JavaScript interview questions | Top 20 questions you must know!

According to the Global Developer Population and Demographic Study conducted by Evans Data Corporation, there are a whopping 26.9 million software developers across the world! This number will continue to increase, reaching about 28.7 million by 2024. Contributing to the study, around 12.4 million software developers have used JavaScript. 

But what exactly is JavaScript? Let’s have a look!

What is JavaScript?

JavaScript is a programming or scripting language that allows the implementation of complex features on web pages. From displaying content timely to providing interactive maps, updates, and 2D/3D graphics, JavaScript can add significantly to the functioning of a website. 

Additionally, having a certification or background knowledge in JavaScript can bring about many job opportunities. These include careers in front-end development, the creation of software-based applications, and web application development. 

Committing to your dream job with a JavaScript qualification can be a great way to stabilize your career! However, with stiff competition, it becomes crucial to prove your knowledge of this language.

Therefore, we have a list of the top 20 JavaScript interview questions to help you crack the coding round!

20 JavaScript interview questions with answers 

Set 1

Q1. What is the difference between JavaScript & Java? 

Answer – JavaScript is an OOP scripting language, while Java is an OOP programming language. JavaScript codes are all in text form. Java codes, on the other hand, require constant compilation. 

Q2. Name the different data types supported by JavaScript. 

Answer – The different data types supported by JavaScript include-

• Object
• Number
• Symbol
• String
• Boolean
• Null
• Undefined

Q3. What does the ‘This’ operator do in JavaScript? 

Answer – The JavaScript operator ‘This’ refers to the object the keyword belongs to. This operator word has different values depending on where it is used. 

Q4. What is the primary difference between var and let? 

Answer – Developers use var and let for variable and method declaration in the JavaScript language. The difference between the two is that var is function-scoped, while let is block-scoped. 

Q5. What does the isNaN function do? 

(This is among the most asked JavaScript interview questions). 

Answer – The isNaN function returns true if an argument is not a number. The function returns false if it is. 

Set 2

Q6. What is the difference between undeclared and undefined variables?

Answer – Undeclared variables do not exist in a program. A runtime error occurs if the program attempts to read the value of an undeclared variable. 

Undefined variables are declared in the software but do not have any assigned value. If the program reads the value of an undefined variable, it returns to the original value. 

Q7. What is a callback? 

Answer – A callback is a simple JavaScript function that is passed as an argument or an option, to a certain extent. It is often executed after another function is complete. 

Q8. How does TypeOf Operator function? 

Answer – TypeOf Operator is primarily used to receive the data type of its operand. The operand can be a data structure such as an object, function, or variable.

Q9. What is hoisting? 

Answer – Hoisting is the default functioning of JavaScript. In hoisting, all variable and function declarations move to the highest echelon. This is irrespective of where the variables and functions are officially declared. 

Q10. Name 3 types of Error Name values. 

Answer – 3 types of Error Name values include-

• URI Error
• Reference Error
• Eval Error

Set 3

Q11. Does JavaScript support automatic type conversion? 

Answer – Yes, JavaScript supports automatic type conversion. This is one of the most common ways of type conversion developers use. 

Q12. What is the Typed language? 

Answer – The Typed language has values that don’t associate with variables. The two types of values associated are- 

• Dynamically 
• Statistically

Q13. What is the key difference between null and undefined?

Answer – Undefined is a variable that is declared but does not have an assigned value. Null, on the other hand, has an assigned value. 

Q14. What are the different ways in which a JavaScript code can involve an HTML file? 

Answer – The 3 different ways a JavaScript code can involve in an HTML file are- 

• External
• Internal
• Inline

Q15. What is the key difference between local storage and session storage?

Answer – In local storage, the data is not sent to the server reversely for each request. This lessens the traffic between the server and the client. In session storage, the data stored immediately clears when the page session ends. 

Set 4

Q16. How are first-class functions defined? 

(JavaScript interview questions also include definitions)

Answer – First-class functions can either be-

• Returned as values from other functions 
• Stored in a variable 
• Passed as arguments to other functions

Q17. What are closures in JavaScript? 

Answer – Closures provide expressive, creative, and concise writing codes for JavaScript programmers and developers. Closures are a combination of lexical function and environment. 

Q18. Name the different looping structures in JavaScript. 

Answer – The different looping structures in JavaScript are-

• Do-While Loops
• While
• For

Q19. What function does the delete operator perform? 

Answer – The delete operator deletes a property, as well as its value. 

Q20. What is implicit type coercion in JavaScript?

Answer – Implicit type coercion in javascript is the automatic conversion of a value from one data type to another. This takes place when the operands of an expression are of different data types.

Key Takeaways 

• JavaScript is a programming language that contributes to the overall functionality of websites. 
• With a background in JavaScript, there are a plethora of lucrative career options to consider. 
• Preparing for interviews with the 20 listed questions and answers can highlight your knowledge & skills and help you stand apart from the crowd. 

We hope you found this blog informative. Don’t forget to comment below and share your thoughts! You can also get in touch with us if you have any doubts. 

Liked this blog? Read: Java Games | 8 Best books aspiring Java developers must know about!

FAQs 

Q1. Can JavaScript be used to animate images? 

Answer – Yes, JavaScript is commonly used to animate images and insert them on web pages. 

Q2. What is front-end web development? 

Answer – Front-end web development is the building of visual, user-facing aspects of a website. This is an excellent career option for those with a JavaScript background. 

Q3. How do automation specialists use JavaScript? 

Answer – Automation specialists run tests on millions of mobile and desktop browsers simultaneously with JavaScript. The language’s unique ability to instruct other programs makes it among the most useful tools for these professionals.