Javascript is one of the most popular programming languages today, and it’s only gaining popularity as time goes on. 

This modern programming language is used in web design and development, native applications, and much more, making it an important skill to have on your resume if you want to be a successful software developer.

If you’re interviewing for Javascript jobs or looking to interview for a Javascript position yourself, this list of 41 Javascript interview questions and answers will help you prepare for your next interview and give you some good tips on what to expect at the interview table.

List of Top 46 Javascript Interview Questions

1. What are Closures in JS and How are they Useful?

Closures are one of JavaScript’s most powerful and unique features. In short, a closure is a function that retains its variables even when executed outside of its original context. This makes closures very useful as callbacks or event handlers because they can retain information needed by other parts of your code even after being called. Here are some questions to help prepare you!

2. List different types of errors possible in Java Script?

• Syntax Error 
• Semantic Error 
• Run-time Error 
• Logic Error 
• Exception 
• Unknown Error 

3. What is the function of an interpreter? 

Interpreters are programs that execute other programs. They read the source code of a program and execute it. The most common type of interpreter is a virtual machine, which executes code written in a bytecode language such as Java. NET. Other interpreters include Perl, PHP, and Python. The interpreter reads the source code of another program and executes it.

4. What are some types of errors in JavaScript? 

There are three main types of errors in JavaScript: syntax, runtime, and logic. Syntax errors are the most common and occur when you make a typo or forget to include a semicolon. Runtime errors happen when your code tries to do something impossible, like divide by zero. Logic errors are usually the hardest to find and fix because they don’t produce any error messages. Instead, they cause your code to produce incorrect results. One example is using an array index that doesn’t exist yet. 

5. What does browser compatibility mean?

Browsers can interpret HTML, CSS, and JavaScript differently from one another. When websites need to be compatible with all browsers on the market, developers will often use browser-specific prefixes (e.g., -WebKit-, -Moz-, -ms-) in their code to ensure that different browsers render their sites similarly.

6. What are some best practices for working with JavaScript?

Here are five best practices for working with JavaScript: 

1) always save your work before refreshing a page; 

2) create function prototypes first; 

3) always indent properly; 

4) comment on each line of code as it’s written; 

5) use meaningful variable names.

7. What do you think about JavaScript? 

JavaScript is a programming language that helps make web pages interactive. It’s usually used alongside HTML and CSS to create websites and web applications. 

8. Do you know what ECMAScript is? 

ECMAScript is the standard that JavaScript is based on, and it’s constantly being updated to keep JavaScript up-to-date. ECMAScript 5 has introduced new features like getters and setters in object literals.

The next version of ECMAScript will introduce modules, which will make writing large programs easier by separating code into separate files.

9. Why would someone use JavaScript instead of Java?

JavaScript and Java are two very different programming languages. JavaScript is a scripting language, while Java is a more robust, object-oriented language. While they can both be used for web development, they each have their strengths and weaknesses. JavaScript is typically used for smaller projects or for adding interactivity to websites, while Java is better suited for larger, more complex applications. For this reason, it’s not uncommon for developers to use both of these languages in tandem.

10. What is the difference between DOM and DHTML? 

DOM stands for Document Object Model. It is a platform- and language-neutral interface that will allow programs and scripts to dynamically access and update the content, structure, and style of a document. DHTML stands for Dynamic HTML. It is a combination of HTML, CSS, and JavaScript that allows for the creation of interactive and animated web pages. DOM is a standard that is supported by all major browsers, while DHTML is not a standard and is not supported by all browsers.

11. What are some disadvantages of using Flash? 

Some disadvantages of using Flash include compatibility issues with various platforms (some devices do not support Flash), long load times, possible security risks (such as cross-site scripting), and slower processing speeds than non-Flash alternatives.

12. What are some advantages of using Flash? 

Some advantages include:

• a large selection of existing templates (available in the Adobe Creative Suite) for animation or video presentations; 
• an extensive library with prebuilt components that can be customized; it does not require programming knowledge to create an animation or video presentation; 
• interactivity can be built into movies created in Flash so users can control what happens on screen; 
• it integrates well with other Adobe products like Photoshop and Illustrator.

13. What is a way to detect whether a browser supports different HTML tags? 

If you want to write code that detects support for a certain HTML tag or CSS feature, then you should use Modernizr. It is a small JavaScript library that helps you build websites that work in all browsers. 

14. How can one determine if JavaScript has been disabled on a webpage or not?

There are a few ways to tell if JavaScript has been disabled on a webpage. 

• One way is to try and click on something that would normally require JavaScript to function. If clicking doesn’t do anything, then it’s likely that JavaScript has been disabled. 
• Another way is to check the source code of the page. If you don’t see any tags, then it’s likely that JavaScript has been disabled. 
• Finally, you can try using a different browser. If you’re able to view the page and use the features without any issues, then it’s likely that JavaScript has only been disabled in the browser you were originally using.

15.  What are some differences between a closure and an anonymous function? 

An anonymous function is a function that was declared without any named identifier to refer to it. A closure is an inner function that has access to the outer (enclosing) function’s variables—the scope chain. The closure has three scope chains: it has access to its scope (variables defined between its curly brackets), it has access to the outer function’s variables, and it has access to the global variables. 

16. What is JSON, and how does it differ from XML?

JSON is a data format that is gaining popularity as an alternative to XML. JSON (JavaScript Object Notation) is a lightweight data-interchange format that is easy for humans to read and write, and easy for machines to parse and generate. JSON is a text format that is completely language-independent but uses conventions that are familiar to programmers of the C-family of languages (C, C++, JavaScript, and so on). These properties make JSON an ideal data-interchange language.

17. Is it possible to have more than one event handler on the same element for the same event type (like click)? 

Yes, it is possible to have multiple event handlers on the same element for the same event type. This can be useful if you want to execute different codes depending on the circumstances. For example, you might want to have a click handler that executes different codes depending on whether the shift key is pressed.

18. What kind of data type should I use if I wanted to assign a variable within an event handler? 

You can use any data type you want when assigning a variable within an event handler in JavaScript. However, keep in mind that some data types are more suited for certain tasks than others. For example, if you want to store a list of items, you might use an array. If you want to store information about a user, you might use an object.

19. What is the difference between == and === in JavaScript?

The == (double equals) operator checks whether the two values are equal in value. The === (triple equals) operator checks whether the two values are equal in value and type. Example 10 == “10” will result in true but 10 === “10” will result in false. As in the first case it just checks the values irrespective of data types, but in === it does strict type comparison. 

20. Can JavaScript programmers count down instead of up like other languages such as C++? 

In JavaScript, we can count down instead of up by using a for a loop. The for loop will start at the number we specify and then count down to 0. This is useful if we want to iterate over an array or string in reverse order. We can also use the forEach method to achieve the same result.

21. Does JavaScript have functions such as case sensitive compare that return -1 when two strings don’t match, 0 when they’re equal, and 1 when they’re different? 

If you’re a programmer, you likely know that JavaScript is a scripting language used to create and control dynamic website content. You may also know that it’s a prototype-based language, meaning objects inherit directly from other objects.

22. What is eval() used for? 

eval() is a function that executes a string of code as if it were JavaScript code. This can be useful for dynamically executing code, but it can also be dangerous because it can run malicious code. eval() should only be used with trusted input and never with user-provided input.

23. What is the order of operations in JavaScript? 

The order of operations in JavaScript is the same as in most programming languages and math. Parentheses first, then exponents, then multiplication and division (from left to right), and finally addition and subtraction (from left to right). This is called the Order of Operations.

24. How many bytes are used to store an integer value on a computer? 

Most programming languages define an integer as a data type that stores whole numbers. Integers can be positive or negative, but they cannot contain decimal points. The size of an integer is dependent on the computer system. 

For example, an int on a 32-bit system can store numbers between -2,147,483,648 and 2,147,483,647. An int on a 64-bit system can store numbers between -9223372036854775808 and 9223372036854775807.

25. What happens when you divide by zero in JavaScript? 

Dividing by zero in JavaScript will return Infinity. This is because when you divide a number by zero, you are essentially asking JavaScript to calculate the number of times that zero goes into the number. Since there’s no answer to this question, JavaScript returns Infinity.

26. Does JavaScript support recursion? 

Yes, JavaScript supports recursion. You can use recursion to call a function within itself. This can be useful for iterating through data structures or performing certain operations on a list of items. However, you need to be careful when using recursion, as it can lead to a stack overflow if not used correctly.

27. When is it best to use a loop vs. a recursive function? 

Loops are best when you know how many times you want the code to run. Recursive functions are best when you don’t know how many times the code will need to run, or when the number of times is unknown but could be very large. 

28. Describe the concept of self-executing functions. 

Self-executing functions are functions that run automatically when they are defined. 

29. Explain what prototypes are and how they relate to object-oriented programming. 

Prototypes are objects that act as templates for other objects. They contain properties and methods (or procedures) that describe what the object should do and how it should behave. 

Object-oriented programming (OOP) is a programming language paradigm that views programs as collections of interacting objects with states and behaviors. 

30. What are closures? 

Closures are a type of function in JavaScript that have access to variables from outside their scope. 

31. What is the difference between strict mode and non-strict mode? 

In strict mode, JavaScript will not allow you to use undeclared variables. This means that you must declare all variables before using them. In non-strict mode, JavaScript will allow you to use undeclared variables. This means that you can declare variables after using them. If the code is running in strict mode, the JavaScript engine throws an error when it encounters an undeclared variable. The non-strict mode may produce unexpected results and should be avoided if possible.

32. What are some of the new features of ECMAScript 6? 

There are a lot of new features coming with ECMAScript 6. Here’s a quick overview of some of the ones that developers find most interesting:

1) The spread operator lets us create arrays from existing objects or constructors. For example, let’s say we have two values – 10 and 20 – and we want to create an array containing those two values; but we don’t want to do this by hand because it would be tedious and error-prone. To make things easier, ECMAScript 6 has introduced the spread operator (three more sentences).

2) Rest parameters enable us to easily write functions that take any number of arguments. They provide a convenient way for avoiding default parameters as well as making our code more readable. Let’s take for example a function called myFunc() which takes three parameters (two sentences).

3) Classes enable programmers to design their custom data types and work with them much as they would work with built-in types such as strings or numbers (four sentences).

4) Restructuring assignment allows you to extract values from arrays into individual variables without needing temporary variables or loops (six sentences).

33. How do self-executing functions work? 

Self-executing functions, also called immediately-invoked function expressions, are pieces of code that run as soon as they are defined.

They are used extensively in JavaScript because they allow you to create local scope and avoid polluting the global namespace.

34. How are numbers represented in JavaScript? 

In JavaScript, numbers are represented in 64-bit floating-point format. This means that they can store a wide range of values, from very small to very large.

However, not all numbers will evaluate to be true no matter what. The best thing about JavaScript is that it is a very versatile language. It can be used for front-end development, back-end development, and even game development.

35. Will any number always evaluate to be true no matter what? 

No, any number will not always evaluate to be true in javascript no matter what. 

36. What is the best thing about Javascript as opposed to PHP?

The best thing about Javascript, as opposed to PHP, is that it is more forgiving when it comes to syntax errors. If a syntax error is found within the first few lines of code written in Javascript, the error can still run and provide a valid result; whereas if there are syntax errors with PHP, an error message is generated without executing any of the code. The downside to this is that there may be unintended side effects of running invalid code.

37. What is Garbage Collection in JavaScript and how does it Work?

Garbage collection is a mechanism of automatic memory management. In layman’s terms, it means that when an object is no longer in use by any part of your code and has become garbage, garbage collection will clean up (that is, delete) that object. This saves developers from having to worry about deleting objects themselves and allows them to focus on other parts of their code.

38. Explain Hoisting in JavaScript?

JavaScript hoisting is a simple concept but can be quite confusing to many people who are not yet familiar with it. Hoisting essentially just moves variable and function declarations to the top of their scope. 

So, when you declare a variable with var or function with function, that code is hoisted to be executed first before anything else in your program runs. Let’s take a look at an example to see how this works. Imagine we have this piece of code:

var a = 5;

function square(x) {

alert(x * x); //this will alert 25

39. Explain Scope in JavaScript?

The scope is all about determining which variables and methods are available to code in a given piece of JavaScript code. We can access local variables and functions, as well as global ones. 

Variables declared outside of any function definition are automatically considered global. Likewise, we can also add some new functionality via an IIFE (Immediately Invoked Function Expression). 

40. How Do You Define a Function in JS? 

A function can be defined in javascript as a block of reusable code that is grouped. Any Javascript code is written within a set of curly braces, )(); is considered to be part of the function definition and will only execute when called from the outside. 

41. What are some examples of built-in functions in JS?

The built-in functions in JS are the standard functions that are available in the JavaScript language. These functions include things like mathematical operations, string manipulation, and Array manipulation. Some of the most commonly used built-in functions are Math. min() and Math. max(), which return the minimum and maximum values of a given array, respectively.

42. What Are the Different Types of Variables Available in JS?

There are three types of variables in Javascript. Variables that start with lowercase characters, variables that start with uppercase characters, and finally some variables have dashes in between. Let us discuss each one of them separately. The variables that start with lowercase letters can contain only letters or numbers while the ones that begin with an uppercase letter can contain only alphabets. Finally, the ones which are called camelCase (a combination of both) may contain both alphabets and numbers.

43. Which JavaScript Frameworks are Popular Right Now? 

There are tons of JavaScript frameworks out there, so it’s no surprise that many developers want to know which ones they should learn. Some consider React and Angular to be more popular than others, but some developers also like Ember and Vue. In general, you need to know at least one of these modern JS frameworks to have a successful career in web development.

44. Commonly Used Tools in JS Development?

Besides just learning JavaScript, there are many tools used in JS development. These include NPM, Browserify, Gulp, and JSHint (just to name a few). Knowing what these are and how they work will make you more qualified as a JS developer. They’re all worth looking into if you’re serious about developing with JavaScript. They can also help expand your toolkit when it comes to building web apps.

45. What Is the Difference Between JavaScript, JQuery, AngularJS, Node.js, and React.js?

JavaScript is an interpreted language that can be written in different ways, such as with Microsoft’s Visual Basic or using a text editor. JQuery is a library of JavaScript functions that makes working with HTML easier. AngularJS, Node.js, and React.js are frameworks in which you write your JavaScript code to make it work more easily and efficiently with your code written in other languages.

46. How Does Learning JS Benefit Us?

JavaScript, often shortened to JS, is a programming language originally developed by Netscape in 1995. Since its inception, it has become one of the most popular and widely used programming languages in use today. 

Learning JavaScript opens up a huge array of job opportunities for people looking to join companies like Google, Facebook, and Microsoft as well as various startups. When compared to other languages such as Java or C++, JavaScript doesn’t require nearly as much setup.

In A Nutshell…

If you’re preparing for a JavaScript interview, you’ve come to the right place. In this post, we’ve answered some of the most frequently asked questions on JavaScript. With these questions in mind, you should be well on your way to acing your interview.
However, if you’re still feeling uncertain about something that was mentioned in this post or has more general inquiries about any type of coding language, don’t hesitate to reach out! Our knowledgeable team at Codersera is always here to help and can answer any questions you may have.