JavaScript OOP Interview Questions
Quick Answer
JavaScript OOP interview questions focus on concepts like prototypes, classes, inheritance, encapsulation, and polymorphism. Understanding how JavaScript implements object-oriented programming helps candidates demonstrate their ability to write modular, reusable, and maintainable code.
Learning Objectives
- Explain the purpose of OOP Interview Questions in a practical learning context.
- Identify the main ideas, terms, and decisions involved in OOP Interview Questions.
- Apply OOP Interview Questions in a simple real-world scenario or practice task.
Introduction to JavaScript OOP Interview Questions
Object-oriented programming (OOP) is a core concept in JavaScript, used to organize code into reusable and modular components.
Interviewers often ask questions about JavaScript's OOP features to assess a candidate's understanding of how objects and inheritance work in the language.
In JavaScript, everything is an object or can behave like one.
Understanding JavaScript Prototypes
JavaScript uses prototypes to implement inheritance. Every object has a prototype object from which it can inherit properties and methods.
This prototype-based inheritance differs from classical inheritance found in other languages.
- Prototype is an object linked to another object.
- Objects inherit properties and methods from their prototype.
- The prototype chain allows property lookup up the chain.
Prototype Chain Example
Consider an object 'child' that inherits from 'parent'. If a property is not found on 'child', JavaScript looks up the prototype chain to 'parent'.
JavaScript Classes and Constructor Functions
ES6 introduced the 'class' syntax as a clearer way to create constructor functions and manage inheritance.
Classes are syntactic sugar over JavaScript's prototype-based inheritance.
- Classes define a blueprint for objects.
- Constructor functions initialize new objects.
- Methods defined in classes are added to the prototype.
Class Syntax Example
Using the 'class' keyword, you can define a class with a constructor and methods.
Core OOP Concepts in JavaScript
JavaScript supports key OOP principles such as encapsulation, inheritance, and polymorphism, though implemented differently than classical OOP languages.
- Encapsulation: Using closures or private fields to hide data.
- Inheritance: Achieved via prototypes or class extends.
- Polymorphism: Methods can behave differently based on the object context.
Practical Example
This example shows class inheritance and method overriding in JavaScript.
Examples
class Animal {
constructor(name) {
this.name = name;
}
speak() {
console.log(`${this.name} makes a noise.`);
}
}
class Dog extends Animal {
speak() {
console.log(`${this.name} barks.`);
}
}
const dog = new Dog('Rex');
dog.speak(); // Rex barks.This example shows class inheritance and method overriding in JavaScript.
Best Practices
- Understand the prototype chain and how inheritance works in JavaScript.
- Use ES6 classes for clearer and more maintainable OOP code.
- Encapsulate data using closures or private class fields.
- Avoid modifying built-in prototypes to prevent unexpected behavior.
Common Mistakes
- Confusing classical inheritance with JavaScript's prototype-based inheritance.
- Not using 'new' keyword with constructor functions, leading to unexpected results.
- Overusing global variables instead of encapsulating data.
- Misunderstanding 'this' context inside methods.
Hands-on Exercise
Create a Class Hierarchy
Define a base class 'Vehicle' and subclasses 'Car' and 'Bike' with overridden methods.
Expected output: Instances of Car and Bike should have their own implementation of a method like 'move'.
Hint: Use ES6 class syntax and the 'extends' keyword.
Prototype Chain Exploration
Create objects linked via prototypes and demonstrate property lookup.
Expected output: Show that properties not found on the child object are accessed from the prototype.
Hint: Use Object.create() to set prototypes.
Interview Questions
What is the prototype chain in JavaScript?
InterviewThe prototype chain is a series of linked objects used for inheritance. When accessing a property, JavaScript looks up the chain until it finds the property or reaches null.
How do ES6 classes differ from constructor functions?
InterviewES6 classes provide a clearer, more concise syntax for creating objects and managing inheritance but are syntactic sugar over the existing prototype-based constructor functions.
Explain encapsulation in JavaScript.
InterviewEncapsulation in JavaScript can be achieved using closures or private class fields to restrict direct access to object data, promoting modularity and security.
What is polymorphism in JavaScript OOP?
InterviewPolymorphism allows methods to behave differently depending on the object calling them, often achieved by method overriding in subclasses.
MCQ Quiz
1. What is the best first step when learning OOP Interview Questions?
A. Understand the purpose and basic idea
B. Skip directly to advanced implementation
C. Ignore examples and practice
D. Memorize terms without context
Correct answer: A
Starting with the purpose and basic idea makes later examples and practice easier to understand.
2. Which activity helps reinforce OOP Interview Questions?
A. Reading once without practice
B. Building or writing a small practical example
C. Avoiding review questions
D. Skipping the summary
Correct answer: B
A small practical example helps connect the topic to real usage.
3. Which statement is most accurate about this topic?
A. JavaScript OOP interview questions focus on concepts like prototypes, classes, inheritance, encapsulation, and polymorphism.
B. OOP Interview Questions never needs examples
C. OOP Interview Questions is unrelated to practical work
D. OOP Interview Questions should be learned without checking results
Correct answer: A
The correct option is based on the available topic explanation.
Key Takeaways
- JavaScript OOP interview questions focus on concepts like prototypes, classes, inheritance, encapsulation, and polymorphism.
- Understanding how JavaScript implements object-oriented programming helps candidates demonstrate their ability to write modular, reusable, and maintainable code.
- Object-oriented programming (OOP) is a core concept in JavaScript, used to organize code into reusable and modular components.
- Interviewers often ask questions about JavaScript's OOP features to assess a candidate's understanding of how objects and inheritance work in the language.
- JavaScript uses prototypes to implement inheritance.
Summary
JavaScript's object-oriented programming is based on prototypes and classes, enabling inheritance and code reuse.
Understanding OOP concepts like encapsulation, inheritance, and polymorphism is essential for writing maintainable JavaScript code and succeeding in interviews.
Frequently Asked Questions
Is JavaScript a class-based or prototype-based language?
JavaScript is fundamentally prototype-based, but ES6 classes provide a syntax that resembles class-based languages.
Can you override methods in JavaScript classes?
Yes, subclasses can override methods defined in their parent classes to provide specialized behavior.
How do you create private properties in JavaScript classes?
Private properties can be created using closures or the new private field syntax with a '#' prefix inside classes.
