JavaScript Interview Questions

JavaScript (JS) is a lightweight, interpreted, or just-in-time compiled programming language with first-class functions. While it is most well-known as the scripting language for Web pages, many non-browser environments also use it, such as Node.js, Apache CouchDB and Adobe Acrobat.

Due to its popularity and usage, there are lot of openings for people having expertise in Javascript. Hence it is important to prepare & read through popular Javascript interview questions. These questions will help you gain and also test your expertise in this area.

Below are the top 100 Javascript interview questions to help you prepare for the interview :

Q 1: Why do we even need JavaScript if we have Java?

Ans – JavaScript is a client-oriented language where Java is specific to object oriented. JavaScript can also be used with HTML. JavaScript will be used specific to client and mostly deal with actions performed in browser. With Java we can handle server side. Java need compilation to at server to do anything.


Q 2: How do you find out the type of a variable?

Ans – There is a unary operator called typeOf which tells about the type of a variable. This is very helpful because sometimes we need to first find out the type of a variable before doing any comparison or any operation.


Q 3: Does Javascript do automatic type conversion? If yes, give an example. For no, give the reason.

Ans – Yes, Javascript supports automatic type conversion. It goes out of its way to accept every problem which is given to it. For example.


//Returns – 0;

Console.log(“20” -1);

//Returns – 19;


Q 4: What is control execution? Does JavaScript support control execution?

Ans – Control execution is the process of executing the statements from top to bottom where it is controlled with conditions. JavaScript also supports the control execution like Java. It has the same keywords like if, for and while.


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Q 5: When should we use do-while loop instead of while?

Ans – “while” and “do-while” loop both servs the same purpose but do-while loop checks the condition after executing the inner code once. Being said, if there is a requirement to run the code at least once without checking the condition then “do-while” is the best.

Q 6: Explain about self-executing functions?

Ans – JavaScript support self-executing functions. These functions are executed by itself. They are executed just after it’s definition.


Console.log(“this is a self-executing function.”);})()


Q 7: How can javaScript help to remove any element from a website?

Ans – JavaScript has 2 methods remove() and removeChild(). Both of these can be used to remove the HTML element from a website. Remove method removes the HTML element whereas removeChild returns the deleted element.


Q 8: How session state is different than view state?

Ans – Session state is an option which is common across all pages where as view state is used for accessing a session only. For different sessions view State will be different and session state can be same.


Q 9: What are escape characters? How are they used in JavaScript?

Ans – Sometimes user has to work with special characters for specific requirement. These are double quotes, single quotes or ampersands. For such cases escape characters are used in JavaScript. These characters can be displayed with a backslash.


Q 10: How to describe arrow functions?

Ans – Arrow (=>) function are also behaves like other function. Only different is with the definition. The arrow functions are written with an arrow in between parameter and implementations.

For example

Var arrowFunction = () => {return true;};

Q 11: What is the concept of optional arguments?

Ans – Java script is a broad-minded language and it doesn’t care much about number of arguments passed. If any method accepts only one argument but user calls it with more than one so java script will ignore other arguments. These parameters are called optional arguments.


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Q 12: What is confirmation box?

Ans – Confirmation box a feature of java script. If we call confirm method then it will show a confirmation pop up with two buttons. As per user response it communicates the selection back to the parent page.


Q 13: What is closures in Javascript?

Ans – The ability to treat functions as values, combined with the fact that local bindings are re-created every time a function is called closure.  Advance feature of this is to bind a particualr value of local binding.


Q 14: If parameters are not known for any function then how can we define it?

Ans – There is an interesting feature called “Rest Parameters”. Whenever there are undefined number of parameters then REST Parameters can be used. It is defined with three dots (…).

Example – function max (…numbers)


Q 15: Is it possible to get the server on client machine?

Ans – Yes, this can be done with JavaScript. This can be done with string. User just need to call it at right place to get it. Sometimes we need to define the host name for IP address to make navigator to work.

Q 16: What is higher order functions?

Ans – Generally creating a big function is not a good idea because there are more chances errors. Also big functions are difficult to understand. To avoid such issues, bigger functions can be divided into smaller one and one outer function can call other small functions. The bigger function which calls all these are known as higher order functions.


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Q 17: What are the ways to define variables in javascript? Is it only var?

Ans – As per latest version of JavaScript, variables can be defined with three types.

  1. Var (This is the old way)
  2. Const – This is introduced recently
  3. Let – This one was introduced after angular 2 and typescript


Q 18: What is alert box? How is it useful?

Ans – As the name suggest, it shows alert to the user with any message. It only contains one button. This is used only for informational or warning purpose. This is very useful to show confirmed warning. Even developers use it for debugging as well.


Q 19: What is is NaN and how is it useful?

Ans – NaN is known as Not a Number. As we already know that JavaScript doesn’t have a lot of data types and it is hard to differentiate between a string and a number. This method can be used to validate if a particular variable is number of not. This will avoid any kind of error which a operation can cause on string or any other datatype.


Q 20: What is Encapsulation and does JavaScript support it?

Ans – JavaScript also follows Object Oriented programming concepts and Encapsulation is one of them. As per it, program should be divided into smaller pieces and each piece should be responsible for single thing. Others don’t need to worry about how it is happening. This is called encapsulaion.