Top 80+ JavaScript Interview Questions and Answers
Hello Friends, How are you? Today I am sharing the top 80+ JavaScript Interview Question and Answers. These JavaScript Interview Questions and Answers are very important for freshers or experienced candidates to get their dream job.
This article provides you with a comprehensive list of common JavaScript interview questions and answers that often come up in interviews. It will also help you understand the fundamental concepts of JavaScript.
These JavaScript interview questions and answers are divided into 3 sections.
1. Beginner Level
2. Intermediate Level
3. Advanced Level
So let's start with the first section of this JavaScript interview question and answer.
Beginner-Level JavaScript Interview Questions and Answers
Q 1. What is JavaScript and what is it used for?
Ans: JavaScript is a programming language that is primarily used to create interactive web applications. It was designed to run in web browsers and allow developers to create dynamic and interactive websites.
JavaScript is a client-side programming language, which means that it is executed by the user's web browser, rather than on the server. This allows for faster and more efficient web applications, as the code is executed on the user's device rather than on the server, which reduces the amount of data that needs to be transferred between the server and the client.
JavaScript is also used for creating mobile applications, desktop applications, and server-side applications. It is a versatile language that is widely used in the development of web applications and is supported by all modern web browsers.
Q 2. Which company developed JavaScript programming language?
Ans: JavaScript was developed by Netscape Communications Corporation in the mid-1990s. It was created to make it easier to add interactive elements to web pages, and it quickly gained popularity due to its simplicity and flexibility. JavaScript was initially known as LiveScript, but it was later renamed to JavaScript to capitalize on the popularity of Java, another programming language that was gaining popularity at the time. JavaScript has since become one of the most widely used programming languages in the world, and it is supported by all modern web browsers.
Q 3. What is the difference between Java and JavaScript?
Ans: JavaScript and Java are two different programming languages that have some similarities but are also quite different in many ways.
Some of the main differences between JavaScript and Java include:
Syntax: While JavaScript and Java share some similarities in their syntax, they are still quite different. JavaScript uses a C-style syntax, while Java has its own syntax that is more similar to C++.
Execution: JavaScript is a client-side language, which means that it is executed by the web browser on the user's computer. Java, on the other hand, is a compiled language that is typically executed on a server or on a user's computer through the use of a virtual machine.
Usage: JavaScript is primarily used for creating interactive elements on web pages, while Java is used for a wider range of applications, including creating mobile and desktop applications, web servers, and more.
Object-Oriented Programming: Both JavaScript and Java support object-oriented programming, but they have different approaches to it. Java is a fully object-oriented language, while JavaScript is a prototype-based language.
Despite these differences, JavaScript and Java do have some similarities, including the fact that they are both high-level programming languages and they both use the same basic programming concepts such as variables, loops, and conditional statements.
Q 4. What are the key features of JavaScript?
Ans: JavaScript is a powerful and versatile programming language that is widely used for creating interactive elements on web pages, as well as for building mobile and desktop applications. Some of the key features of JavaScript include:
Dynamic typing: JavaScript is a dynamically-typed language, which means that the type of a variable is determined at runtime rather than at compile time.
Object-oriented programming: JavaScript is a prototype-based language, which means that it uses objects and prototypes to create new types and to define their behaviour.
Support for functional programming: JavaScript supports functional programming concepts such as first-class functions, which means that functions can be passed as arguments to other functions, stored in variables, and so on.
Event-driven programming: JavaScript is an event-driven language, which means that it is designed to respond to user events such as clicks, hover, and so on.
Asynchrony: JavaScript has support for asynchronous programming, which allows it to handle multiple tasks concurrently without blocking the execution of other tasks.
Cross-platform compatibility: JavaScript is supported by all modern web browsers, which means that it can be used to create interactive elements that work on a wide range of devices and platforms.
Extensibility: JavaScript can be extended through the use of libraries and frameworks, which allows developers to build complex applications more quickly and easily.
Overall, these features make JavaScript a powerful and popular language for building a wide range of applications.
Q 5. What are the different datatypes supported by JavaScript?
Ans: JavaScript supports several different data types, which can be divided into two main categories: primitive data types and reference data types.
The primitive data types in JavaScript are:
Number: This data type represents numeric values and includes both integers and floating-point values.
String: This data type represents a sequence of characters and is used to store and manipulate text.
Boolean: This data type represents a logical value and can only be either true or false.
Undefined: This data type represents a value that has not been defined.
Null: This data type represents a null or empty value.
The reference data types in JavaScript are:
Object: This data type represents a collection of key-value pairs and can include both primitive and reference data types.
Array: This data type represents an ordered collection of values and can include both primitive and reference data types.
Function: This data type represents a block of code that can be executed and is often used to define reusable functions.
Symbol: This data type represents a unique and immutable value that can be used as an identifier for objects.
Overall, JavaScript has a flexible type system that allows it to handle a wide range of data types and to support various programming paradigms.
Q 6. Is JavaScript a case-sensitive language?
Yes, JavaScript is a case-sensitive language, which means that the language treats variables, functions, and other language elements differently based on the case of their names.
For example, the following variables are considered to be different in JavaScript:
let x = 10;
let X = 20;
In this case, the variables x and X are considered to be different because they have different cases.
In general, it is a good practice to be consistent in the case that you use your variables, functions, and other language elements in JavaScript to avoid confusion and potential bugs.
Q 7. What is the use of the isNaN function?
The isNaN() function in JavaScript is used to determine whether a value is a number or not. It stands for "is Not a Number," and it returns a Boolean value indicating whether the value is a number or not.
For example:
console.log(isNaN(10)); // returns false
console.log(isNaN('10')); // returns false
console.log(isNaN(NaN)); // returns true
console.log(isNaN(undefined)); // returns true
console.log(isNaN('hello')); // returns true
In the example above, the isNaN() function returns false for the first two values because they are both numbers. It returns true for the third value because it is the special NaN value, which indicates that a value is not a number. It also returns true for the fourth and fifth values because they are not numbers.
The isNaN() function is often used in JavaScript to validate user input or to check the type of a value before performing certain operations on it. It is also useful for handling cases where a value might be null or undefined, as these values are considered to be non-numeric by the isNaN() function.
Q 8. What is the difference between undeclared and undefined values?
Q 9. How can you create an object in JavaScript?
Q 10. How can you create an array in JavaScript?
Q 11. What are some of the commonly used built-in methods in JavaScript?
Q 12. What is the scope of variables in JavaScript?
Q 13. what is the variable naming convention in JavaScript?
Q 14. What are the global variables and how it is declared?