Best Javascript Interview Questions And Answers

To build a career in JavaScript programming, candidates need to pass an interview. At the same time, people looking to hire JS devs need to prepare for a dialog. 

Below is a list of JavaScript interview questions and answers that may be asked during a job interview. Candidates are likely to be asked javascript basic interview questions to advance the interview, depending on their experience and various other factors.

The list below covers both JavaScript questions for beginners and javascript interview questions for experienced developer. This JS Interview guide will help you get through your interview and get your dream JavaScript coding job.

Also, the list of advanced javascript interview questions will help people looking to hire PRO devs to sort out the available options.

Feel free to use the below-listed questions and answers for the next seniority level interviews:

Junior Javascript Developer Interview Questions


What is JS?

It is a client-side and server-side scripting language that is inserted into HTML pages and understood by web browsers. JavaScript is also an object-oriented programming language.


List the differences between Java and JavaScript?

The first one is a complete programming language. Unlike, JavaScript is a coded program that can be rendered in HTML pages. These 2 languages ​​are not at all interdependent and are intended for different purposes. Java is Object Oriented Programming (OOPS) or structured programming languages ​​such as C++ or C, whereas JavaScript is a client-side scripting language.


What are JavaScript data types?

Below are the JavaScript datatypes:

  • Number
  • String
  • Boolean
  • Object
  • Undefined


What is the use of the isNaN function?

The isNaN function returns true if the argument is not a number; otherwise, it is false.


What's faster between JavaScript and ASP scripting?

JavaScript is faster. JavaScript is a client-side language and therefore does not require the help of a webserver to run. On the other hand, ASP is a server-side language and therefore always slower than JavaScript. Javascript is now also a server-side language (nodejs).


What is negative infinity?

Negative infinity is a number in JavaScript that can be obtained by dividing a negative number by zero.


Can JavaScript be split across multiple lines?

Breaking inside a string operator can be done with a backslash '\' at the end of the first line.


document. Write ("This is \a program,");

And if you go to a new line when not in a string expression, then javaScript ignores the line break.


var x=1, y=2,

The above code is fine but not recommended as it makes debugging difficult.


Which company developed JavaScript?

Netscape is a software company that developed JavaScript.


What are undeclared and undefined variables?

Undeclared variables are variables that do not exist in the program and are not declared. If the program attempts to read the value of an undeclared variable, a runtime error occurs.

Undefined variables are those that are declared in the program but have no meaning. If the program tries to read the value of an undefined variable, an undefined value is returned.


Write code to dynamically add new elements?

  <script type="text/javascript">      function addNode () {
      var newP = document. createElement("p");
      var textNode = document.createTextNode(" This is a new text node");
     document.getElementById("firstP").appendChild(newP); }
  <p id="firstP">firstP</p>

Mid Javascript Developer Interview Questions


What are global variables? How are these variables declared?

Global variables are available throughout the length of the code, so it has no scope. The var keyword is used to declare a local variable or object. If the var keyword is omitted, a global variable is declared.


// Declaring global: globalVariable = "Test";

The problems encountered when using global variables are conflicts between local and global variable names. Also, it isn't easy to debug and test code based on globals.


What is a prompt box?

A prompt box is a field that allows the user to enter data by providing a text box. A label and field for entering text or number will be provided.


What is this keyword in JavaScript?

The 'This' keyword refers to the object it was called from.


How do timers work in JavaScript?

Timers are used to execute a piece of code at a given time or repeat code at a given interval. This is done using the setTimeout, setInterval, and clearInterval functions.

The setTimeout (function, delay) function is used to start a timer that calls a specific function after a specified delay. The setInterval (function, delay) function re-executes the given function at the specified delay and stops only when canceled. The clearInterval (id) function instructs the timer to stop.

Timers run on the same thread, so events can queue up, waiting to be executed.


What character is used for comments in Javascript?

// for one-line comments and
/ * Multi
* /


What is the difference between ViewState and SessionState?

ViewState refers to the page in the session.

"SessionState" refers to user data that can be accessed across all pages of web applications.


What is the === operator?

=== is called the strict equality operator, which returns true if two operands have the same value without conversion.


How do I submit a form using JavaScript?

To submit a form using JavaScript, use:

document.form [0] .submit ();
document.form [0] .submit ();


Does JavaScript support automatic type conversion?

Yes, JavaScript supports automatic type conversion. This is a common type of conversion method used by JavaScript developers.


How can I change the style/class of an element?

This can be done as follows:

document.getElementById ("myText"). style. fontSize = "20";


document. getElementById ("myText"). className = "any class";

Senior Javascript Developer Interview Questions


How can I read and write a file using JavaScript?

There are two ways to read and write a file using JavaScript.

  1. Using JavaScript extensions
  2. Using File System Access API and HTML input element


What are all looping structures in JavaScript?

Below are the looping structures in Javascript:

  • For
  • While
  • Do-while loops


What is called variable typing in Javascript?

The variable type is used to assign a number to a variable. The same variable can be assigned to a string.


i = 10;
i = "string;"

This is what we call variable typing.


How to convert a string of any base into an integer in JavaScript?

The parseInt () function is used to convert numbers between different bases. parseInt () takes the string to be converted as its first parameter. The second parameter is the base of the given string.

To convert 4F (or base 16) to integer, the code used would be -

parseInt ("4F", 16);


Difference between "==" and "==="?

"==" only tests equality by value, whereas "===" is a more stringent test for equality and returns false if the value or type of the two variables is different.


What will be the result of 3 + 2 + "7"?

Since 3 and 2 are integers, they will be added numerically. And since 7 is a string, it will be concatenated. So the result will be 57.


How to determine the operating system on the client machine?

To discover the operating system on the client machine, navigator. A platform string (property) should be used.


What do you mean by NULL in Javascript?

NULL is used to represent the absence of a value or object. It implies neither an object, nor a null string, nor a valid boolean, nor a number, nor an array object.


What is the function of the delete operator?

The delete keyword is used to delete a property as well as its value.


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


What is an undefined value in JavaScript?

An undefined value means:

  • The variable used in the code does not exist
  • Variable not assigned any value
  • The property does not exist.
Hiring Javascript developers? We have the people you are looking for!
Get in touch
Looking for vetted Javascript developers to join your team?

There are hundreds of battle-proven software development experts in our Talent Network.

Are you a Javascript developer looking for amazing projects? Join as a Talent