That’s the reason an increasing number of aspiring developers and programmers are learning it in an individual manner or as part of superior web development courses in India. As a versatile language, JS brings plenty of new opportunities and possibilities.
While it is famous as one of the top front-end development languages, it can also be used for back-end development using NodeJS. Furthermore, it plays a good role in the development of mobile apps, desktop apps, and PWAs (progressive web apps) as well.
This write-up is dedicated to you, if you have been searching for:
- JS interview questions on specific topics like OOPS, promise, closure, hoisting, etc.
Let’s get started!
Over the years, it has become an in-demand language, and an increasing number of people are learning it with reliable web development course online or offline.
When you need access to the features of a function that is located outside the inner function, the role of JS closure comes into play. It is such an option that it helps even when the function located outside is closed.
In JS, an array is a variable that can store multiple values. In simple terms, you can say that an array has the ability to store a set of more than one item with the name of a single variable.
The following are the main characteristics of the JavaScrip array:
- Ability to build shallow copies
- Feature to store collection of different items or data types
The function keyword in JS is used to execute the functions.
An object in JS is like a thing in real life, having some properties, methods, behaviour, etc. For example, a bike is an object with properties like color, brand name, model, etc. The methods of this bike can be a brake, stop, start, drive, etc.
If the value is Not a Number, it will show true. It should be noted that this function first changes the values to numbers and then checks whether it is NaN or not.
9. What is the difference between client-side and server-side JS?
|Users can see the source code||Users can’t see the source code|
|It serves the requests of the users and shows the content accordingly.||It is used to manage the database and other backend aspects.|
|Dependent on browser||No dependency on browsers|
|Runs on the web browsers||Runs on the web servers|
|Less data security||Strong data security|
Yes. It is a case-sensitive language.
DOM stands for Document Object Model. It is an interface in coding that is used to represent the data and content of a web document. It helps in enabling programs to modify the style, content, and structure of a web document.
An operator in JS is simply a symbol that is utilized to do specific operations, like addition, subtraction, and more. The operations are done on values and variables, or you can call them operands.
Let’s understand it with an example. If you want to add two values, then the “+” operator will be used.
10 + 45: // 55
Here, + is an arithmetic operator, which has added the given values. 10 and 45 are the operands here.
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a) Assignment Operators
Used when you need to assign values to variables.
b) Comparison Operators
Role of these operators is when you want to do a comparison of two values and then have a result. The result will be a boolean value, showing true or false.
|!=||Not equal to|
|===||Strict equal to|
|!==||Strict not equal to|
|>=||Greater than or equal to|
|<=||Less than or equal to|
c) Arithmetic Operators
The role of these operators is to do arithmetic calculations.
d) Logical Operators
Used to do logical operators. After the operation, it shows a boolean value (true/false) as the result.
|| |||Logical OR|
e) String Operators
Used for concatenation of strings.
|typeof||To know the type of the variable|
|instanceof||To know whether an object is an instance of an object type|
f) Bitwise Operators
Used when you need to do operations with binary numbers.
|>>>||Zero-fill right shift|
Since it is passed in functions to call back the arguments in more functions as well, its name is justified. It must be noted that you can use callback in other functions only if they are already executed.
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16. How to write hello world in JS?
// write hello world program in JS
// write hello world program in JS
// writing hello world program in JS
JS comments are simply used to represent a comment or message related to the code. For example, we can use comments to mention details of the code, recommendations, warnings, etc. It should be noted that the JS engine will neglect the comments.
The runtime environment is the place to run or execute a program. This environment can access the objects, libraries, etc. and hence runs the code accordingly.
Generally, there are two types of JS environments used by a developer. One is the browser-based runtime environment, and another is the NodeJS-based runtime environment.
The browser-based runtime environment makes use of a JS engine, web APIs, an event loop, and the callback queue.
a) Fast Performance
Since JS is an interpreted language, you don’t have to compile it every single time it runs. As a result, it speeds up the development process. Furthermore, it runs on the client side, where it doesn’t depend on the server. This again optimizes the performance of the website or web app by avoiding the need for requesting resources from the web server.
b) Easy to Understand and Simple
Even if you are a beginner with no knowledge of coding, you will find this language easy to learn. Opting for an offline or online web development course further makes things effortless for you so that you can become well-versed with the complete website and web application development skills.
c) Works Well With Other Languages and Systems
d) Cross-platform Development
e) Frameworks and Libraries
a) Creating Interactive Web Pages
Using JS, developers can make web pages more interactive and appealing. For example, new features can be added that bring some functionalities to the users and also enhance the overall UX.
- Hover colours on buttons
- Carousel slider
- Images zoom in/out
- Countdown timer
- Hamburger menu
b) Web-based Game Development
c) Web App Development
e) Web Servers
- User input validation when a user is about to submit a form
- Show pop-ups and dialog boxes
- Change the look of HTML docs
- Setting up dynamic forms
- Show date and time
b) Tricky Debugging
Since JS runs on the client side in the browser, the debugging options and features are limited.
|Object-oriented programming language||Object-based scripting language|
|Can be used for complicated tasks and processes||Can’t be used for complicated tasks|
|Needs code compilation||Text-based code|
|Independent language||Needs to be used with HTML|
|Strongly typed programming language. Need to declare variables before using them in the program.||Loosely typed language. No issues whether data types are declared or not|
|It’s statically-type||It’s dynamically-typed|
|High memory consumption||Low memory consumption|
|Saved as byte code||Saved as source code|
|For concurrency, it uses threads||For concurrency, it uses events|
|.java extension used to save programs||.js extension used to save programs|
|Supports multithreading||Doesn’t support multithreading|
|Objects are based on class||Objects are based on prototype|
|Need JDK or Java Development Kit to run the code||Need text editor to run the code|
|Primarily used for backend development||Can use for both front-end and back-end|
It was developed in 1995.
28. What is a named function in JS?
29. What is an anonymous function?
Unlike named functions, anonymous functions are the ones that do not have any names. Such functions are declared without the use of identifiers.
|A trademark of Oracle Corp.||Microsoft owns JScript|
|No support for active content creation||Supports active content creation|
|Can manage multi-browser compatibility||Supports Internet Explorer only|
|Code runs in browser||Code runs only in Internet Explorer|
|Can’t access objects of the browser||Can access objects of Internet Explorer|
|Widely used||Not popular as it is supported by Internet Explorer only|
Must Read: Top 7 Front-End Development Languages 2023
31. Which are the different states of a JS Promise?
- rejected: when the operation fails
- pending: when it’s neither rejected nor fulfilled
- fulfilled: when the operation is successful
a) preventDefault() method
It is used when you want to prevent the default action of elements in the browser. You must know that not all actions can be cancelled, and is applicable to only cancellable events.
b) stopPropagation() method
It is used to stop the propagation of the parent element so that it can not find access to specified events.
|Using new keyword||var objectname=new Object();|
|Using object constructor||var objectname=new Object();|
34. Which method can we use in order to find out the character from a particular index?
For this, we can use the string charAt() method.
var str=”WsCube Tech”;
For doing so, we need to add the below-mentioned script tag to the header:
Here, wscubetech.js is the name of the external JS file we are looking to use.
A prototype in JS is one of the most useful objects using which developers can inherit one object’s features to another. You can say that a prototype is linked to all the objects and functions in JS. That’s what it makes capable of inheriting the features of an object.
Since numerous actions in JS are asynchronous, these actions are not capable of holding a program from running before completion. Using the callback function, we can define when to run the code.
38. What is the use of window object in JS?
Moreover, you can say that the global variables of JS are considered properties, whereas the global functions are considered methods of the window.
The use of the JS history object is to find the web URLs that a user has visited in the browser. In addition, it also helps in browsing the last and next pages, as well as specific pages that a user has visited.
As an important property of the window object, the history object is accessible by:
For adding JS code to HTML code, the <script> tag is used. The JS code that you want to add to the HTML is used within the <script> tag, whether you want to place it in the <head> or <body> section.
- Adding information about the program or specific lines of code, helping the users to get the scope or use of that code.
- There are several instances when you need to write a few lines of code on a temporary basis. On such occasions, you can add a comment to the code like “to be removed later on”. And when you want to remove it, the comment helps you easily find that code, instead of searching or reading every line of code.
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42. What are the different types of JS comments?
To answer, start by saying that there are two types of comments.
a) Single-line comments
These comments are written with two forward slashes (//) before your message of the comment. You can use single-line comments before and after the statement.
// This is your single-line comment
//adding the value of variables x and y
document.write(c);//add 15 and 30
b) Multi-line comments
Usually, the developers prefer multi-line comments more because these can be used to represent single lines and multiple lines of comments both.
For using it, you need to use a forward slash with an asterisk (at the opening of the comment), and an asterisk with a forward slash (at the closing of the comment).
/* This is your multi-line comment.
Browser won’t display it */
null in JS is an object which is used to represent that a variable doesn’t have any value. This is not available by default. The developer needs to do it intentionally.
Whereas, undefined is a global object in JS which means that the declared variable has not been assigned any value.
As you might know, the meaning of void is empty. In the case of JS, the void is actually an operator with its role when a function doesn’t return any result. The result it returns will be undefined.
45. Why do we use strict mode in JS?
Using it, we can modify the normal JS semantics, such as converting the silent errors to throw errors. As a result, silent errors can be avoided. In addition, the performance of the code can be improved in several cases when we use the strict mode.
It is a technique in JS that helps in using a variable, function, or class before the code execution or declaring these things.
In simple words, the role of JS hoisting is to have the feature to call functions, variables, or classes even if you have not defined them in your code.
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48. What is an argument object?
It is an in-built JS object that can be accessed only inside a function. Also, this function needs to have the values of the arguments being passed.
49. How can you use JS to find the operating system of the client machine?
We can make use of the navigator.appVersion string to find the client OS version.
50. How can you use JS to modify the background color of an HTML document?
We can use the following code to do so:
In place of “blue”, we can use any colour that we intend to use.
JS exception handling is performed using try, catch, finally, and throw keywords. Developers can use try or catch blocks to handle exceptions easily.
52. Explain the role of the debugger keyword in JS.
// Function Body
The below syntax can be used for creating a function in JS:
55. Is it possible to assign an anonymous function to a variable?
Yes. We can do it.
a) Web Developer
c) Front-end Developer
d) Full-Stack Developer
e) Senior Web Developer
f) Senior Full-Stack Developer
e) Explain the uses and applications of JS.
These are the most common questions:
a) Differentiate between attributes and property.
b) How to access HTML elements in JS code?
c) Explain the concept of event bubbling.
d) What is the difference between undefined and undeclared?
e) How to add JS code to an HTML file?
f) How to define a variable?
h) Explain the difference between undefined and null.
i) What is isNan in JS?
j) How many types of comments are there in JS? Which symbols are used for comments?
Please find the frequently asked JS interview questions:
a) What is negative infinity?
b) Explain the concept of imports and exports in JS.
c) How to add new elements dynamically?
d) How can you check the OS of the client system?
e) Explain the difference between SessionState and ViewState.
f) How to convert the string to an integer?
g) Explain the difference between call and apply.
h) What is variable typing?
i) How to make an array empty?
j) Explain the concept of strict mode and ways to enable it.
k)What are escape characters?
Below are a few of the top JS interview questions:
a) What are arrow functions? When do you use them?
b) Explain the difference between ES6 Map and WeakMap.
c) What is equality?
d) What is Function.prototype.bind?
e) The this operator in JS is inconsistent. Explain the reason behind it.
f) Explain the difference between cont and object.freeze().
g) What is a deep freeze object?
h) Explain the temporal dead zone in ES6.
i) What is Prototype Design Pattern?
j) What is IIFE?
b) Why do we use vanilla script?
c) How to create an object in vanilla JS?
d) Explain the difference between event bubbling and event delegation.
f) Tell about the instances when you should use vanilla JS rather than frameworks.
h) How to delete a property from a vanilla object?
i) How to do substitution in vanilla?
j) What is string interpolation in vanilla JS?
b) Basic knowledge and skills in HTML and CSS
c) Know functional programming in JS
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