Top 10 Most Important JavaScript Interview Questions

JavaScript Interview Questions

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Top 10 Most Common JavaScript Interview Questions.

Looking for JavaScript Interview Questions? Here are the Top 10 Most Common JavaScript Interview Questions.

Alongside HTML and CSS, JavaScript is one of the core technologies of the World Wide Web. JavaScript enables interactive web pages and is an essential part of web applications. The vast majority of websites use it, and major web browsers have a dedicated JavaScript engine to execute it. Source: Wikipedia

JavaScript Interview Questions.

Q1. What is JavaScript?

JavaScript is a scripting language, primarily designed for creating web pages as well as adding interactivity to web applications.

Q2. How does JavaScript work?

This is the common JS Interview Questions asked in an interview. Every browser has three prime components to work. The first one is the DOM (Document Object Model) interpreter. This will take your HTML document and convert it and displays it in the browser. The other small program that is part of the browser is a CSS interpreter, which will style the page and make it look better. The last one is a mini-program in the browser called the JS engine.

  1. Browser loads the HTML file/JS file
  2. JavaScript is an interpreted language(means no compilation required)
  3. Browser(JavaScript engine) executes line by line and waits for events(like clicks, mouseovers, etc) to happen.

Q3. Mention some of the features of JavaScript?

Below are the different features of JavaScript:

  • JS is a lightweight programming language, with interpreted functionality
  • JS is open-source and cross-platform
  • JS is integrated into HTML and Java
  • Designed to create network-centric applications

Q4. Regarding JS what are the different types of JavaScript data?

  1. Strings
  2. Functions
  3. Boolean
  4. Object
  5. Number
  6. Undefined

Q5. Define the common errors which occur in JavaScript?

In general, there are 3 types of error we find in JS which are as follow.

  1. Runtime error: this is the outcome of the misuse of the commands within the HTML language
  2. Load tie error: this is a syntax error and is generated dynamically
  3. Logical error: this error occurs when the logic of the functions are badly performed.

Must Read: 10 Effective and Essential Angular Javascript Interview Questions

Q6. Explain why JS is a case-sensitive language?

JavaScript is a case-sensitive programming language. In JS we use different types of variables, functions and various other identities which should be consistent throughout.

Q7. List down some of the Advantages and disadvantages of JavaScript?


  1. Rich user interface
  2. Increased interactivity(when a mouse hovers on elements like buttons or keyboard accessibility)


  1. Lacks multithreading activities
  2. Not suitable for networking applications
  3. Client-side JavaScript cannot be read or write

Q8. Types of objects in JS and define them?

There are 2 types of objects in JS:

  1. Date Object: This is built within the JS programming. These are created with the use of a new date and can be operated with the help of an available bunch of methods once it is created. This includes the year, month, day, hour, minutes, seconds and even milliseconds of the date object. These are set with the help of local standards of the universal time.
  2. Number Object: these include the dates as it solely represented by integers and fractions. The literals of the numbers get converted to number class automatically.

Let us move to the next JavaScript Interview Questions

Q9. What is closure in JavaScript?

When we define a function within another function (a.k.a. parent function) and are accessing the variables that are defined in the parent functions. The closure accesses the variables in three scopes:

  • Variables declared in their own scope
  • Variables declared in a parent function scope
  • Variables declared in the global namespace
[wpsm_codebox style=”2″]

var globalVar = “abc”;

// Parent self invoking function
(function outerFunction (outerArg) { // begin of scope
// Variable declared in outerFunction function scope
var outerFuncVar = ‘x’;
// Closure self-invoking function
(function innerFunction (innerArg) { // begin of scope
// variable declared in innerFunction function scope
var innerFuncVar = “y”;
“outerArg = ” + outerArg + “\n” +
“outerFuncVar = ” + outerFuncVar + “\n” +
“innerArg = ” + innerArg + “\n” +
“innerFuncVar = ” + innerFuncVar + “\n” +
“globalVar = ” + globalVar);

}// end of scope innerFunction)(5); // Pass 5 as parameter
}// end of scope outerFunction )(7); // Pass 7 as parameter


innerFunction is a closure that is defined inside outerFunction and has access to all variables declared and defined in the outer function scope. In addition, the function defined inside another function as a closure will have access to variables declared in the global namespace.

outerArg = 7
outerFuncVar = x
innerArg = 5
innerFuncVar = y
globalVar = abc

Q10. How to empty the array in JavaScript?

This is the popular JS Interview Questions asked in an i nterview.By following any of the given methods –

  • arrayList = [ ]

Above code will set the variable ArrayList for a new empty array.

  • length =0;

The above code, first of all, clears the existing array by setting its length to 0. This way is useful when you want to update all the other references variables pointing to ArrayList.

  • splice(0, ArrayList.length);

This way of empty the array will also update all the references of the original array.

  • while(ArrayList.length){


This is one of the ways to empty the array.

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