Top 30 JavaScript Interview Questions and Answers for 1 to 2 years Experienced Developers

Building a career as a developer in the IT industry can be challenging if you lack stream-specific preparation. For instance, if you want to be a JavaScript developer, you have to dig deep into its concepts and learn every bit about JavaScript, but then also, I would say, you are 50% prepared for the Job. I'm not saying that you aren't capable enough to get that JavaScript developer's Job; instead, I mean to say that you aren't ready enough to face the recruiters. Recruiters usually don't ask straightforward questions while recruiting a developer. They have their set of tricks to manipulate the easiest question, and that confuses the candidates. Besides this, if someone bombards a couple of questions back to back, it is natural that you (candidate) will feel nervous and could end up making a mistake.

Since you are reading this guide, let me tell you that you will step in with confidence and crack that interview. Wait! I'm not just saying it to you for the sake of your morale; I'm saying it because I'm going to prepare you like that.

In this guide, I have listed the top 30 JavaScript Interview questions and answers, aka JS interview questions, that are most common and frequently asked by recruiters worldwide.

Don't get confused with Python and Java; it was just a, for example, we will learn about 30 frequently asked interview questions in JavaScript.

Note: At the end of this guide, there is a small bonus for you, but there is a catch, you have to scroll down after going through each and every question from the list. It is not recommended to skip the question, even if you know the answers. Revision is a very powerful tool.

30 Frequently Asked JavaScript Interview Questions

Let's jump into these popular JavaScript interview questions and answers for beginners and 1 to 5 years experienced web developers and JavaScript professionals. These questions have been collected from both telephonic and face-to-face rounds of interviews so that you can use them similarly for your preparation.


1. How is JavaScript different from Java? 

Java and JavaScript are two very different programming languages. Java is an I-independent language, whereas JS needs to be integrated with some other front-end language.

JS is a scripting language, and it doesn't completely work on OOP concepts. At the same time, Java is an object-oriented programming language that deeply works on OOPs concepts. Both of these languages are designed for different purposes.

Java works best as a backend language, whereas JS works best as a client-side scripting language.

However, both these languages also have some similarities, such as some libraries and syntax of JS and Java are identical. You can also see my article about Java vs JavaScript to learn more differences between these two. 


2. What is JavaScript, according to you?

JavaScript or JS is a scripting client-side programming language. It works best when integrated with some front-end language like HTML, and it only works on browsers. JS works on some concepts of OOPs, like objects.


3. What is the isNaN function in JavaScript?
isNaN is a JavaScript's inbuilt function that returns a boolean value; it returns True if the argument is not numeric, whereas False in other cases.



4. What is ASP script, and How is it different than JavaScript?
ASP stands for Active Server Pages; it is a backend language like PHP and is used for scripting. It was first introduced in 1998.  ASP is different from JS because JS is a client-side programming language, whereas ASP is a backend language.


5. Which one is faster, JS or ASP script?
Since ASP is a server-side programming language and JS is a client-side browser-based language, ASP is slower than JS.


6. What is Nodejs?
Nodejs is a backend, open-source platform that basically allows developers to run JS outside a web browser. Through Nodejs, developers can run JS in the backend.


7. List some JS Data types?
Data types of JS are as follows:

• Boolean
• Number
• Object
• String
• Undefined

8. Who developed JS?
JS was developed by Netscape back in 1995.


9. What is the concept of global variables?
JS global variables are the highest scope variables. Global Variables are declared at the top of the script or outside a function. Global variables can be accessed from anywhere by any function within the script.  If you want to learn more, I also recommend joining these best JavaScript courses for beginners from online learning websites. 

10. What are the JavaScript paradigms?
JS supports functional, event-driven, imperative programming paradigms.


11. What percentage of Websites are currently using JS as their client-side language?
A total of 97% of websites live on the internet are using JavaScript as their client-side language.


12. What do you mean by "pass by value" and "pass by reference"?
Pass by value* in JavaScript enables us to invoke the functions by simply passing a variable’s value as an argument. When using the pass-by-value concept, functions create their own copy of the variable. Therefore any modification made in the variable’s value doesn’t modify the actual value of the variable.

For instance,
Var x is passed as an argument to a function sum(), then the first thing sum() will do is create a copy of variable x. Later, when the function sum() modifies the variable, x doesn’t get affected by the modification.

Apart from this, Pass by reference in JS enables us to invoke a function by passing the address of the variable as an argument. In pass-by-reference, any modifications made in the argument’s value are reflected in the actual variable.

Considering the above example, if var x would have been passed as an argument using pass by reference concept, and the method sum() modifies the variable, variable x will be affected.


13. What does the "this" keyword mean?
In JavaScript, whenever a developer wants to refer to the object from its source, "this" keyword is used.

14. Explain the bind() method.
bind() is an inbuilt method of JavaScript that is used to create a new function with the value of "this" keyword bound with the object that owns it.


15. Explain type conversion in JS.
Type conversion, in general, enables developers to convert a particular data type to another data type.

For instance,
Conversion of Numbers to the string can be done using the following statement:

String(x);

Here, x can be any number.

Apart from this, these statements are used for the following type of conversion:

• Converting a String to Number: Number(x);
• Converting Dates to Numbers: Number(x);
• Converting Dates to String: String(x);

You can also see these online JavaScript courses and classes from Pluralsight to learn more about key JAvaScript concepts like data types and more. 

16. Write the syntax of submitting a value in a form.
In JavaScript, to submit a value to a form, we have to use the following statement:

document.form[0].submit();

17. Which extension is used to read a file?
JavaScripts extends its support to two different ways that enable developers to read a file. One way is by using an extension, the name of the extension is JavaScript. At the same time, another one is by using a web browser and ActiveX objects.


18. How many loops does JS have?
JavaScripts supports all the loop structures that Java supports. Looping structures of JS are as follows:

• For
• While
• Do-While

9. Explain the "===" operator in JavaScript?
The "===" is a comparison operator but with some additional features. The traditional comparison is "=="; it compares the values of two variables. Whereas the "===" comparison operator is used to compare values and the data type of the variables.


20. List the difference between "==" and "==="?
Both of these operators are used for comparison. However, the difference between these two is that "==" is to compare values, whereas "===" is used to compare values and the data types of the variables. Only a limited number of programming languages support "===," JavaScript is one of them. You can further see my article about == vs === in JavaScript to learn more differences between the two. 



21. What will be the output of the following expression? alert(2+6+"3")
Usually, the output of the following statement is:

alert(2+6+3);

11

But since the last integer is placed inside double quotes, JS will treat 3 as a string, and the final output of the statement alert(2+6+"3") is 83.

Here, JS has concatenated the string 3 with the sum of 2+6, i.e.8; thus the output became 83 instead of 11. If you want to learn more then you can also see these websites to learn JavaScript in detail. 

22. What does undefined data type mean in JS?
Undefined data types are the variables that are declared in the JS script but never assigned with a value. So whenever the program tries to access the undefined value, it returns an undefined value.


23. List the escape characters of JS.
Like every programming language, there is a list of escape characters that JavaScript supports. We use JavaScript for letting the system that a different character needs to be printed.

Usually, while trying to display single quotes, double quotes, apostrophes, or ampersands, escape characters are used. Without escape characters, these special characters won't be displayed.

JS has "\" backslash as the escape character.


For instance,

document.write "This is a "JS" Script"

In this statement, we want to print the string JS with double quotes, but the compiler will get confused and throw an error because the double-quote before J will be considered as the final quote for the first quote, and the compiler will think that the string ended.

To print the string JS with double quotes, we need to use a backslash before and after the double quotes.

document.write "This is a \"JS\" Script"

24. Explain the concept of currying?
Currying is one of the special features of JavaScript, which allows us to change the way to invoke a method. Technically, it enables us to convert a particular function with x arguments into x functions with either one or less than one argument. Currying doesn't change the function's working; it just changes the way of invoking that function.


25. In JS, what does the break statement mean?
In JavaScript, the Break statement helps the developer to step out of the current loop. Whenever the user wants to terminate the loop and move the control flow out of the loop, a break statement is used.

For instance,

function demofunc() {

var day;

switch (new Date().getDay()) {

case 0:

day = "Sunday";

break;

case 1:

day = "Monday";

break;

case 2:

day = "Tuesday";

break;

case 3:

day = "Wednesday";

break;

case 4:

day = "Thursday";

break;

case 5:

day = "Friday";

break;

case 6:

day = "Saturday";

break;

}

document.getElementById("demo").innerHTML = "Today is " + day;

}

26. What do you mean by closure in JS?
In JavaScript, closures are the variables that stay in the function's memory even after returning their value.


27. What does callback mean?
In JavaScript, there is a function called Hoisting, and as per the concept, all the declarations are considered a top priority and get executed in the first place.

To counter the effect of hoisting, the concept of callbacks comes into play. Wherever the callback is used, that function automatically gets executed after another function gets executed.

For instance,

function disp(some) {
   document.getElementById("demo").innerHTML = some;
}

function subtractor(int1, int2, callbacks) {
  let sub = int1 - int2;
   callbacks(sum);
}

subtractor(5, 5, disp);

You can further see these free JavaScript courses to learn more about callback and other key Javascript concepts in depth. 

28. Explain memorization?
Memoization is a caching technique that is often used by JavaScript developers. In this technique, a particular function is cached, and its return value is mesmerized.

Now, whenever that function is called with the same parameters as earlier, cached data is used, and the return is re-return without actually computing the value.

This makes the script more efficient and time-saving. But you must note that memoization might seem like a very useful concept, but it actually isn't. Memoization needs a large amount of memory for caching and storing the value.

29. What will be the output of the following script?

<!DOCTYPE html>
<html>
<body>
<p>JavaScript Script</p>
<p>This is going to display the result</p>
<p id="dem"></p>
<script>
  function disp(result) {
     document.getElementById("dem").innerHTML = result;
  }
function sum(num1, num2) {
  let disp = num1 + num2;
  return disp;
 }
  let finalResult = sum(65, -10);
 disp(finalResult);
</script>
</body>
</html>

The output will be:

JavaScript Script
This is going to display the result
55

30. Should you use innerHTML in a JS script? Why?
It is never advised to use innerHTML in a JS Script; there are multiple reasons behind it. A few of them are as follows:

• innerHTML makes JS much slower.
• innerHTML's content is automatic every time.
• Missing the Scope makes validation quite tough in innerHTML.
• It makes the webpage unstable.

Quick Bonus
I know, I had mentioned that there will be a bonus for you and guess what! It's time for that bonus.

Here are the two additional questions that could play an important role in the JavaScript interview.

Note: Both these questions are the most common, I would say, these are asked in every developer job interview.

31. What do you mean by anonymous function in JavaScript?
Anonymous function in JS is a special function that is declared without any name. Normally, we use the function keyword followed by the function name, but there is no function name in an anonymous function, just the function keyword.

Besides that, the anonymous function can’t be accessed anywhere in the script except after its declaration. It is because, to implement the anonymous function, the function expression the concept is used, and function expression doesn’t support hoisting.

Anonymous function declaration:

var disp = function(){
   alert(“This is anonymous function”);
};
disp();

Since an anonymous function doesn’t have any name thus, to invoke it, a variable name is used.

The anonymous function might seem similar to function expression, but both of these have some differences. For example, anonymous functions can only have one expression in the body, whereas function expressions can have multiple expressions. Though the number of arguments in both these types can be multiple. See these JavaScript online courses to learn more about anonymous functions and other key JavaScript concepts

.

32. What do you mean by the concept of recursion in programming?
Recursion is a very important concept of programming; almost every programming language extends its support to recursion.

In recursion, a function calls itself for n number of times or until the result is achieved. Function calling can be direct or indirect. The function that is involved in recursion is termed a recursive function.

For instance,
To calculate the sum of the first numbers, one can add 1+2+3+…+n using a simple loop. Whereas the other way around is to use the concept of recursion and solve the problem with the least effort and in an optimal way.

int sum(int n){
  if(n < 1)
     return 0;
   else {
    return n + sun(n-1);
  }
}

While solving problems, it is recommended to consider recursion if possible. Using recursion in programming is considered a good thing and is often referred to as clean programming.

Final Notes

If you have gone through these questions thoroughly, I can assure you that you will tackle whatever question your recruiter throws towards you easily. Apart from this, if you want, you can practice some programming questions so that you can confidently write a particular script if the recruiter asks to write a particular script.

If you want me to add or omit any question, drop it down in the comments, we will discuss it. Rest, sit back and go through these JavaScript Interview Questions multiple times. Apart from this, it is expected that you have already gone through the fundamental concepts of programming.

You can also go through Agile technology concepts because you might end up getting placed in the team that delivers complete software. You must have the basic information of software development terms.

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In case of any queries, you can drop them down in the comments and let someone else answer them; you can have a discussion too.

P. S. - If you are new to JavaScript world and want to learn JavaScript in depth then I also suggest you to join a comprehensive Java course which covers both JavaScript essentials and advanced ES6 to ES8 concepts. If you need recommends you can checkout these 14 Best JavaScript courses to choose the one you like.