Frequently Asked JavaScript Developer Interview Questions

JavaScript has become one of the most popular programming languages, and it’s no wonder that many organizations are looking for skilled JavaScript developers. The demand for professionals who can write clean and efficient code, understand various JavaScript frameworks and libraries, and have a deep knowledge of the language’s syntax, is on the rise. 

To help you prepare for your next JavaScript developer interview, I have compiled a list of the most frequently asked questions that you can expect to encounter during the interview process and these were asked in my interview as well. These questions will help you demonstrate your knowledge and skills to potential employers. Whether you’re a seasoned JavaScript developer or just starting your career, this guide will provide valuable insights into what to expect in a JavaScript developer interview.

Question. Differentiate between Java and javascript?

Answer. Below is the table given to differentiate between Java and javascript languages.

Question. What is JavaScript?

Answer. Javascript is a lightweight, client-side scripting and interpreted programming language. And object-oriented functionalities are also supported by the javascript programming language. The main application of the javascript programming language is to increase user interaction with the website. And widely it is used for the development of mobile applications, games, and client-side validation.

Question. Give some javascript features?

Answer. Features of javascript are given below:

• It is an interpreted programming language.
• Cross-platform
• Open-Source
• Integrated with the Java programming language.
• Used for network-centric application creation.
• Lightweight

Question. List out the data types that are supported in JavaScript.

Answer. Javascript supports the given below data types:

• Null
• Object
• String
• Undefined
• Symbol
• Boolean
• Number

Question. Give some disadvantages of Javascript.

Answer. The following are the disadvantages of Javascript:

• Multithreading is not supported.
• Multiprocessing is Not supported. 
• Files reading and writing are not allowed.
• Networking applications are Not supported.

Question. Name and Explain the function types available in the Javascript.

Answer. There are two types of functions in the javascript which are given below::

• Named function – Named functions are those functions by which a name is included at the time definition of the function. For Example:

function namedFunctionExample(){  
   document.writeln(“Demonstration of the Named Function”);
}  
namedFunctionExample();  

• Anonymous function – Anonymous functions are those functions that are declared at the runtime dynamically and these functions do not include any name at the time of their declaration. For Example:

var anonymousFunctionExample=function() {  
    document.writeln("Demonstration of the Anonymous Function");  
}
anonymousFunctionExample();

Question. Whether javascript is a case-sensitive language or not?

Answer. Yes. Javascript is a case-sensitive language. So any identifiers, variables, keywords and names of the function in the javascript language must be typed with consistent letters capitalization.

Question. What are javascript named functions?

Answer.  Named functions are those functions by which the name is included at the time definition of the function.

For Example:

function namedFunctionExample() {  
    document.writeln("Demonstration of the Named Function"); 
}  
namedFunctionExample();

Question. What are javascript anonymous functions?

Answer.  Anonymous functions are those functions that are declared at the runtime dynamically and these functions do not include any name at the time of their declaration. For instance:

var anonymousFunctionExample=function() {  
  document.writeln("Demonstration of the Anonymous Function");  
}  
anonymousFunctionExample();

Question. What do you mean by the argument objects in JavaScript and how we can get the argument types of a function?

Answer.  All the arguments that are passed to the function are represented by the javascript variable argument. The type of arguments that are passed to a function is provided by the type of operator.

For Example:

function argumentObjectExample(x){
 console.log(typeof x, arguments.length);
}

argumentObjectExample(); // print "undefined", 0 as an output

argumentObjectExample(9); // print "number", 1 as an output

argumentObjectExample("9", "7", "4"); // print "string", 3 as an output

Question. Which method is used for returning the specific index character?

Answer. charAt() method is provided by the javascript for finding the value of the specific index of the string. And it takes the index as a parameter and the value index varies from 0 to n-1 and here n denotes the length of the string. Negative index values and index values greater than the string length are not allowed.

For example:

var strExample="Hello!! Welcome";    

document.writeln(strExample.charAt(4)); // print o as an output

Question. Give a simple program example for printing hello world in javascript.

Answer. A program to print hello world in javascript is given below.

<script type="text/javascript">  
document.write("Hello World!");  
</script>

 Question. Differentiate between “ == “ and “ === “ operators used in javascript.

Answer.  “ == “ and “ === “ operators both are used for comparison. The only difference between these operators is for values comparison we use the “==” operator and for both data types and values comparison we use the “===” operator. For Example:

var a = 7;
var b= "7";
document.write(a == b); // prints true as the value of x and y are the same
document.write(a === b); // prints false as the value of x and y are the same but both have different data types.

Question. Differentiate between the let and var keywords of the javascript?

Answer. Below are some points given for differentiating between the let and var keywords:

• let keyword was introduced in javascript in 2015 but the var keyword is used in Javascript from the start. 
• Variables declared with the var keyword have the function scope and on the other hand, Variables declared with the let keyword have the block scope.
• Redeclaration is not allowed for the let keyword declared variables and requires declaration before using it whereas var keyword declared variables are hoisted.

Question. What is javascript hoisting?

Answer. In javascript, hoisting is a default behavior by which all the statements of the declaration of the function and variables are moved to the top, irrespective of where it is declared. For Example:

hoistedVariableExample = 8;

console.log(hoistedVariableExample); // 3 is printed even hoistedVariableExample is declared after this statement

var hoistedVariableExample;

Question. Explain the concept of javascript Implicit Type Coercion.

Answer. In javascript, Implicit type coercion automatically converts the data type of the value into another data type. Generally, Implicit Type Coercion takes place at the time of operating on the different data type operands. 

Example 1: When the number data type value is added to the value of the string data type. Then the data type of number value is also converted into a string data type.

var a = 7;

var b = "7";

document.write(a + b); // 77 is printed as an output

Example 2: When the number data type value is subtracted from the value of the string data type. Then the data type of string value is also converted into a number data type.

var a = 7;

var b = "7";

document.write(a - b) // 0 is printed as an output

Question. Whether javascript is a dynamically typed language or statically typed language.

Answer. Javascript language is a dynamically typed language. In a dynamically typed language, the type checking of the variables is done at the run-time, instead of that type checking of the variables is done at the compile-time in the statically typed language. Javascript variables are not associated with any particular data type because javascript is a loosely(dynamically) typed language. So any data type value is allowed to be stored in the variable. 

For Example: If the number value is assigned to the variable, then its data type is converted into a number and the data type is converted into a string if a string value is assigned to the variable.

var x = 888;

var y = "Hello!! Welcome..";

Question. Tell me something about the javascript NaN property.

Answer. The “Not-a-Number” value is represented by NaN in javascript. NaN is used to represent values that are not legal numbers. The number is returned by the typeof NaN. isNaN() function is used in javascript to check whether the value is NaN or not. Any data type value is firstly converted by the isNaN() function into a Number data type and then it checks the NaN for that value.

isNaN("Hello Welcome") // true is returned by it

isNaN(987) // false is returned by it

isNaN('1') // false is returned by it, as its value is 0 when it is converted into number data type and 0 is a number 

isNaN(true) // false is returned by it, as the result of the conversion of true value to the Number data type is 1 and which is a number

isNaN(false) // false is returned by it, as the result of the conversion of false value to the Number data type is 0 and which is a number

isNaN(undefined) // true is returned by it

Question. Differentiate between the javascript test() and exec() methods?

Answer. Below are some points given for differentiating between the test() and exec() method.

• exec () and test () methods both are the type of RegExp expression methods of the javascript.
• exec () method is used for searching patterns in the string, and the pattern is directly returned if it is found in the string, otherwise, an empty result is returned by it.
• test() method is used for searching patterns in the string, and the boolean true value is returned if the pattern is found in the string, otherwise, the boolean false value is returned by it.

Question. Explain the typeof Operator used in the javascript.

Answer. The typeof operator is used in javascript for getting the data type of the value of the operand. The string value is returned by the javascript typeof operator which represents the data type of the operand.

For Example:

document.write(typeof(8)) // number is printed by it

document.write(typeof("9")) // string is printed by it

document.write(typeof(true)) // boolean is printed by it

Question. What is the difference between undefined and undeclared variables?

Answer. The variables that are not declared in the program are the undeclared variables and the run time error is thrown if the program tries to read the undeclared variable value.

The variables that are declared in the program but no value is assigned to them, then it is undefined variables. An undefined value is returned if the program tries to read an undefined variable value.

Hope this small guide will be helpful for securing internships and full time javascript developer jobs.