JavaScript Interview Questions and Answers

What is JavaScript Language?

JavaScript works based on OOP (Object Oriented Programming). For creating web pages, it is a client scripting language. To make a webpage is dynamic and to insert special effects like roll-out and roll over, programmers prefer this JavaScript. JavaScript can be inserted into HTML pages and it is used in web browsers. Let’s discuss JavaScript interview questions and answers.

In this article, you will get the answers to top JavaScript Questions asked in the JavaScript Interview.

What is the name of the company which developed JavaScript?

Netscape Communications Corporation created the JavaScript in September 1995.

What are the characteristics of JavaScript?

Some of the unique and distinguishing features of JavaScript are the following.

  • HTML Supported
  • Good for web browsers
  • Lightweight and high efficiency
  • Interpreted Programming Language
  • Fully controlled browsing
  • Scripting language
  • Object-Oriented based language
  • Within scripts New function creating language

How Java is different from JavaScript?

Java is compiled form and as compared to it JavaScript is interpreted. Java is backend used language and JavaScript is frontend used language.  Applications of Java can run in a virtual machine and on the other hand JavaScript executes in the browser. Java is for different apps and its syntax is like C++ and JavaScript is for browsing apps and its syntax resembles C language.

Discuss the data types of JavaScript?

Here are the six data types of JavaScript language.

  • String
  • Boolean
  • Number
  • Object
  • Array
  • Function

What is the function of an operator?

Like other programming languages, JavaScript operator performs any kind of operation on single or multiple data values and give the output of that operation.

What is an anonymous function?

An anonymous function is a function having no name and it’s the argument about other functions. Such functions are helpful to save from name collisions.

Is JavaScript faster than ASP scrip?

Yes, JavaScript is too much faster than an ASP script because JavaScript language is written for client-based browsers and it does not need a web server’s support and has better execution. ASP scripting is server-side scripting and that’s why it’s slow.

What is Callback function in and JavaScript why we use?

This is the passed argument and it’s a function in this language and it passes to another function. Because of this callback function can take another. The basic reason for this function is to execute code in response to an event.

What is negative infinity?

It’s the fixed value which shows the available lowest value. It means there is no smaller value than it. It can be formulated by dividing a negative number just by zero.

What’s the difference between undeclared and undefined variables?

Undeclared variables are simply those which are not declared in the program and if the program captures and tries to read it then automatically, a runtime error appears.

Undefined variables are the variable that is declared in any program but they are not assigned any value or number. If the program captures and tries to read it then automatically, the undefined value is returned.

What is a debugger in JavaScript?

In all browsers like Mozilla, Firefox, and Chrome, etc. have built-in debugger which stops the execution and performs the debugging function. Normally, by using the F12 key you can generate the debugger and it replicated the function of breakpoints. But make sure your debugging setting must be enabled in browser settings.

What is a prompt box?

A prompt box is used to enter the inputs by users. This input is provided by a text box.

What is “this” keyword?

Keyword “This” is a reference variable that refers to something. It avoids the conflicts of naming in the object method.

What is a JavaScript timer?

During the running of the program, a timer is used to execute the program at a given specific time. This process is done by SetTimout and SetIntercal functions.

What is the drawback of JavaScript?

Due to a single thread, there might be queue up and it can be awaiting process to use a timer in JavaScript.

What is a comment in JavaScript Context?

A comment is used to give additional information about your program.

What is a multi-line Comment?

Multiline comments are used for the description of code and compilers ignore these comments.

What is the symbol of comments in JavaScript?

This is the symbol for single line //

/* Multiline

What is JavaScript hoisting?
Variables and functions are moved into the hoisting process.  These are moved according to their top of the scope.

What is the Strict mode or mistake reducer mode?

In this mode, JavaScript throws back errors and it minimizes the risks of mistake. In the end, the process of debugging becomes so easy when a strict mood is used.

What is == operator?

== it’s called equality operator and it gives true response when the operands have equal value.

What does the !! operator do?

This operator simply converts the value into Boolean.

Discuss what’s an automatic type of conversation?

Because JavaScript is “Loosely typed” language and whenever there is a need for the specific data type, this language automatically converts the data accordingly to that data type which it needs. For example, integer value can be changed into a floating one.

What kind of loops are used in JavaScript?

  • While
  • For
  • Do while

What is the difference between the confirmation box and the alert box?

Ok, and the cancel button is showed by confirmation box while on the other side, only Ok button is showed.

What is Scope?

Scope gives access to variables. JavaScript has the following three types of scopes.

  • Global Scope
  • Functional Scope
  • Block Scope

What is a closure?

This is the feature of JavaScript language in which an inner function gets the access of outer function variables.

What is the purpose of the Boolean object in JavaScript?

Boolean shows the values in two stances; either true or false. The boolean object can be created by Boolean constructor.

function display ( )


  1. document.writeln(10<20);//true    
  2. document.writeln(10<5);//false     
  3. }     
  4. display();  

What are JavaScript Cookies?

Cookies are small files that are created when we visit different sites and these cookies store the username and other shopping details as well.

What are the errors of JavaScript?

The following three types of error occur in JavaScript.

  • Logical Errors
  • Loading Errors
  • Run time errors.


We have discussed some famous JavaScript Interview Questions and Answers. You should not stop your learning journey because as you know changes are made to the languages and should master all the JavaScript functionalities to pass the interview with a good result. If you like this article share it with your friends on Facebook, Twitter, etc.

Muhammad Nauman

Muhammad Nauman a Linguistics Scholar, Content Writer, and Teacher of English as a Second Language is a passionate and dedicated person to his interests in language. He has been teaching for three years in various institutes and has a keen interest in language learning and teaching and is a researcher as well.

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