside of an object so that they can only be accessed by other code inside the object. Inheritance is the process of creating a new object that inherits the properties of an existing object. Polymorphism is the ability of an object to take on different forms depending on the context in which it is used.

3. How would you describe encapsulation in JavaScript?

Encapsulation in JavaScript refers to the idea of wrapping up data and functionality into a single unit, usually an object. This allows for greater control over how that data and functionality is used and accessed, as well as a way to hide implementation details from outside code.

4. How would you describe inheritance in JavaScript?

Inheritance in JavaScript is when one object is able to inherit the properties and methods of another object. This is typically done by using the prototype property of an object to point to another object, which will then be used as the blueprint for the first object.

5. How do you use composition to achieve code reuse in JavaScript?

Composition is a way of combining small pieces of code to create larger pieces of code. In JavaScript, this can be done by using functions to create new objects. For example, if you have a function that creates a new person object, you can use that function to create a new employee object by adding properties and methods that are specific to employees. This allows you to reuse the code that creates the person object, and only write the code that is specific to employees once.

6. What’s the difference between prototypical and class-based inheritance in JavaScript?

The main difference between prototypical and class-based inheritance is that in class-based inheritance, classes are used as templates to create objects, while in prototypical inheritance, objects are used as templates to create other objects. Class-based inheritance is more like a traditional inheritance model, where objects are created by instantiating classes. In prototypical inheritance, objects are created by cloning other objects.

7. Is it possible to instantiate “abstract” classes in JavaScript? If not, why?

No, it is not possible to instantiate “abstract” classes in JavaScript. The reason for this is that abstract classes are meant to be extended, and therefore cannot be used on their own.

8. Do objects in JavaScript have private fields or methods? If yes, how can they be accessed?

No, JavaScript objects do not have private fields or methods. However, you can create a closure around an object which will give you some level of privacy.

9. Why is it important to avoid using global variables in JavaScript?

Global variables are variables that are available throughout the entire program, and as such, can be overwritten or deleted by any part of the code. This can lead to unexpected behavior and errors, and make it difficult to debug the code. It is generally considered best practice to avoid using global variables in JavaScript, in order to keep the code clean and organized.

10. What are the main benefits of using Object-Oriented Programming in JavaScript?

The main benefits of using Object-Oriented Programming in JavaScript are:

1. It helps to keep code organized and easy to understand.
2. It makes code reuse easier, which can lead to more efficient code.
3. It can make code more flexible and extensible.

11. What are some common pitfalls of Object-Oriented Programming in JavaScript?

One common pitfall is that because JavaScript is a prototype-based language, it can be easy to create objects that have unintended side-effects on other objects. Another common issue is that because JavaScript is loosely typed, it can be easy to create objects that have methods that are incompatible with each other.

12. Which design patterns should I be familiar with when working with Object-Oriented Programming in JavaScript?

The three design patterns that are most important to know when working with Object-Oriented Programming in JavaScript are the Singleton, Factory, and Decorator patterns. The Singleton pattern is used to ensure that only one instance of a given object exists at a time. The Factory pattern is used to create new objects, and the Decorator pattern is used to add new functionality to existing objects.

13. When developing an application that uses Object-Oriented Programming in JavaScript, what are some best practices you follow?

Some best practices for Object-Oriented Programming in JavaScript include encapsulating data and methods inside of objects, using inheritance to extend functionality, and using polymorphism to allow objects to behave in different ways depending on the context. Additionally, it is important to keep in mind that JavaScript is a prototype-based language, so understanding how prototype chains work is crucial to effective Object-Oriented Programming in JavaScript.

14. Is there any tooling available for refactoring my existing JavaScript classes into something more manageable?

Yes, there are several tools available that can help with this process. JSLint is a popular JavaScript linter that can help identify potential problems with your code. JSHint is another popular tool that can help with both code quality and code style issues. Finally, the Closure Compiler is a tool that can help optimize and minify your JavaScript code.

15. What are some ways to make libraries compatible with multiple versions of JavaScript?

There are a few ways to make libraries compatible with multiple versions of JavaScript:

– Use feature detection to check for the presence of specific features before using them.
– Use a JavaScript library like Underscore.js or lodash that provides compatibility shims for older JavaScript engines.
– Use a JavaScript transpiler like Babel to convert newer code into older code that will run in older JavaScript engines.

16. What do you understand about modules in JavaScript? How can they be used to improve maintainability?

Modules in JavaScript are essentially self-contained pieces of code that can be reused throughout a project. This helps to improve maintainability by keeping the codebase more organized and manageable. Additionally, modules can be used to load only the code that is needed for a particular task, which can help to improve performance.

17. How does type checking work in JavaScript? Are there any tools that can help?

Type checking in JavaScript is a way of ensuring that a variable is of the correct type before using it. This can be important in preventing errors, and there are a few different ways to do type checking. One way is to use the typeof operator, which returns a string indicating the type of the variable. Another way is to use the instanceof operator, which checks whether the variable is an instance of a particular object type. There are also a few tools that can help with type checking, such as the JSLint and JSHint static code analysis tools.

18. How can you ensure that a particular method gets called only once?

In order to ensure that a particular method gets called only once, you can use the JavaScript method .bind(). This will bind the method to the object, so that it can only be called from that object.

19. What are mixins and traits in context with Object-Oriented Programming in JavaScript?

Mixins and traits are both ways of reusing code in object-oriented programming. A mixin is a class that contains methods that can be reused by other classes. A trait is a collection of methods that can be reused by other classes.

20. How do you solve version conflicts if two npm packages need different versions of the same library?

One way to solve this problem is to use a tool like nvm, which stands for node version manager. This tool allows you to install and manage multiple versions of node.js on the same computer, and you can easily switch between them as needed.