Basic & Tricky Top JavaScript Interview Questions And Answers

JavaScript interview questions
 
JavaScript has been around since 1995. Developed by Brendan Eich, it is a popular language employed by top tech giants like Facebook and Google. JavaScript helps build desktop-like web applications. However, with the launch of Node.js, JavaScript has become a top choice for creating server-side software.

 

It is an interpreted, high-level programming language that follows a multi-paradigm approach. The programming language has multiple traits of functional programming. It is widely employed to build server-side software and dynamic web pages.

 

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Today, even the Internet is not expansive enough to capture JavaScript’s versatility. So, as a programmer, if you wish to commence a career in JavaScript, there is no better time than now. You can start looking for a job in the field.

 

However, given the booming career prospects, you may encounter stiff competition. Hence, it is vital to be well-prepared to excel in the interview. We are here to help you ace the interview. This guide will discuss the top 100 JavaScript programming interview questions and answers.

 

Basic + Advanced JavaScript Interview Questions And Answers

A. Basic JavaScript Interview Questions And Answers

Ques 1. Can you tell me the difference between Java and JavaScript?

It is one of the most JavaScript important interview questions and answers.

Ans.

  • Java is an Object-Oriented Programming language, and JavaScript is an Object-Oriented Scripting language.
  • Java code requires compilation, but the JavaScript code is all in text format.
  • Java builds apps that run in a browser or virtual machine, but JavaScript codes run solely on a browser.

 

Ques 2. List the different kinds of JavaScript data types.

These core JavaScript interview questions and answers are common in several interviews.

Ans. Broadly, JavaScript supports seven data types:

  • Null – Represents Unknown, nothing, and empty values.
  • String – Represents alphanumeric, multi-character, and single-character values.
  • Boolean – Represents the true and false values.
  • Number – Represents floating-point and integer values.
  • Undefined – Represents unassigned value. If a variable is declared but not assigned in JavaScript, it implies an undefined data type.
  • Object – Used to store collections of complex entities or data.
  • Symbol – Used to create a unique identifier for objects.

 

Ques 3. Between JavaScript and an ASP script, which is faster?

This is a very basic question to check your foundational knowledge.

Ans. Of ASP script and JavaScript, the latter is faster. It is a client-side language. So, it would not require the webserver’s help for execution. ASP, on the other hand, is a server-side language. Thus, it is slower than JavaScript.

 

Ques 4. Who developed JavaScript?

It is one of the more JavaScript important interview questions and answers.

Ans. In September 1995, a Netscape programmer, Brendan Eich, created JavaScript. He developed this scripting language in only ten days.

 

Ques 5. What was it originally called?

Ans. At the time of its launch, JavaScript was called Mocha. After this, the developer named it Live Script before it was called JavaScript.

 

Ques 6. Why should we all study JavaScript?

How can you not encounter these basic JavaScript interview questions and answers in an interview?

Ans. JavaScript is one of the top three languages that all developers must learn.

  • HTML to define the web page content. It is also known as the structure of web pages.
  • CSS to give styling or specify the layout of the web pages. It is known as the cover or the body of the skeleton.
  • JavaScript to boost the workability of the web pages. It gives them a behavior.

 

Ques 7. What is the spread operator in JavaScript?

It is one of the top JavaScript interview questions and answers.

Ans. The spread operator allows the iterables to expand to a single element or argument.

 

Ques 8. List out the features of JavaScript.

It is one of the most frequently asked JavaScript programming interview questions and answers.

Ans. Some top JavaScript features include:

  • Interpreted programming language
  • Complementary to HTML
  • Open-source
  • Complementary to Java
  • Good for the network-centric applications
  • Lightweight
  • Cross-platform

 

Ques 9. Mention the different kinds of JavaScript data.

Ans. JavaScript data are of the following types –

  • String
  • Function
  • Boolean
  • Object
  • Number
  • Null

 

B. Top JavaScript Interview Questions And Answers For Freshers

We have discussed the main basic JavaScript interview questions and answers above. Now, let us discuss some core JavaScript interview questions and answers for freshers.

 

Ques 10. State the difference between == and ====.

It is one of the JavaScript tough interview questions and answers. An incorrect answer here could be a clear red flag for the interviewers.

Ans. == performs any vital conversions before comparison, but ==== does not. The equality table in JavaScript is pretty infamous. Hence, it is a modus operandi to use === when you compare values for equality.

 

Ques 11. List some of the advantages of JavaScript.

It is not one of the JavaScript technical interview questions and answers. But, if you have studied JavaScript, you must know the reply to this.

Ans. Some pros of JavaScript are:

  • It has rich interfaces.
  • It has high interactivity.
  • Server interaction is less.
  • Immediate visitor feedback.

 

Ques 12. Now, talk about some disadvantages of JavaScript.

You are applying for a job in JavaScript. So, should you be bothered about its negative aspects? Yes, these are JavaScript tricky interview questions and answers. So, please do not ignore them.

Ans. The top JavaScript limitations include:

  • No multi-threading support
  • No multi-processing support.
  • File writing and reading are not allowed.
  • No networking applications support.

 

Ques 13. What variables do we use in JavaScript?

It is one of the advanced JavaScript interview questions and answers for freshers.

Ans. We use the data type variables in JavaScript.

 

Ques 14. Explain Session state and View State.

Ans.

  • Session state is where the user or browser spends on all application pages.
  • View State is the point of time where the user spends on only the session’s page.

 

Ques 15. List the comparison operators supported in JavaScript.

It is one of the most frequently-asked JavaScript important interview questions and answers.

Ans. JavaScript supports the following comparison operators:

  • > Greater than
  • < Less than
  • <= Less than or equal to
  • >= Greater than or equal to
  • == Equal to
  • != Not Equal to
  • === Equal to with datatype check
  • !== Not equal to with datatype check

 

Ques 16. List the different kinds of popup boxes in JavaScript.

It is one of the standard JavaScript programming interview questions and answers.

Ans. The three kinds of Popup boxes in JavaScript include:

  • Alert
  • Confirm
  • Prompt

 

Ques 17. How can you get the primitive value of a string in JavaScript?

Ans. ValueOf() method gives the primitive value of a string in JavaScript.

 

Ques 18. How to declare variables in JavaScript?

Ans. JavaScript shows variables with the var keyword. So, the variable should start with the A letter, $ or _.

 

C. JavaScript Coding Questions And Answers For Interview

Now, from the fresher questions, let us move ahead and discuss a few coding questions in the JavaScript interview.

 

Ques 19. Find the output.

You cannot avoid JavaScript tricky interview questions and answers when you want a job in the field.

console.log(“0 || 1 = “+(0 || 1));

console.log(“1 || 2 = “+(1 || 2));

console.log(“0 && 1 = “+(0 && 1));

console.log(“1 && 2 = “+(1 && 2));

Ans. The output will be:

0 || 1 = 1

1 || 2 = 1

0 && 1 = 0

1 && 2 = 2

 

Ques 20. How can you create an array in JavaScript?

It is one of the top JavaScript coding questions and answers for interviews.

Ans. Array stores several values in a single variable.
Creating an array:

var mobile = [“Oppo”, “ Samsung”, “Vivo”]

By using a new keyword:

Var mobile = new array(“ Oppo”, “Samsung”, “Vivo”)

 

Ques 21. Explain callback function

Ans. It is a function passed to another function as an argument, executed after completion of the operation.

 

Ques 22. What happens when an Infinite while loop runs in the browser?

It is one of the advanced JavaScript interview questions and answers, to which the interviewee must say – It will cause the program to crash in the browser.

 

Ques 23. What do you understand by NaN property in JavaScript?

Ans. NaN is the Not-a-Number value. It signifies a value that is not a legal number.

typeof of a NaN will give a Number. However, to assess if the value is NaN, use the isNaN() function.

 

Ques 24. Find the output.

var a=3; var b=a++; var c=++a; console.log(a,b,c)

Ans. Output – 5 3 5

 

Ques 25. How to create objects in JavaScript?

It is one of the most-asked JavaScript coding questions and answers for interviews to which you can calmy reply, since, JavaScript is an object-oriented scripting language, it is easy to create objects.

var std = { name: “ John”, age: 18, id: 90 };

 

Ques 26. What output will you receive for undefined * 2 in JavaScript?

Ans. Undefined * 2 will give nan output.

 

Ques 27. List some ways to create an array in JavaScript.

You will find several JavaScript tough interview questions and answers. It is one of them.

Ans. Typically, there are three different ways to create an array in JavaScript. These include:
1. By array literal
usage:

var myArray=[value1,value2…valueN];
2. By creating instance of Array
usage:

var myArray=new Array();
3. By using an Array constructor
usage:

var myArray=new Array(‘value1′,’value2′,…,’valueN’);

 

D. JavaScript Technical Interview Questions And Answers

Below, we will discuss a few JavaScript tricky interview questions and answers.

 

Ques 28. How to read the properties to JavaScript?

It is one of the core JavaScript interview questions and answers.

Ans. You can write and read the JavaScript properties with the dot(.) notation.

 

Ques 29. How can you achieve primitive values of a string in JavaScript?

Ans. In Javascript valueOf() method is used to get the primitive value of a string.

For instance,

var myVar= “Hi!”

console.log(myVar.valueOf())

 

 Ques 30. Is it possible to redirect a page to another with JavaScript?

Ans. You can redirect the page to another URL or page with JavaScript by using the location, replace, and location.assign.

 

Ques 31. How to submit a form in JavaScript?

Even though it is not one of the JavaScript tough interview questions and answers, you will find such questions in several interviews:

Ans. To submit a simple form, use the following code:

document.form[0].submit();

 

Ques 32. Talk about some prevalently used JavaScript testing frameworks.

Ans. Some prevalently used JavaScript Frameworks include:

  • Nightwatch
  • Mocha
  • Puppeteer
  • Karma
  • Jasmine
  • Jest

 

Ques 33. Point out HTML DOM mouse events.

Ans. Here are the HTML DOM mouse events:

  • onclick
  • ondblclick
  • mousemove
  • mousedown
  • mouseover
  • mouseout
  • mouseup

 

Ques 34. How can you formulate a cookie with JS?

Ans. A cookie is a data set saved on the computer and accessed by the browser.

You can create a JavaScript cookie with the following:

document.cookie = “cookiename=John”; expires = date”;

 

Ques 35. Can you tell me the result for 3+2+”7″?

Ans. 2 and 3 are integers, added numerically. Since 7 is a string, system performs concatenation.

The outcome will be 57.

 

E. Advanced Javascript Interview Questions And Answers

Now, let us discuss some advanced questions asked in the interview.

 

Ques 36. You want to move an element from lowercase to uppercase from an array. Tell us how you will do it?

Ans. toUpperCase can return the string value to uppercase.

 

Ques 37. What will the statement declare?

var myArray = [[[]]];

Ans. It will declare that it is a three-dimensional array.

 

Ques 38. What are the looping structures in JavaScript?

Ans. In JavaScript, the looping structures are:

  • For
  • While
  • Do-while loops

 

Ques 39. Is JavaScript case-sensitive?

Ans. Yes, JavaScript is a case-sensitive language.

 

Ques 40. Does JavaScript have a concept-level scope?

Ans. No, there is no concept-level score. The variable inside the operation has the scope inside the function.

 

Related: Interview question & answers for: SQL, Web designing, Selenium, Python

 

Bonus Basic & Advanced JavaScript Interview Questions And Answers!

Ques 41. How would you check if a number is an integer?

 

Ques 42. What is a name function in JavaScript & how to define it?

 

Ques 43. What is Object Destructuring?

 

Ques 44. What are the rest parameter and spread operators?

 

Ques 45. Write the code to force a page to load another page in JavaScript.

 

Ques 46. What are escape characters in JavaScript?

 

Ques 47. How are object properties assigned?

 

Ques 48. Could you explain escape() and unescape() functions?

 

Ques 49. How to convert an Object {} into an Array [] in JavaScript?

 

Ques 50. What is the difference between Call & Apply?

 

Ques 51. What is recursion in a programming language?

 

Ques 52. How to check if an object is an array or not? Provide a code.

 

Ques 53. Please write JavaScript code for adding new elements dynamically.

 

Ques 54. Write about the errors shown in JavaScript?

 

Ques 55. Explain WeakSet in javascript.

 

Ques 56. What are the scopes of a variable in JavaScript?

 

Ques 57. What are object prototypes?

 

Ques 58. How to use Loop in JavaScript?

 

Ques 59. How will you remove duplicates from a JS array?

 

Ques 60. What are Exports & Imports?

 

Ques 61. What is QuickSort Algorithm in JavaScript?

 

Ques 62. Discuss examples of non-bulean value coercion to a boolean one.

 

Ques 63. What are the different states of a Promise?

 

Ques 64. What is OOPS Concept in JavaScript?

 

Ques 65. What are the ways to define a variable in JavaScript?

 

Ques 66. What is hoisting in JavaScript?

 

Ques 67. What are screen objects? State their various properties.

 

Ques 68. What is the difference between a web garden and a web frame?

 

Ques 69. What is the role of deferred scripts in JavaScript?

 

Ques 70. How to empty an array in JavaScript?

 

Ques 71. What is Promise chaining?

 

Ques 72. What is the unshift() method?

 

Ques 73. What are the decodeURI() and encodeURI()?

 

Ques 74. What is the difference between innerHTML & innerText?

 

Ques 75. How are event handlers utilized in JavaScript?

 

Ques 76. What is DOM?

 

Ques 77. How to print Statements in JavaScript

 

Ques 78. Can you draw a simple JavaScript DOM (Document Object Model)?

 

Ques 79. Explain Closures in JavaScript.

 

Ques 80. What do you mean by blur function in JavaScript?

 

Ques 81. Name some of the built-in methods and the values returned by them

 

Ques 82. What is the use of the Push method in JavaScript?

 

Ques 83. What’s a pure function, and why should you care?

 

Ques 84. What is the use of promises in javascript?

 

Ques 85. Give an example of combining multiple Promises.

 

Ques 86. How to handle exceptions in JavaScript?

 

Ques 87. Are ECMAScript and javascript related? How?

 

Ques 88. Name some of the JavaScript Frameworks

 

Ques 89. Explain async/await in JavaScript.

 

Ques 90. What is JavaScript Unit Testing?

 

Ques 91. What are the challenges in JavaScript Unit Testing?

 

Ques 92. What is Promise.all?

 

Ques 93. What is memoization?

 

Ques 94. What is External JavaScript?

 

Ques 95. How would you create a private variable in JavaScript?

 

Ques 96. Define the use of Void(0) in JavaScript?

 

Ques 97. What is the difference between Window & Document in JavaScript?

 

Ques 98. What are classes in javascript?

 

Ques 99. State the differences between declaring variables using var, let, and const.

 

Ques 100. What are the various functional components in JavaScript?

 

How Do I Prepare For An Interview With JavaScript?

Here are some tips to prepare for a JavaScript interview:

  • Know all that you need to study.
  • Formulate a plan.
  • Do not forget the behavioral interview
  • Prepare all the top JavaScript interview questions and answers
  • Be thorough with them.
  • Enroll in mock interviews
  • Be updated about the latest trends and skills
  • Stay confident
  • Go for it.

 

Frequently Asked Questions

Ques 1. What is JavaScript?

Ans. JavaScript is an interpreted, lightweight programming language capable of object-oriented functions. It enables you to build interactivity into static HTML pages. Its general-purpose core is embedded in Internet Explorer, Netscape, and other browsers.

 

Ques 2. Who invented JavaScript?

Ans. Brendan Eich created JavaScript.

 

Ques 3. What are the advanced topics in JavaScript?

Ans. The advanced topics in JavaScript include:

  • Scope and the JavaScript compiler
  • Execution of function code
  • Lexical scope
  • IIFE Pattern
  • Dynamic scope
  • Explicit binding
  • Closures
  • Object orienting prototypes
  • Async patterns
  • Solving callback problems

 

Ques 4. How do you practice JavaScript coding?

Ans. To practice JavaScript coding, you need to:

  • Build JavaScript projects
  • Improve some existing projects
  • Complete code-related challenges
  • Join an open-source project
  • Be a part of the coding community
  • Share your learning experience and projects with others for feedback.
  • Write articles on coding to share and learn well.

 

Ques 5. Is JavaScript synchronous or asynchronous?

Ans. JavaScript is asynchronous.

 

Ques 6. What is ES6 in JavaScript?

Ans. It is the newer version of JavaScript introduced in 2015.

 

Ques 7. Is JavaScript case sensitive?

Ans. Yes, JavaScript is case-sensitive.

 

Ques 8. What is a callback in JavaScript?

Ans. It is a function passed to another function as an argument, executed after completion of the operation.

 

Ques 9. What is the Arrow function in JavaScript?

Ans. It was introduced in the ES6 version of JavaScript and helps build functions in a cleaner format than the regular functions.

 

Ques 10. Is JavaScript multithreaded?

Ans. JavaScript is a single-threaded language.