Top 20 Javascript Interview Questions for 2020

October 15,2020 - 5 min read
Javascript Interview questions.

Over the years, the market and the technology stack for the IT industry has significantly changed. There is a significant increase and improvement in the number of frameworks, libraries, tools and languages that can be used to build technological applications. Javascript is one such language that has emerged as one of the most popular languages in the IT industry.

If you are interested in the world of technology then Javascript is not only a prominent and useful language but it also offers a promising field in terms of career and remuneration.

As expected, the job market for Javascript developers is competitive, it’s easy to learn the basics but it’s not easy to get your first job as a Javascript developer. But hold on, we aren’t scaring you. There is a way you can ace your Javascript development interview.

In today’s blog we have compiled a list of top 20 Javascript interview questions that will help you get a job as a Javascript developer in your dream company. The blog consists of combination of questions that can be asked from a Javascript developer. We have tried to touch upon some important topics related to every domain in which Javascript is used. We hope you make the most of these questions!

Q1). What is JavaScript?

A1) Brenden Rich in 1965 built Javascript as a scripting language. Initially, it was used to add dynamic content on a webpage. After the advent of Javascript, websites were no longer a collection of static web pages but they have now become dynamic with advanced complex features that Javascript adds to the website. Javascript basically makes the website interactive and dynamic.

Q2) What are JavaScript functions?

A2) JavaScript functions are blocks of code that executes a task as many times as you want. The function keyword defines a Javascript function. Javascript functions allows Code reusability as the code is only written once but used several times in a particular application. They make the function compact and efficient.

Q3) What are JavaScript objects?

A3) Javascript objects are a collection of properties and methods. The properties of an object are defined as a key-value pair where the key is the name of the attribute and value is the answer of that attribute. For example, key is color and its value is red. Property of a Javascript objects can be accessed with the help of a dot operator.

Q4) Explain the relevance of JavaScript in front end development

A4) Javascript as a frontend language is used to make websites more interactive. Javascript is used to make a webpage along with languages like HTML and CSS. In a technology stack of HTML, CSS and Javascript, HTML (Hypertext mark-up language) is responsible for the basic content and structure of the website, CSS (Cascading Style Sheet) tells the browser how the content should be displayed by determining things like colour, font, layout etc. and Javascript makes the website interactive by adding elements that the user can interacts with, such as drop-down menus contact forms etc.

Q5) How is JavaScript used in back end development.

A5) Javascript can also be used as a backend language because of a Javascript framework called Nodejs. Nodejs is built on Chrome’s V8 engine and written in C++. It is basically a runtime environment that uses Javascript as its scripting language.

Javascript and its frameworks and libraries are used in a number of stacks as a part of the backend process. The MERN (MongoDb, Express, React, Nodejs) stack consists of a database, both server side and client-side framework and the MEAN (MongoDb, Express, Angular, Nodejs) stack are popular examples.

Q6) List some Frameworks and libraries in JavaScript.

A6) Some of the most popular Javascript libraries are jQuery, React, D3.js. There are a bunch of Javascript frameworks also like Angular, Vue.js etc. Some of the top-notch companies have come up with Javascript frameworks and libraries. For example, React was developed by Facebook and Angular was developed by Google.

Q7) What are JavaScript datatypes?

A7) There are mainly two types of data types in JavaScript and they are Primitive data type and Non-primitive data type. Primitive data type in Javascript are Number, String, Boolean, Undefined, Null and Symbol. On the other hand, Non-primitive datatype include objects in Javascript.

Q8) What is the is isNaN() function in JavaScript?

A8) The isNaN() function is used to determine if the type of a value is number or not. This function returns true if the type of the value is number Otherwise it returns false.

Q9) What is the difference between undefined value and null value?

A9) Undefined means that a variable has been declared but it has not been initialised or assigned any value yet. On the other hand, null is an assigned value which means nothing. Another difference is that null is an object whereas undefined is of type undefined. In simple terms, undefined is a type and null are an object.

Q10) What are the different types of pop up boxes in JavaScript?

A10) Popup boxes are basically used to display some kind of message or notification to the user. In Javascript, there are three types of pop up boxes in JavaScript namely Alert Box, Confirm Box and Prompt Box. An alert dialog box is used to provide a necessary message to the users. An example of alert dialog box is when if one input field requires to enter some text of a particular datatype but the user does not provide any input or enters the value of wrong datatype, then you can use an alert box to give a warning message. A confirm box does what it says. It is used to verify or validate something from the User. When this pop-up box appears, the user will have to confirm to proceed. The prompt dialog box enables you to interact with the user.

Q11) Define exception handling in JavaScript

A11) Exception Handling comes in use whenever any statement generates an error (throw an exception). Instead of going to the next line the Javascript interpreter checks for exception handling.

The exception handling mechanism contains the try, catch, throw and finally statement. The try statement lets you test the code, catch statement handles the error, the throw statement lets you create customised errors. The code is executed in the finally statement.

Q12) Explain 'this' keyword in JavaScript.

A12) ‘this’ keyword in Javascript points to a particular object. It refers to the object it belongs to. The object is determined on how a function is called. The function which is called contains ‘this’ keyword.

Q13) What are prototypal inheritance in JavaScript?

A13) Javascript, just like any other object-oriented programming language, supports all OOP’s concept. The difference is that the but their implementation is a bit different in Javascript. Instead of a class implementation Javascript has a prototype-based inheritance. The Prototype chain is the central concept in Inheritance. The prototype chain consists of the first prototype of the object in the beginning Null as the last prototype in the chain.

Q14) What are built-in method in JavaScript?

A14) Built-in functions in Javascript are functions that already defined and ready-to-use. It actually means that you don’t have to define the function but just call them when required. Developers can use the built-in methods in their code without actually defining them. Javascript has a bunch of built-in methods.

Q15) Explain some built-in methods in detail.

A15) There are various built-in methods in Javascript for objects, arrays, strings etc. For example, the function Includes () check for the existence of a particular property in an array. Another function is Find () which finds any element in an array. The function findIndex() does the same thing except that it returns the Index instead of the value.

Q16) What are view and session state in JavaScript?

A16) The ViewState is maintained only at the page-level, the information in view state cannot be transferred to any other place as it is stored for the clients only. The SessionState is maintained at the session level and the data is accessed across all pages in the web application and can be accessed by any person.

Q17) Explain the '===' operator?

A17) In Javascript, the ‘===’ operator is a strict equality comparison operator. This operator returns false if the values are not of similar type. For example, if 2 and “2” are compared with ‘===’ then it would return false because the type of both the values are different.

Q 18) What is DOM?

A18) The DOM is an acronym for Document Object Model. DOM defines the logical structure of a document. DOM also describes the way a document can be accessed and manipulated.

Q 19) What are cookies and how can they be used in JavaScript?

A19) Cookies are basically data that is stored in small text files on the user’s computer. Cookies are used to remember information about the user whenever he/she visits a particular webpage. Javascript uses the document.cookie property to create, read and delete cookies.

Q 20) What are escape () and unescape () function in JavaScript?

A 20) The escape () and unescape () function in Javascript is used to encoding and decoding of a string. The escape () function is used to encode a strong. It basically makes the string portable to transmit it over any network. The unescape () function then decodes the string. It is used to get back to the original string.

These were the important Javascript interview questions. However, if you feel there are any questions or concepts Javascript that we omitted or if you have any doubts regarding the topics discussed above, please feel free to ask questions and we would be elated to guide you in your journey as a Javascript developer.

This blog was a comprehensive compilation of some possible questions that can be asked in any interview that requires you to know Javascript. If Javascript interests you and you really want to become a successful Javascript developer then it would be best if you go the extra mile and take professional training in which you would get a sound understanding of Javascript along with some interesting projects.

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