Ace your JavaScript Interview by practicing these 11 questions

Top 11 JavaScript Interview Questions You Must Know

Brandon Eich developed Java Script in just ten days and changed the way websites were developed. 98% of the websites use JavaScript on the client side to determine how a web page behaves. 

Moving on, despite a shaky start Java Script after several evolutions have now become the most popular programming language in the world with the highest number of code repositories than any other language. 

How Java Script got its name has an interesting angle. With Java being the most sought-after program back then and to steal some of that attention, the makers got inspired to use Java Script. The official version however remains that Java applets were supported by Netscape within its browser and thus the name was born.

With new frameworks and languages like Ember, Angular, React, and Vue, complicated web applications can be written using small teams in the shortest time possible. And let’s not forget to mention that JavaScript can also be used to write native mobile apps, making it possible to share codes between the world of mobile and the web. 

And since it is such a dynamic, versatile scripting language, the technology sector is always on the lookout for talented web developers and software engineers who are proficient in JavaScript and its various frameworks. With interesting work profiles and attractive salary packages, these jobs are sought after. 

So, how to nail a JavaScript interview?

Well, the easy way is to know the important questions and learn the method of answering them covering all important aspects and writing the correct codes.

Let’s go through some of the most important questions and most often asked questions.

1. What is JavaScript?

JavaScript is a client-side and server-side scripting language inserted into HTML pages and is understood by web browsers. It can also be defined as an Object-based Programming language.

2. Are Java and JavaScript the same?

The two languages are not at all interdependent and are designed for different intents. While Java is a complete programming language in itself, Java Script is a coded program that can be introduced to HTML pages. Java is an object-oriented programming (OOPS) or structured programming languages like C++ or C, whereas JavaScript is a client-side scripting language.

3. What are the different data types present in Javascript?

  • Number: It represents a number and can be written with or without decimals.
  • String: It represents a series of characters and is written with quotes. A string can be represented using a single or a double quote.
  • Boolean: It represents a logical entity and can have only two values: true or false. Booleans are generally used for conditional testing.
  • Object: Used to store collection of data 
  • Undefined: When a variable is declared but not assigned, it has the value of undefined and its type is also undefined.

4. Explain Hoisting in JavaScript?

JavaScript Hoisting refers to the process whereby the interpreter appears to move the declaration of functions, variables, or classes to the top of their scope, prior to execution of the code. Hoisting is the default behavior of javascript and allows functions to be safely used in code before they are declared. 

The variables and functions irrespective of where they are declared, they are moved on top of the scope. The scope can be both local and global.

5. Explain the meaning of “Debugger” in JavaScript?

Errors in codes can create a lot of ambiguity in the logic and understanding of both users and programmers. Sometimes the errors remain invisible even to the programmer’s eye. Debuggers can go through the entire code, identify the errors and fix them. 

The debugger keyword is used in the code to force stop the execution of the code at a breaking point and calls the debugging function.

6. Difference between “ == “ and “ === “ operators?

Both are comparison operators. The difference between both the operators is that “==” is used to compare values whereas, “ === “ is used to compare both values and types.


var x = 2;

var y = “2”;

(x == y)  // Returns true since the value of both x and y is the same

(x === y) // Returns false since the typeof x is “number” and typeof y is “string”

7. What is a “closure” in JavaScript? Provide an example?

A closure is an inner function that has access to the variables in the outer (enclosing) function’s scope chain. The closure has access to variables in three scopes; specifically: 

(1)  variable in its own scope, 

(2) variables in the enclosing function’s scope

(3) global variables.

var globalVar = “xyz”;

(function outerFunc(outerArg) {

    var outerVar = ‘a’;

    (function innerFunc(innerArg) {

    var innerVar = ‘b’;


        “outerArg = ” + outerArg + “\n” +

        “innerArg = ” + innerArg + “\n” +

        “outerVar = ” + outerVar + “\n” +

        “innerVar = ” + innerVar + “\n” +

        “globalVar = ” + globalVar);



In the above example, variables from innerFunc, outerFunc, and the global namespace are all in scope in the innerFunc. The above code will therefore produce the following output:

outerArg = 123

innerArg = 456

outerVar = a

innerVar = b

globalVar = xyz

8. Is JavaScript statically typed or dynamically typed language?

JavaScript is a dynamically typed language. In a dynamically typed language, the type of a variable is checked during run-time in contrast to a statically typed language, where the type of a variable is checked during compile-time.

Static Typing is a relatively risk-averse approach to coding as the type checking occurs at the compile time. This translates into the fact that it runs checks to catch things like missing functions, invalid type arguments, or a mismatch between the data value and type of variable assigned to it, even before the program has a chance to run and put the program at risk of crashing. 

In dynamically-typed languages like Python, JavaScript, PHP, and Perl, type-checking happens regularly at runtime without halting operations. If the type checker detects an error, it alerts the developer and gives them an opportunity to correct the code before the program crashes.

Since javascript is a loosely(dynamically) typed language, variables in JS are not associated with any type. A variable can hold the value of any data type.

For example, a variable that is assigned a number type can be converted to a string type:

var a = 23;

var a = “Hello World!”;

9. What is an Immediately Invoked Function in JavaScript?

An IIFE (Immediately Invoked Function Expression) is a JavaScript function that runs as soon as it is defined.

It is a design pattern that is also known as a Self-Executing Anonymous Function and contains two major parts:

  1. The first is the anonymous function with lexical scope enclosed within the Grouping Operator (). This prevents accessing variables within the IIFE idiom as well as polluting the global scope.
  2. The second part creates the immediately invoked function expression () through which the JavaScript engine will directly interpret the function.

10. What do you mean by strict mode in javascript and what characteristics of JavaScript strict mode?

ECMAScript 5 introduced the concept of “strict mode”. It allows you to place a program, or a function, in a “strict” operating context. This strict context prevents certain actions from being taken and throws more exceptions. 

In normal JavaScript, mistyping a variable name creates a new global variable. In strict mode, this will throw an error, making it impossible to accidentally create a global variable.

In ‘Strict mode, all forms of errors, including silent errors, are thrown. As a result, debugging becomes a lot simpler.  Thus there are fewer chances of a programmer making an error.

To sum up characteristics of strict mode in javascript
  • Duplicate arguments are not allowed 
  • The JavaScript keyword cannot be used as a parameter or function name.
  • The ‘use strict’ keyword is used to define strict mode at the start of the script. Strict mode is supported by all browsers.
  • The creation of global variables is not allowed in ‘Strict Mode

11. Mention some advantages of javascript?

  • Javascript is executed on the client-side as well as server-side also. 
  • To use JavaScript on the backend, one needs to learn NodeJS. It is the only JavaScript framework that may be used on the backend
  • Javascript is a simple language to learn
  • Web pages now have more functionality because of Javascript
  • Javascript is quick to use for the end user

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