75 Key JavaScript Interview Questions And Answers: You Must Ask First

75 Key JavaScript Interview Questions And Answers: You Must Ask First

JavaScript has been used in website building since 1993, which makes it one of the oldest languages on the Internet. Alongside HTML and CSS, JavaScript was originally intended to make dynamic changes to Web pages without reloading new content from the server. It also made possible what became popularly known as AJAX (Asynchronous Javascript And XML).

Nowadays, all browsers support their latest version and an increasing number of developers are incorporating more advanced features into their projects. This explains why JavaScript is one of the most popular programming languages in today’s world.

As JavaScript is a widely-used programming language, the demand for developers has been steadily increasing. The number of job openings in this field is expected to rise by more than 24% through 2021.

With many opportunities on offer and huge potential for growth within the industry, it’s no wonder that so many people are starting their careers as programmers with JS. And, many companies are looking for experienced engineers with a knowledge of JavaScript. Many of these companies will offer very high salaries to qualified candidates.

So, depending on what role you are interviewing the JS candidate for, as well as their level of experience, there is a range of questions you should ask. The following 75 JavaScript interview questions should be enough to weed candidates out from good to bad depending on your needs.

JavaScript Interview Questions You Must Ask

Let’s dive into some basic background on JavaScript frameworks to help you prepare for your interview.

What does a JavaScript developer do?

A JavaScript developer’s main responsibility is to develop both the front-end and back-end logic of a web application. They are usually supported by back-end developers who work on server-side application logic, which includes connecting the JS with other services for example using REST requests. Front-end developers typically specialize in markup and styling; when working with JS these responsibilities fall primarily upon JavaScript engineers.

Some popular JS frameworks include ReactJS, Angular, EmberJS.

Angular JS

Angular is a JavaScript-based open-source front-end web application framework mainly maintained by Google and the community. It lets you use HTML as your template language and lets you extend HTML’s syntax to express your application’s components simply and elegantly. Angular combines declarative templates, dependency injection, end to end tooling from AOT to DOM manipulation.

What is Angular JS used for?

There are tons of applications built using Angular JS out there! The most popular ones include Instagram’s “Explore” page (built with angular), YouTube Red, UberEats app for Android and iPhone.


EmberJS is an open-source client-side data binding library for developing single-page applications using only JavaScript on top of Handlebars.js templating engine with influences from SproutCore JS Framework (now defunct). Ember also provides a routing facility that makes it possible for different parts of your site or app to be served statically without any page reloads.

Where is Ember JS used?

The primary goal of the framework is to build user interfaces for apps or sites quickly without sacrificing quality code; more specifically, building composable components from reusable parts that can be mixed together like LEGO bricks.

EmberJS is used by many companies like Groupon, LinkedIn, and Walmart.com to build scalable and maintainable applications that are required for their business.


ReactJS was developed by Facebook to build large and complex web applications that are fast, incremental, composable, fluid yet robust enough for a single page application. It provides the performance of an imperative library with a declarative programming approach which can be optimized in various ways such as using immutable data structures or memoization.

What is the purpose of React JS?

The main purpose of React JS is to create a UI that can be used in different browsers and devices. It also reduces the effort needed for cross-browser compatibility issues.

Now that you have learned the basics, you should be able to interview the candidate with confidence. We have split out the interview questions into 3 sections: Beginner, Intermediate, and Advanced.

Let’s go!

25 Interview Questions for Beginner Candidates

1. Where did JavaScript come from?

JavaScript was developed by Netscape Communications Corporation as part of their browser software in 1995 because at that time all other browsers only supported Java applets and Sun Microsystems refused to license their technology. JavaScript also became popular due to it being a lightweight scripting language that could be embedded into any web page without so much fuss. It has since been used extensively on most websites, especially large ones.

2. What is the difference between Java and JavaScript?

Java is a programming language whereas JavaScript is an object-oriented scripting language.

3. What is JavaScript?

JavaScript is a programming language that can be used on web pages. A lightweight interpreted programming language with object-oriented capabilities that you can use to add interactivity into otherwise static HTML pages. The general-purpose core of the language has been embedded in Netscape, Internet Explorer and other web browsers.

4. What are the advantages of JavaScript?

JavaScript has many features such as being easy to learn and use, having interactive features, and enabling fast development time for web pages. It also enables programmers to add animations, videos, or games into their Web pages easily because it does not have any compilation step. And lastly, JavaScript provides an API so programmers can manipulate browser objects like cookies.

5. What are some disadvantages of using JavaScript?

The code created with this scripting language may become unmanageable quickly if not written properly; therefore an organized coding style needs to be followed closely when using it. Another disadvantage would be the lack of static typing which means there will need to be an extra step for checking the data type before passing it into a function.

6. How is JavaScript being used in web pages?

JavaScript can manipulate browser objects like cookies; provides animations, videos, or games to Web pages easily due to its interactive features and no compilation needed; and can add functionality such as changing colors with one click of a button. It also has many other features that make it ideal for use with HTML and CSS.

7. What types of files do most websites need to have content on them?

Most websites will contain an index file named “index.html” which points towards other necessary components like style sheets and scripts that help show up the desired webpage properly instead of all being scattered throughout different folders on the server.

8. What are some of the properties and methods of objects in JS?

Objects have properties and methods, but not all objects will necessarily include both types of property or method. The object’s type can define what it includes among its members (properties or methods). Similarly, any new values created within the script using Object() would be considered to be instances of that type; with their own set of inherited values for those attributes if they exist on that particular kind of object. Beyond these basics, there are also many different built-in functions provided by the JS engine which may vary from browser to browser.

9. Can you compare how JavaScript compares to other languages?

JavaScript is a high-level programming language, which means it focuses on the building blocks of computer programs and uses fewer low-level features than many other languages. It is also not strictly typed; this can be seen as both an advantage and drawback depending on your perspective. JavaScript’s syntax will feel familiar to programmers who have experience with server-side languages like PHP or Ruby—especially if their interest in JS comes from a desire to use that knowledge for front-end development work.

10. What are some of the best practices when using object literals?

When possible, try to avoid creating nested objects where only one property needs changing because this causes unnecessary repetition within code. Instead, create separate variable names for each individual piece of information you need so you can create individual objects when needed.

11. What are some of the best practices for JavaScript modules?

JavaScript modules allow you to use local variables, functions, and constants within a single file. They also make it easier to compartmentalize your code so that different parts don’t overwrite each other’s values by default. However, this ease comes with its own set of drawbacks; because the scope is limited in modules- meaning they only affect what they’re inside of – there’s no way for one module to access another without explicitly using “require” or “import”. This means if two files need coordinates from the same map object but have their own version of it defined then both will be loaded together into memory which could cause performance issues as the file is loaded from disk when only one need be, which can be solved by using constants.

12. What are the data types supported by JavaScript?

The data types supported by JavaScript are undefined, null, boolean (true or false), string, symbol, number, and object.

13. What are the features of JavaScript?

JavaScript features are the following: being a prototype-based language, automatic memory management without programming manual cleanup code such as garbage collection like Java or C++, concurrency (thanks to asynchronous functions), and dynamic typing which means variables can change types based on their values. Here are others:

  • It is a  lightweight, interpreted programming language.
  • It is used to create applications that are more focused on networks.
  • It is a complementary complement to and integrated with Java
  • It is an open and cross-platform scripting language.

14. Is JavaScript a case-sensitive language?

Yes, JavaScript is case-sensitive. The language keywords, variables, function names, and any other identifiers must be typed with a consistent capitalization of letters.

15. How to define a variable?

To create a new variable you need to assign it using the “var” keyword followed by the name of your new variable/object that you want to be assigned with capital letters. The type needs to be specified as well when assigning variables such as: var myNumber = 55; When creating objects use curly brackets instead like this: var areasOfInterest= {name:”Vienna”, population: 67000}; Here is what is going on under the hoods… Similar languages work similarly – when we compare two Java strings for example they have to be matched in the same case, or one of them has to convert to all uppercase.

16. How can you create an object in JavaScript?

In JavaScript, we create an object using the keyword new. For example:

– var person = new Object();

17. How can you create an Array in JavaScript?

In JavaScript, we create an array using the keyword new. For example:

– var person = [“John”, “Mary”];

18. What is jQuery?

jQuery is a JavaScript library designed to simplify HTML document traversing, event handling, animating, and Ajax interactions for rapid web development. It takes care of cross-browser inconsistencies in DOM compliance so you don’t have to do it yourself!

19. What is Callback?

A callback is a function that is passed by an argument or option so that it can be executed after some other function has finished executing. In JavaScript, functions are objects, and because of this functions can take arguments as well as being called from another function.

20. What are the variable naming conventions in JavaScript?

The naming conventions for variables in JavaScript are pretty loose. You can use any letter of the alphabet, a number, or even an emoji as long as it doesn’t start with a character that’s not allowed (such as $).

  • JavaScript variable names are case sensitive; for instance, “test” and “Test” are two different variables.
  • Do not use any of the JavaScript reserved words to name your variables. For example, keyword break or boolean is invalid variable names.
  • JavaScript variables should not start with a numeral and must begin with either a letter or the underscore character. For example, 123name is invalid but _123name or name123 are valid.

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

JavaScript has three ways to define a variable.

  • Var – JavaScript variables are declared to declare a variable and optionally give an initial value. Here’s an example: var a = 10; Variable declarations happen before other program code is processed.
  • Const – Functions that are declared as ‘const’ cannot modify the object on which they are called. Functions marked with ‘const’ can be called on any type of object.
  • Let – Variables that are set to a temporary value, then changed later in the code, may be declared as “let” or function-specific.

22. What are the scopes of a variable in JavaScript?

A variable’s scope is the region of a program in which it can be referred to by its unqualified name. It determines where that variable will be visible and what other variables can refer to it. A top-level or global function has as many scopes for local variables as there are enclosing blocks, which means you could have a different value for everyone!

23. What is the difference between a For loop and a While loop?

A For Loop executes the code block for each iteration of an index, from its starting value to its ending one. A While Loop checks if this condition satisfies – If TRUE it will continue executing until FALSE or some kind of break happens (exit() statement).

24. How does TypeOf Operator work?

TypeOf operator checks the type of a variable. It expects an input parameter (variable or object) and returns true if it is, for example, a string or boolean.

The Typeof Operator also works with objects as well by returning “object” instead of the name of their respective types in this case.

25. How do I create classes with JavaScript?

JavaScript doesn’t really have actual classes but you can make your own pseudo-classes using functions (or call them what they are – objects). This way not only does one object keep its properties when copied like so: newObject = Object(oldObject), but also creates a prototype chain where any changes made to either oldobject or newobject affect both of them since they’re based off each other through function calls.

25 Interview Questions for Intermediate Candidates

26. What is closure in JavaScript?

A closure is one of those things that might not be altogether familiar to you when you first see them but are actually fairly simple once explained. Closures can best be thought of as blocks of code with two parts – outer and inner functions. The outer function has access to the variables defined within its scope (function). But since it’s also being executed from outside of this function’s scope, any changes made there will affect only other areas on the same level or lower levels which have been passed into it by way of assignment or parameter declaration. This means that if the variables are changed within the outer function, they will not affect any changes made inside the inner function.

27. What does “this” refer to in JavaScript?

This refers to a specific object and it is typically used when referencing methods that are defined as part of said object. This is often used inside callback functions where you need access to members on the same this or parent scope level (such as another method). It should not be confused with a window that represents the DOM browser environment in the current page load context.

28. What is the JavaScript module pattern?

The module pattern in JavaScript creates private functions that are not exposed externally but rather defined as part of a self-executing anonymous wrapper with public methods for other code modules to use. All top-level variables are declared either inside this context (to be protected) or on outer levels so they can be returned from the module if needed. Modules should also expose a self-identifying interface that defines the public methods.

29. When can you assign an anonymous function to a variable and pass it as an argument to another function?

Yes! Somewhere in the code, an anonymous function can be assigned to a variable. It can also be passed as an argument to another function.

30. How to create a cookie using JavaScript?

To create a cookie in JavaScript, we can use the document.cookie property to set and get cookies. A simple way to store basic information about a user is by using an unquoted session ID as follows:

document.cookie = “username=Julian”;

It will then be available for all scripts on that domain until updated or deleted.

31. How to read a cookie using JavaScript?

Reading a cookie is even easier than writing one since the value of the document.cookie object is the cookie, so you can use this string anytime you want to access it.

32. How to delete a cookie using JavaScript?

To delete a cookie in JavaScript, we can use the document.cookie property to set and get cookies as follows:

document.cookie = “username=”;

It will then be deleted for all scripts on that domain until it is updated or written again by an external script.

33. How do Let Operators work?

The LET Operator (also known as Lambda expressions) allows you to assign values without declaring them explicitly with var keyword but rather invoking anonymous functions inline which are locally scoped inside of where they’re declared resulting in cleaner code often referred to as functional programming style when applied correctly due to its ability to reduce complexity simplifying things like conditional statements because.

34. What is the difference between Attributes and Property?

Attributes are attached to an object (e.g., window or document) and Property is the state of a property on that object.

35. What are Objects, Methods, Properties?

Objects generate information that can be used by methods; properties store data about objects for later use.

36. Why should you avoid using global variables in jQuery code?

If JavaScript has loaded into your webpage before loading jQuery’s scripts then it might overwrite any variable names allocated with global scope within this page when running some operations such as $(document).ready().

37. In how many ways a JavaScript code can be involved in an HTML file?

There are three ways in which JavaScript code can be embedded into an HTML file: Inline, Internal, External

The JavaScript inline function behaves like a normal one. The difference is that the former is created during runtime while the latter is concatenated to the script when creating, and it cannot be reused or replaced. When we need JS for a function, you can either integrate in the file you are working on or create an external script that will be called by your code whenever necessary. There are also two types of scripts: internal and external scripts.

Internal scripts are just functions that are declared by our code, and they can’t be called externally. External scripts have the same name as your script but must not start with a ‘.’. For example: myscript = function () {};

38. What is the difference between var foo = “bar” and new String(“bar”)?

The first will just create a variable with the name of “foo” and assign it to an equal bar, whereas the second creates a string object which stores data about that particular instance of the bar without having any need for assigning it within the scope or outside of its own context.

39. What is the difference between the operators ‘==‘ & ‘===‘?

One big difference between the “==” and “===” operators is that former makes type correcting comparisons e.g., if you compare a number with a string with numeric literal, == allows for it but === doesn’t because it both checks the value and type of two variables; if two values are not of the same type “===” returns false whereas “==” will return true.

40. What is an IIFE?

IIFE stands for Immediately-Invoked Function Expression – it’s a function that executes right away as soon as it’s defined, using ‘()’ instead of curly brackets:

(function () { return “I’m in!”; })(); // this produces I’m in! on-screen when executed

41. How do you dynamically change DOM elements with JavaScript?

There are two ways to use Dynamic HTML (DHTML) to alter page content at runtime. The first uses scripting code within body tags or table cells; the second involves adding CSS classes based on user input or other events.

42. What is the event loop in JavaScript?

The Event Loop is a mechanism for handling messages that require asynchronous attention such as mouse clicks, page scrolling and keyboard presses. When a message arrives, it gets queued at one of several queues: Message Queue, Window Messages Queue, Set Timers queue (in case you need to do something after some time), or Idle tasks queue which executes idle functions like updating scrollbars every 300ms when nothing else needs your attention.

43. How are objects created with JavaScript?

This can be done using two methods – by declaring an object literal or by constructing an instance of an object prototype via the new operator.

44. Is there any way to debug JS code on- a browser?

Yes, you can use Chrome DevTools for debugging JavaScript.

45. What is the difference between an object and a function in JS?

A Function creates new objects by providing values that are assigned to properties of the Object. The value property stores these values. An object literal or prototype chain can also be used to create Objects without functions being involved at all. Functions follow prototypal inheritance which means they inherit from their parent’s constructor while Objects with prototypes inherit directly from Object(). They have other differences too – see this answer on Quora.

46. How does one debug code written in JavaScript?

You could use Google Chrome Developer Tools (DevTools) for debugging your javascript code on browsers like Firefox, Safari, IE 11, Edge, Internet Explorer and Opera. And you could use Chrome DevTools for debugging your javascript code on Google Chrome browser only.

47. What is the difference between null & undefined?

There is a difference between null and undefined in JavaScript. Null indicates the lack of any value, while Undefined means that we have no idea what type or value to give it.

48. What is the difference between undeclared & undefined?

Variables that have not to be set exist in the program and are not assigned. The runtime will encounter a problem when an undeclared variable is used if it does not exist in the program. Variables that were declared but haven’t been assigned values also exist in the program, but with no value shown when they’ve been executed.

49. What is the difference between innerHTML & innerText?

innerHTML is an attribute of the HTML element that holds a string representing the content to be displayed within the tag. innerText, on the other hand, is not a property or method and it’s used only when writing scripts in JavaScript.

50. What are the differences between primitive and object types in JavaScript?

There are a few small but important differences between primitives and objects. Primms are passed by value while Objects are passed by reference.

25 Interview Questions for Advanced Candidates

51. Does JavaScript support automatic type conversion?

Yes, JavaScript does support automatic type conversion.

52. What is the difference between implicit and explicit casting?

Explicit casts are used to convert from one data type to another, for example converting an integer into a string or vice versa.

Implicit casts can happen automatically when passing values in certain contexts such as binary operations where two operands of different types produce a result that has the same type as its leftmost operand, but it will not change if you explicitly specify what kind of operation should be done with these kinds of numbers using operators like & (bitwise AND), |( bitwise OR), etc., even though they would be implicitly converted by maths operators on primitive number integers.

53. What are arrow functions? How are they used in JavaScript?

Arrows (->) is a synonym for the function keyword that can be used as an alternative. It’s not really necessary to use this, but it makes code more readable by adding parentheses around arguments. Use -> when declaring functions instead of () or =() because arrow syntax looks nicer and does not need parens

54. Explain the differences between declaring variables using var, let, and const.

  • Var: Declares a variable with the given name. The scope is from within the function to outside of it. Let’s you declare multiple variables on one line and share their scopes, but they cannot be accessed by other functions or through dot notation. Cannot be used for declaring objects in contrast to const
  • Const: Declaration that makes the assigned value immutable–cannot change after being initialized, even if using assignment operators such as = (equals), += (plus equals) -+=( plus equals and store result back into same location). This means when creating an object literal we use “const” instead of “let”, because object literals are mutable. It can only contain primitive values like strings, numbers, null or undefined.
  • Let: Declares a block of code for function invocations, loops etc. The variable is local to the current scope–cannot be accessed outside this function or through dot notation

55. What is the rest parameter and spread operator?

The rest parameter allows a function to return multiple values. This is particularly useful for functions that take arrays as arguments because it can mean you don’t have to do any extra work in order to get the values out of your array and into an object or other container (such as when returning from nested loops). The spread operator lets us use destructuring within our parameters.

56. What is the use of promises in JavaScript?

Promises are an object-based mechanism for asynchronous programming. Using promises, you can write code that looks synchronous but acts asynchronously. Promises abstract the process of working with async results and make it easier to work in a callback-based world

57. What are generator functions?

Generator functions are a way of creating iterators, which is an object that can be used to generate sequential objects. Generators allow us to create code where we don’t need the full set of data before executing it and in some cases keep track of how much has been generated

58. Explain WeakSet in javascript.

WeakSet is a new type of collection introduced in ES2017, it allows us to store objects without them being inserted into the observable array.

59. Explain how you can access an object on WeakSet?

When getting an object from a weak set using its key, we need to check if there’s at least one shared reference to it instead of checking for presence inside the Array prototype chain since that would always return false because they’re not present in any arrays

60. Explain WeakMap in javascript.

WeakMap is a new type of collection introduced in ES2017, it allows us to store objects without them being inserted into the observable array.

61. Explain how you can access an object on WeakMap?

When getting an object from the weak map using its key, we need to check if there’s at least one shared reference to it instead of checking for presence inside the Array prototype chain since that would always return false because they’re not present in any arrays.

62. What is Object Destructuring?

Object destructuring is a JavaScript language feature that allows us to extract data from objects and assign it local variables.

63. What are Maps?

Maps are an object with key-value pairs, they can be used as associative arrays (i.e., the indices indicate what values should be associated with each indexed element) or sets of unique keys. The value for any given map item might not exist in all maps; this does not make them sparse, because nulls still count towards their size.

64. What is a Temporal Dead Zone?

A Temporal Dead Zone is a JavaScript error that occurs while using setTimeout or clearTimeout to execute the callback function.

65. What are Promises?

Promises provide an easy way of dealing with asynchronous events, for example when we perform an AJAX request and want to be able to handle its success or failure. They encapsulate everything needed in order to fulfill our promise into one object: they accept a single parameter that contains data (e.g., text), and they return another Promise object if fulfilled or reject it otherwise; this enables us to chain operations on the result as well as catch errors easily!

66. What is a prompt box in JavaScript?

A “prompt box” is a box that provides a text field. The prompt() method displays an input dialogbox with two options: “OK” or “Cancel.” A prompt box often springs up when you want to create an input-before proceeding dialogue, but it can also be used for formatting and remembering inputs.

67. What is the difference between Call & Apply?

Call calls the function and passes in its argument, while apply does not. This is often used to call a function using an object rather than a string or other data type. The following example demonstrates how Call & Apply differ:

function myFunction(param) { console.log(“Hello World”); } //Calling with arguments var greeting = new String(“Howdy”) + “!”; myFunction.call(greeting); //Prints out “Hello World” //Applying with arguments var greetingObj = {} greetingsObj[“name”]=”Nara”; myFunction.apply(greetingObj); //Prints out Hello Nara!

68. How to empty an Array in JavaScript?

With JavaScript, it is possible to remove all of the values in an array using a for loop. The following example demonstrates how:

var colors = [“red”, “green”]; for (var i=0 ;i <=colors.length-l) { console.log(“removing color “+colors[i]); }

69. What are the benefits of including ‘use strict’ at the beginning of a JavaScript file?

To ensure your JavaScript code is stricter than normal, it’s a good idea to enable the use strict command. Code errors that would have otherwise gone unnoticed will now generate an error or trigger an exception. This makes debugging much easier for programmers and saves time in the future.

70. Why do you wrap the entirety of a JavaScript source file in a function?

Developers have been employing a technique that creates a closure around the entire contents of the file in order to create a private namespace. The new name is often required since JavaScript modules and libraries might otherwise clash with others.

Another feature of this technique is to allow for an easy alias for a global variable. This is often used in jQuery plugins.

71. What are the two ways of storing JavaScript variables?

There are only two types of storage for values in JavaScript: temporal and permanent. The most commonly seen type, one that occupies temporally-based space as long as it needs to be accessed, is called variable (or “local”). On the other hand, global variables can exist anywhere on any page or document within your site’s domain name but should not be used because they have adverse effects such as affecting performance and security issues.

72. What are escape characters in JavaScript?

Escape characters are a series of special codes, used to tell the JavaScript interpreter that you want it to take what follows as some other type of value. The most common use is for strings containing text with an embedded apostrophe or quotation mark character; these need to be preceded by a double-quote and followed by another one: “This string contains ‘double quotes’.”

73. What is the significance or reason for wrapping the JavaScript source code in a function block?

With the increasing popularity of JavaScript for web application development, it is more and more common to use closures. This technique allows you to create a namespace around your code within a file, which can avoid name conflicts with other libraries and modules in your project.

74. What role do referred scripts play in JavaScript?

JavaScript allows you to incorporate code written in other languages into your script. This is accomplished by including the language’s code within a “script” tag as type=”text/javascript”. The inclusion of scripts from different sources can lead to errors, but these are usually caused not by JavaScript itself but rather by the differing behavior of representations like Unicode and regular expressions between scripting engines.

75. What do you mean by Event Bubbling and Event Capturing?

There are two ways to accomplish event propagation in the HTML DOM API: “bubbling” and “capturing.”

Event bubbling occurs when an event goes up the DOM and is fired on each target it encounters. Event capturing happens when the event travels to its intended target. The event is then fired on the target, but it never travels up the DOM tree to find any other targets.

In closing …

You should find these 75 interview questions helpful in finding a top JavaScript developer. These questions should be enough for you to determine the candidates’ competency as an early consideration.

Remember, top talent takes time to find, so it’s important to get started early.

As always, we are here for you. If you need help in locating these candidates, don’t hesitate to contact us.

Good luck!

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