Frequently Asked JavaScript Interview Questions and Answers

JavaScript, is among the popular programming languages . It is a high-level, interpreted programming language. 

According to the StackOverflow Developer Survey, it is one of the most preferred programming languages. Therefore, JavaScript is a language worth learning.

If you want to advance in your career as a JavaScript Developer, you should be prepared for every question that may arise about it. We have listed the most popular JavaScript interview questions and their answers for you.

 

The 37 Most Frequently Asked JavaScript Interview Questions and Answers

 

 

1. What is JavaScript?

JavaScript is one of the most popular scripting languages used for both client-side and server-side development. JavaScript code can be added to HTML pages that browsers can understand and execute.

 

2. What is the difference between JavaScript and Java?

Java is a programming language. JavaScript is actually a scripting language. While Java is used to create applications that can run on the operating system or in a browser, JavaScript only runs within a browser. JavaScript can be used to create standalone desktop applications. 

 

3. What are the data types in JavaScript?

There are 7 basic data types supported by JavaScript. These data types are as follows:

• Boolean – Represents true or false values.
• Null – Represents empty or unknown values.
• Number – Represents numeric values.
• Object – Used to store data or more complex contents.
• String – Represents single-character and multi-character strings.
• Symbol – Values used to identify objects.
• Undefined – If a variable has not been assigned in JavaScript, it is undefined. It is used to represent these undefined data types.

 

4. What are the standout features of JavaScript?

Some basic and important features of JavaScript are as follows:

✅ JavaScript is object-oriented.

✅ JavaScript is open-source.

✅ JavaScript is a scripting language.

 

5. How is the distinction between “let” and “var” made in JavaScript?

In JavaScript, both “let” and “var” are used for variable declaration. The most important difference between them is that “var” is function-scoped, while “let” is block-scoped.

 

6. What are the escape characters in JavaScript?

In JavaScript, special characters such as (&), ('), (" ") and (' ') are written using the backslash (\\), which is the escape character.

The JavaScript escape characters are as follows:

• \b – Backspace
• \f – Form feed
• \n – New line
• \r – Carriage return
• \t – Tab
• \v – Vertical tab

 

7. How to see the output in JavaScript?

In JavaScript, you need to print the result to the screen to see the outcome of calculations. There are 4 different ways to see the operations performed by JavaScript:

• ✅  alert() -  display in a message box
• ✅  document.write() - display on the HTML page
• ✅  innerHTML -  display inside an HTML tag
• ✅  console.log() - display in the browser console

 

📍 Example syntax for using innerHTML: 

It is a method used to select HTML objects.

 

📍 Example syntax for using document.write(): 

It is a method used to test JavaScript commands and print the results to the screen. 

 

📍 Example syntax for using alert():

A method used to print JavaScript data as an alert message on the screen. 

 

📍 Using console.log():

It is a method used to view operations on objects. 

 

8. What is DOM? 

DOM stands for Document Object Model. It represents the HTML document. It is used to access and modify the HTML content. 

 

9. How to create a cookie in JavaScript?

Creating a cookie in JavaScript is quite simple. A cookie can be created using the following method:

document.cookie = "key1 = value1; key2 = value2; expires = date"

 

10. Is it possible to split JavaScript code into several lines?

If the code is extended over a very long line, it becomes difficult to work because the entire code is not visible. In this case, the backslash '\'\' can be used to split the code line. 

 

📍 Example: 

 

11. What are undeclared and undefined variables?

➡️ Undeclared variables are variables that do not exist in a program. If the program tries to read the value of an undeclared variable, a runtime error occurs.

➡️ Undefined variables are variables that are declared in the program but have not been given any value. 

 

12. What is the 'this' keyword in JavaScript?

The 'this' keyword in JavaScript, when used inside a function, refers to the object (or context) to which the currently executed function is bound. 

The this keyword can take different values depending on where it is used. For example, it can have a different value when used inside an object than when used inside a function.

For example, this operation's output will be “Anıl Yıldız.” 

 

13. How do timers work in JavaScript?

Timers are used to execute a piece of code at a predetermined time or repeat the code at a certain interval. This is done using setTimeout, setInterval, and clearInterval functions.

 

• ⏳ setTimeout (function, delay) function is used to start a timer that calls a specific function after the specified delay. 
• ⏱️ setInterval (function, delay) function repeatedly executes the given function at the specified delay and only stops when canceled. 
• 🛑clearInterval (id) function sends a stop command to the timer.

 

14. How are comments made in JavaScript, and which symbol is used?

There are 2 symbols for comments in JavaScript.

1- Single-line comments use “//”. 

 

2- Multi-line comments use “/*”. 

 

15. What is the difference between ViewState and SessionState?

• 🆚 'ViewState' is specific to a single page in the session.
• 🆚 'SessionState' is specific to user-specific data that can be accessed from all web application pages.

 

16. What is the “===” operator? What is the difference between “==” and “===” operators?

The "===" (triple equals) operator in JavaScript is a comparison operator that compares both the values and types of the operands. It checks whether the two given values are equal.

 

When two variables are compared with the “==” operator, if the variables have different types but the same value, the result is true. If the comparison is done with “===”, both the type and the value of the two variables must be the same for the result to be true.

 

📍 Example: 

 

17. How to submit a form using JavaScript?

To submit a form using JavaScript, the submit() method is used. 

 

18. Does JavaScript support automatic type conversion?

Yes, JavaScript supports automatic type conversion. When operations are performed on different data types, it automatically tries to convert them to the correct data type.

 

19. How to change the style/class of an element in JavaScript?

or

 

20. What are the loop structures in JavaScript?

The loop structures in JavaScript are as follows:

1️⃣ For - Repeats the code block multiple times.

2️⃣ While - Repeats the loop while a specified condition is true.

3️⃣ Do-while loops - It runs the loop block at least once before checking the condition. It repeats the loop as long as the condition is false.

4️⃣ For in - Repeats the loop for all properties of an object.

 

21. How to convert a base-based string to an integer in JavaScript?

🔖 The parseInt() function is used to convert numbers between different bases. 

To convert 4F (or base 16) to an integer, you would use the following code:

 

22. Is JavaScript case-sensitive?

Yes, JavaScript is a case-sensitive scripting language.

 

23. How many types of functions does JavaScript support?

In JavaScript, there are two main functions: named and anonymous.

 

24. What does NULL mean in JavaScript?

The NULL value is used to represent no value or no object. 

 

25. What is the function of the delete operator?

The delete keyword is used to remove properties and values.

 

26. What is the difference between an alert box and a confirm box?

In JavaScript, the difference between an alert box and a confirm box is as follows:

 

⚠️ An alert box is a single box with only a “OK” button. When the user clicks the "OK" button, the alert box disappears.

 

✅ A confirm box has “OK” and “Cancel” buttons. When the user clicks "Yes" or "No", the confirm box displays different values based on the user's selection.

 

27. How to create generic objects in JavaScript?

Generic objects can be created as follows:

 

28. Which keywords are used to handle exceptions in JavaScript?

In JavaScript, the try, catch, and finally keywords are used to handle exceptions (errors). 

 

29. What are the types of errors in JavaScript? 

There are three types of errors in JavaScript:

 

1- Syntax Errors: These are syntax-related errors that occur when a web page is loading. They appear at interpretation time in JavaScript. 

When there is a syntax error in JavaScript, only the code on the same thread as the syntax error is affected, and the code on other threads continues to execute assuming there is no error.

For example, in the line below, a closing parenthesis is missing, so a syntax error occurs. 👇

 

 

2- Runtime Errors: These errors occur due to incorrect use of a command in the HTML language. They occur during execution (after compilation/interpretation).

For example, in the following example, a runtime error occurs because a method that does not exist is called. 👇

 

3- Logical Errors: These are errors caused by faulty logic with a different operation performed on a function. Logical errors can be the most difficult type of error to track.

 

30. How is the push method used in JavaScript?

In JavaScript, the push method is used to add elements to the end of an array. Using this method, multiple elements can be added.

 

📍 Example syntax: 

 

31. What is the unshift method in JavaScript?

The unshift method is used to add one or more elements to the beginning of an array. This method adds the specified elements as the first element of the existing array and shifts the other elements of the array.

 

📍 Example syntax:

 

32. What is 'Strict Mode' in JavaScript and how is it enabled?

In JavaScript, 'Strict Mode' ensures the code works more securely and consistently and treats certain improper usages as errors. Strict mode can be applied to an entire JavaScript file or just a specific block of code.

Strict mode can be enabled by adding the string "use strict" at the top of the file. 

 

To enable Strict mode in a function: 👇

 

33. What are window.onload and onDocumentReady?

In JavaScript, window.onload and document.ready() are two different methods used during page loading. 

1️⃣ The window.onload function executes after the entire web page has loaded. It includes all DOM-related elements such as the title tag, style sheets, images, and videos.

Example syntax:

 

In this code example, you can see how window.onload works in JavaScript. 👇

 

Output:

This is the output displayed in the console when the code above is executed. 👇

 

2️⃣ onDocumentReady is a jQuery method for executing a block of code. It loads the code immediately after the DOM is loaded. This allows early manipulation of the code.

Example syntax:

 

34. What are decodeURI() and encodeURI() in JavaScript?

decodeURI() and encodeURI() are two methods used in JavaScript for handling URLs.

•  

1️⃣ encodeURI(): This method encodes the characters used in a URL into valid ones by converting invalid characters in a URL (space, &, ?, #, etc.) into valid characters, ensuring the URL has a valid structure.

2️⃣ decodeURI(): This method is used to decode coded characters in a URL, obtaining the original characters.

 

35. What are the advantages of using JavaScript?

The usage area of JavaScript is quite broad. The reason JavaScript is so popular is naturally the advantages it offers. Some of the advantages of JavaScript are:  

• Easy to use - JavaScript is quite easy to understand. Even if you are new to programming, learning JavaScript is not as difficult as you might think. 
• Fast - JavaScript is an "interpreted" language. Thus, it reduces the time needed for compilation. Also, JavaScript speeds up program execution by eliminating server-side waiting times.
• Compatible - JavaScript works seamlessly with other programming languages. Therefore, many developers prefer to use JavaScript to create various applications.
• Simple - JavaScript is easy to understand and learn. The structure is simple for both users and developers.
• Popular - Since the most commonly used browsers today support JavaScript, it is used in almost every web application. Technology giants such as Google, Amazon, and PayPal use JavaScript.
• Rich Interface - JavaScript offers various interfaces for creating web pages. Drag-and-drop components provide an enhanced user experience on the web page.

 

36. What can be done with JavaScript?

• ✅ JavaScript is used to build web pages and applications. Google Maps is developed using JavaScript.
• ✅ JavaScript is also used in smartwatches. 
• ✅ Websites that receive millions of hits every day such as Google, Facebook, Netflix, Amazon benefit from JavaScript.
• ✅ Mobile applications can also be developed using JavaScript. In addition, game development is another area where JavaScript is used. 

 

37. What are the JavaScript frameworks?

The most commonly used JavaScript frameworks are as follows:

1. AngularJS: A web development framework that provides fast application development.
2. ReactJS: A high-level JavaScript framework supported by Facebook. 
3. MeteorJS: A JavaScript framework used on the backend side. It helps save time. 
4. jQuery: A framework used to develop and make websites more interactive. Technology companies such as Google, WordPress, and IBM use jQuery.