50+ Top JavaScript Interview Questions and Answers

By | May 15, 2022
JavaScript Interview Questions

Web development offers a galore of job opportunities to developers. When we talk about front-end development, it is mostly about JavaScript. If you are looking forward to being a front-end web developer and want to land a job in any company, then you must face some interviews. Here in this article, we have covered the best JavaScript interview questions, which will help you crack the interview.

JavaScript is one of the top 22 programming languages of 2022. It is a scripting language that is primarily used for client-side software development, but with the release of Node.js, now it can be used on the server-side.

Every webpage or application you see on the internet includes JavaScript for the development of a better user interface. Big companies such as Facebook and Google spend millions of dollars on JavaScript to build interactive, intuitive, and responsive web applications that can run on any device.

Top JavaScript Interview Questions and Answers

We have divided our list of frequently asked JavaScript interview questions into three different levels, namely basic, intermediate, and advanced. Let us begin with the basic level JavaScript interview questions.

Basic JavaScript Interview Questions

1. What is JavaScript?

Answer: JavaScript is a programming language; more specifically, it is a client- as well as a server-side scripting language. By scripting language, we mean that it can be embedded in HTML pages. It is considered a programming language because it follows the concept of object-based programming.

2. Name the tech company which developed JavaScript?

Answer: Netscape.

3. Is JavaScript related to Java?

Answer: No, JavaScript has nothing to do with Java. Both are used for different purposes though somehow, both of them intersect when it comes to front-end development frameworks. Apart from this, both serve different purposes. Java is a general-purpose programming language, whereas JavaScript is all about front-end web development.

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4. Give some differences between Java and JavaScript?

Answer:

  • Java is a general-purpose programming language in which code runs on a virtual machine, whereas JavaScript code runs inside the web browser.
  • Java is an object-oriented programming language, whereas JavaScript is an object-oriented scripting language.
  • We need to compile the code in Java to run the program, whereas JavaScript code is all in the form of text.

Further reading starts here.

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5. What do you understand by ECMAScript?

Answer: ECMAScript (European Computer Manufacturers Association) is an association for scripting language standards. JavaScript is the best implementation of this association.

6. What is the latest version of JavaScript?

Answer: The latest version of JavaScript is ECMAScript 2021 (ES2021), which was released in June 2021. It is also known as the 12th edition of JavaScript.

7. Name some of the main features of JavaScript?

Answer:

  • JavaScript is a lightweight scripting language for developing web applications.
  • It is an interpreted language and not a compiled programming language. A browser in which you are running JavaScript code interprets it and executes it line by line.
  • JavaScript is highly case-sensitive.
  • It supports control statements.
  • It is a dynamically-typed language.
  • JavaScript is a platform-independent language.

8. Name all the data types in JavaScript.

Answer: 

9. What does the isNaN function do in JavaScript?

Answer: isNaN is a built-in function of JavaScript that is used to check whether the passed argument is a number or not. If the argument is a number, it returns True. Else, it returns False.

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10. Are attributes and properties in JavaScript the same?

Answer: No, they both are different. Attributes can be defined as the ones that can give more details on elements like id, type, value, and so on. On the other hand, properties are those values that are assigned to elements like type=”text”.

11. Is JavaScript case-sensitive?

Answer: Yes, JavaScript is case-sensitive. Every keyword, function name, variable, or any other identifier is written in JS with a consistent capitalization of letters.

12. Is JavaScript faster than ASP script? If yes, why?

Answer: Yes, JavaScript is faster as compared to ASP script because JavaScript is a client-side language too, which makes it faster, unlike ASP script, which is just a server-side language. JavaScript does not need any extra accessories such as a compiler or interpreter to run its code. It can directly run inside the user browser.

13. Give Some advantages of using JavaScript.

Answer:

  • JavaScript always executes on the client environment, which makes execution faster.
  • It provides a rich user interface interactivity.
  • It is a very easy language to learn, and the outputs are more satisfactory as compared to other programming languages’ console black and white outputs.
  • Every browser supports JavaScript, so you do not need any heavy IDE to write JavaScript code. Thus, you can simply use Notepad.

14. Comment on the negative infinity in JavaScript.

Answer: In JavaScript, if we divide a negative number by zero, then it is termed negative infinity.

15. How do we perform breaking a string statement in JavaScript?

Answer: In JavaScript, we use backslash ‘\’ at the end of the line to break a string statement.

For example:

document.write(" Hello  \ world");

16. What are the undefined variables in JavaScript?

Answer: In the program, if we declare a variable and do not assign any value to it, then the variable will be known as an undefined variable. If we try to access it to return something, then it returns an undefined datatype.

17. What are undeclared variables?

Answer: The undeclared variable situation occurs when we get an error during the runtime. This is because of those statements in which we try to access variables that we had not declared.

18. Create an object in JavaScript.

Answer: Though JavaScript supports the concept of an object, here is how you can create an object in JavaScript:

var stu = {
name: "Sahil",
age: 16
};

19. What are name functions in JavaScript?

Answer: In JavaScript, when we declare a function, we give a name to it so we can call it whenever we want. The name given to such a function is known as the name function, and to declare a function, we use the function keyword.

Code example:

function rank () 
{ 
return "Captain"; 
}

20. What are global variables in JavaScript?

Answer: In JavaScript, if we define a variable outside the function, which can be accessed by any function throughout the program, then it is known as a global variable. If a global variable and local variable share the same name, we need to use the window object to access a global variable.

Example Code:

<script>
<!--
var myVar = "global_variable";   // Declare a global variable
function writeglobal() {
document.write(myVar);
}
//-->
</script>

OR

<script>
window.myVar = "global_variable";   // Declare a global variable
function writeglobal( ) {
alert(myVar);
}
</script>

21. What is the prompt box in JavaScript?

Answer: A prompt box in JavaScript is used to get input from the user. It provides a box in which the user can pass the inputs, which can be numerical or text.

Code Example:

window.prompt("sometext","defaultText");

22. What are timer functions in JavaScript? Name some of them.

Answer: In JavaScript, there are some built-in functions that are used to set a time period to execute some code. The functions that come under timer functions are setTimeout, setInterval, and clear interval.

Intermediate JavaScript Interview Questions

23. What are comments in JavaScript, and which symbols do we use to comment on something?

Answer: In JavaScript, if we comment on a line, it means that it is not going to execute. We use comments to give some additional information about what the particular code is supposed to do. We use // to comment on a single line and /* to comment out multiple lines */

24. What are argument objects in JavaScript?

Answer: The argument which is passed along the function is known as a variable argument, and it could be of any data type.

Code Example:

console.log(typeof x, arguments.length);
}
func(); //==> "undefined", 0
func(7); //==> "number", 7
func("1", "2", "3"); //==> "string", 3

25. What type of data does the typeof operator return in JavaScript?

Answer: The typeof operator returns the data type of variable which is passed along it.

Code Example:

var x= 20;
console.log(typeof x);

26. What does the ‘this’ operator does in JavaScript?

Answer: ‘this’ is using an object, which tells the functions to execute the current values of JavaScript code. In simple words, it is used to refer to the current variable of the function.

27. What does === operator do in JavaScript?

Answer: It is also known as an identity operator, and it checks whether the values are equal and have the same data types. If the values are the same and of the same datatype, then it returns true. Otherwise, it returns false.

28. What is the callback function in JavaScript?

Answer: A callback function is a function that is passed as an argument to another function and which can be called even after the outer function is finished.

Code Example:

function whatToSay(greeting) {
  return greeting
}

function greeting() {
  myDisplayer("Hello");
}

greet = whatToSay(greeting);
greet();

29. Give the difference between ViewState and SessionState.

Answer: In ViewState, when another page has loaded, the data of the previous page is lost. However, in SessionState, data remain available until the user closes the browser or the session ends.

30. Does JavaScript automatically convert the type of data?

Answer: Yes, it does. It is also known as implicit type conversion.

31. What are the rules for naming a variable in JavaScript?

Answer: When we name a variable, we should keep these points in our minds:

  • Do not use a reserved keyword as a variable name.
  • Do not start the variable name with digits (0-9). Always use alphabets or underscore to start a variable name.
  • Variable names are case sensitive, which means the car is not Car. Therefore, always use lowercase as a variable name.

32. Name all the loops of JavaScript.

Answer: JavaScript has 3 flow controls for loop

  1. For loop
  2. While loop
  3. Do-while loop

33. What are cookies in JavaScript?

Answer: When a user visits a website, then some of the test files are stored in the user system or browser. Those small test files are known as cookies.

34. Create a cookie using JavaScript.

Answer: We use the document.cookie object to create a cookie.

Code Example:

document.cookie = "key1 = value1; key2 = value2; expires = date";

35. Name the ways by which we can read a write file in JavaScript.

Answer: There are two ways of doing so:

  1. By using the JS extension.
  2. By using the webpage and ActiveX objects.

36. How to change an element’s style or class in JavaScript?

Answer: 

document.getElementById("myText").style.fontSize = "20?;
or
document.getElementById("myText").className = "anyclass";

37. What is variable typing in JavaScript?

Answer: The type of a variable is only known when it is defined. In JavaScript, the type and value of the variable can be changed if the variable appears in a different context.

Code Example:

n = '1000000'        // Set 'n' as a string
n = 10000*100;     // n changed from string to number

38. Automatic type conversion is possible in JavaScript or not?

Answer:  Yes, it is possible, and it is the common way of type conversion in JavaScript.

39. Which code is used to submit a form in JavaScript?

Answer:  document.form[0].submit();

40. What type of language is JavaScript?

Answer: JavaScript is a dynamically typed language because its variable can hold multi-type values. In the 37th question, for example, we changed the type of n from a string to a number, which means that n is not restricted to only one type.

Advance JavaScript Interview Questions

41. How is session storage different from local storage?

Answer: Session storage is similar to local storage, but the difference is the data stored in the session storage is not available when the session ends, or the browser is closed. Unlike session storage, the local storage is always there until it gets cleared manually.

42. JavaScript and JScript are the same or not?

Answer: No, they both are different. JavaScript is a product by Netscape and JScript is a product by Microsoft.

43. Is the ‘==’ operator similar to the ‘===’ operator?

Answer: ‘==’ operator is used to check whether both operands have the same value or not, while ‘===’ checks whether both operands have the same value and type or not. Both the operators return Boolean values true and false based on the conditions.

44. Are null and undefined the same?

Answer: No, both are different. We call a variable undefined if we have not assigned any value to it, but null itself is a value. So, if a variable is null, it means it is a defined variable.

45. What is NULL in JavaScript?

Answer:  NULL means no value or no object.

46. Find the result of 6+3+”9”?

Answer: Here, 6 and 3 are the integer values, and 9 is a string, so it will be concatenated with the result.

So the result is: 99

47. Name some famous frameworks of JavaScript.

Answer: Here are some famous JavaScript frameworks:

48. What will happen if we use + operand between two strings?

Answer: It will concatenate the two strings.

49. What does the delete operator do in JavaScript?

Answer: Delete is a reserved keyword in JavaScript, which is used to delete the property and value of any object.

50. How to perform the delete operation in JavaScript?

Answer: By using the delete operator, the delete operation is performed in JavaScript.

Code Example:

var student= {age:20, batch:"ABC"};
delete student.age;

51. Name all the types of pop-up boxes in JavaScript?

Answer: JavaScript has three types of pop-up boxes, as follows:

  1. Alert box
  2. Confirmation box
  3. Prompt box

52. How is the alert box different from the confirmation box?

Answer: Alert box has only one button, whereas the confirmation box has two buttons.

53. What does the pop() method does in JavaScript?

Answer: The pop() method usually operates on arrays. It pops out or removes the last element of the array and returns it.

54. Name the exception handling keyword used in JavaScript?

Answer: try, catch, and finally. If the block of try executes and fails due to some exception, then the block of catch will execute. The finally block of code will execute whether there is an exception or not.

Code Example:

Try{
Code
}
Catch(exp){
Code to throw an exception
}
Finally{
Code runs either it finishes successfully or after catch
}

55. When do we use .call() and .apply() methods?

Answer: We can use the .call() method when the numbers of the argument are known and the .apply() method when the number of arguments is unknown.

56. If arr is an array having 10 elements, what happens if we write arr.length = 0?

Answer: The arr.length = 0 will remove all the elements of array arr, and it will become an empty list.

57. How to create an array in JavaScript?

Answer: Defining the array:

var x = [];
var y = [1, 2, 3, 4, 5];

58. How many ways are there to add JavaScript code to an HTML file?

Ans: There are 3 ways to do this:

  1. Inline
  2. External
  3. Internal

59. Enumerate different Mouse Events in JavaScript.

Ans: There are nine different mouse events in JavaScript, as follows:

  1. click
  2. dblclick
  3. mousedown
  4. mouseup
  5. mouseenter
  6. mouseleave
  7. mouseover
  8. mouseout
  9. mousemove

60. Write a script to reverse a string.

Answer:

let string = "Welcome to TechGeekBuzz"; 
str_rev = string.split("").reverse().join("");

"zzuBkeeGhceT ot emocleW"

61. Mention the various techniques we can use to empty an array.

Answer: Suppose we have the following array:

array=[1,2,3,4,5,6,7,8,9,10]

The following are the four different methods to empty an array:

1. Reassign an empty array to the variable array.

array = []

2. Set the length of array =0.

array.length = 0

3. Use the splice method to set the array length 0.

array.splice(0, array.length)

4. Use a loop and pop all the elements from the array.

while(array.length)
{
  array.pop();
}

62. Check if a variable is an integer or not?

Answer: 

num =20
console.log(isInt(num))

63. Write a function that accepts two strings and check if they are anagrams of one another.

Answer: 

function check_anagram(str_1, str_2)
{
    let a= str_1.toLowerCase();
    let b = str_2.toLowerCase();

    a= a.split("").sort().join("");
    b= b.split("").sort().join("")
    
    return a===b;
}

64. What would be the output of this code:

0.1+0.2 === 0.3;

Answer: 

false

This happens because of floating-point errors in the internal representation.

Conclusion

With this, we conclude our list of the most commonly asked JavaScript interview questions and answers. We hope these questions will help you during the interview. If you have appeared in any JavaScript interview in the past few months or years, feel free to leave the questions you encountered in the comments section below, and we will add them to the list with the appropriate answers.

We also recommend you go through these top front-end interview questions to refresh your knowledge of other front-end technologies. Wish you luck!

If you like this article or have any suggestions or corrections, please comment down below.

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