50+ JavaScript Interview Questions and Answers (2025) – MTA Skills

Ace your JavaScript interview! This article features 50 in-depth questions and answers covering core JavaScript concepts, from fundamentals to advanced topics. This is part-1 of 150 questions series.

Welcome, developers! Whether you’re a beginner or an experienced coder, acing JavaScript interviews is key to advancing your career. This blog presents a curated list of top 50 essential JavaScript questions, completed with clear explanations and practical examples. Let’s dive in and get you ready to shine in your next interview.

1. What are the differences between Java and JavaScript

Java: It is one of the most popular programming languages. It is an object-oriented programming language and has a virtual machine platform that allows you to create compiled programs that run on nearly every platform. Java promised, “Write Once, Run Anywhere”
JavaScript: It is a light-weighted programming language (“scripting language”) for developing interactive web pages. It can insert dynamic text into the HTML elements. JavaScript is also known as the browser’s language

2. What are Data Types in JavaScript?

JavaScript data types are categorized into two parts
1. primitive
2. non-primitive
Primitive Data Type: The predefined data types provided by JavaScript language are known as primitive data type. Primitive data types are also known as in-built data types
– Numbers
– Strings
– Boolean
– Symbol
– Undefined
– Null
– BigInt
Non-Primitive Data Type: The data types that are derived from primitive data types are known as non-primitive data types. It is also known as derived data types or reference data types
– Objects
– Functions
– Arrays

3. Which symbol is used for comments in JavaScript?

Comments prevent the execution of statements. Comments are ignored while the compiler executes the code. There are two types of symbols to represent comments in JavaScript:
Double slash: It is known as a single-line comment

Slash with Asterisk: It is known as a multi-line comment

4. What are the differences between JavaScript variables created using let, var or const?

In JavaScript, let, var, and const are all keywords used to declare variables, but they differ significantly in terms of scope, initialization rules, whether they can be redeclared or reassigned and the behavior when they are accessed before declaration:

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

• == is the abstract equality operator while === is the strict equality operator
• The == operator will compare for equality after doing any necessary type conversions
• The === operator will not do type conversion, so if two values are not the same type === will simply return false

6. What would be the result of 3+2+”7″?

Here, 3 and 2 behave like an integer, and “7” behaves like a string. So 3 plus 2 will be 5. Then the output will be 5+”7″ = 57

7. What is the use of the isNaN function?

The number isNan function determines whether the passed value is NaN (Not a number) and is of the type “Number”. In JavaScript, the value NaN is considered a type of number. It returns true if the argument is not a number, else it returns false.

8. Is it possible to break JavaScript Code into several lines?

Yes, it is possible to break the JavaScript code into several lines in a string statement. It can be broken by using the backslash n ‘\n’

Example:

9. What are global variables? How are these variables declared, and what are the problems associated with them?

Global variables are the variables that define outside of functions. These variables have a global scope, so they can be used by any function without passing them to the function as parameters. 

Example:

Note: It is difficult to debug and test the code that relies on global variables.

10. What do you mean by NULL in JavaScript?

The Null value represents that no value or no object. It is known as empty value/object.

11. What is the difference between null and undefined in JavaScript?

A deeper comparison between the two special values, null (intentional absence of value) and undefined (uninitialized variables).

12. What is a prompt box?

The prompt box is a dialog box with an optional message prompting the user to input some text. It is often used if the user wants to input a value before entering a page. It returns a string containing the text entered by the user, or null.

13. What is typeof operator?

JavaScript provides a typeof operator that can examine a value and tell you what type it is:

Example:

14. Explain is Scope in JavaScript?

In JavaScript, each function gets its own scope. The scope is basically a collection of variables as well as the rules for how those variables are accessed by name. Only code inside that function can access that function’s scoped variables.

A variable name has to be unique within the same scope. A scope can be nested inside another scope. If one scope is nested inside another, code inside the innermost scope can access variables from either scope.

15. Which company developed JavaScript?

Netscape developed JavaScript and was created by Brenden Eich in the year of 1995.

16. What are undeclared and undefined variables?

• Undefined: It occurs when a variable is declare but not assign any value. Undefined is not a keyword.
• Undeclared: It occurs when we try to access any variable which is not initialized or declared earlier using the var or const keyword. If we use ‘typeof’ operator to get the value of an undeclare variable, we will face the runtime error with the return value as “undefined”.

17. Explain arrays in JavaScript.

An array is an object that holds values (of any type) not particularly in named properties/keys, but rather in numerically indexed positions:

Example:

18. What is the purpose of the array slice method?

The slice() method returns the selected elements in an array as a new array object. It selects the elements starting at the given start argument, and ends at the given optional end argument without including the last element.

Example:

19. What is the purpose of the array splice method?

The splice() method adds/removes items to/from an array, and then returns the removed item. The first argument specifies the array position/index for insertion or deletion whereas the optional second argument indicates the number of elements to be deleted.

Example:

20. What is the difference between slice and splice?

21. Explain the concept of hoisting in JavaScript.

Hoisting in JavaScript is the behavior where variable and function declarations are moved to the top of their scope during the compile phase

• Variables: var is hoisted and initialized with undefined; let and const are hoisted but not initialized, causing a ReferenceError if accessed before declaration.
• Functions: Function declarations are fully hoisted and can be called before their definition; function expressions are not fully hoisted and result in a TypeError if called before being defined.

22. What is a closure in JavaScript?

A closure is when an inner function has access to the outer function’s variables, even after the outer function has finished executing. This allows the inner function to remember the scope in which it was created.

23. How can you create an object in JavaScript?

You can create an object in JavaScript using:

1. Object Literal
2. New Keyword
3. Constructor Function
4. ES6 Classes

1. Object Literal:

2. New Keyword:

3. Constructor Function:

4. ES6 Classes:

24. What is event delegation?

Event delegation is a technique that allows you to handle events at a higher level in the DOM tree, rather than on individual elements. This is done by taking advantage of event bubbling, so a single event listener on a parent element can handle events for all its child elements. This improves performance and simplifies event handling for dynamic content.

25. Explain how “this” works in JavaScript

Certainly! The “this” keyword refers to the context in which a function is called. Here are the main points:

1. Global Context: In the global scope, this refers to the global object (window in browsers)

2. Object Method: Inside a method, this refers to the object that the method is a property of

3. Constructor Function: In a constructor, this refers to the newly created instance

4. Constructor Function: In a constructor, this refers to the newly created instance

26. Describe the difference between a Cookie, Session Storage and Local Storage in browsers

All of the following are mechanisms of storing data on the client, the user’s browser

• Cookies: Suitable for server-client communication, small storage capacity, can be persistent or session-based, domain-specific. Sent to the server on every request.
• localStorage: Suitable for long-term storage, data persists even after the browser is closed, accessible across all tabs and windows of the same origin, highest storage capacity among the three.
• sessionStorage: Suitable for temporary data within a single page session, data is cleared when the tab or window is closed, has a higher storage capacity compared to cookies.

27: What are lambda expressions or arrow functions?

An arrow function is a shorter/concise syntax for a function expression and does not have its own this, arguments, super, or new. These functions are best suited for non-method functions, and they cannot be used as constructors.

Example:

28. What is a higher order function?

A higher-order function is a function that accepts another function as an argument or returns a function as a return value or both. The syntactic structure of higher order function will be as follows,

• You can also call the function which you are passing to higher order function as callback function
• The higher order function is helpful to write the modular and reusable code

29. What is a unary function?

A unary function (i.e. monadic) is a function that accepts exactly one argument. It stands for a single argument accepted by a function.

Example:

30. What is the currying function?

Currying is the process of taking a function with multiple arguments and turning it into a sequence of functions each with only a single argument. Currying is named after a mathematician Haskell Curry. By applying currying, an n-ary function turns into a unary function.

Example:

Curried functions are great to improve code reusability and functional composition.

31. What is a pure function?

A pure function is a function that, given the same inputs, always returns the same output and does not have any side effects (like modifying variables outside the function or interacting with external systems). This makes pure functions predictable, easier to test, and reliable.

Think of them like mathematical functions—clean and consistent!

32. What is the purpose of the let keyword?

The let statement declares a block scope local variable. Hence the variables defined with let keyword are limited in scope to the block, statement, or expression on which it is used. Whereas variables declared with the var keyword used to define a variable globally, or locally to an entire function regardless of block scope.

Example:

33. What is an IIFE (Immediately Invoked Function Expression)?

IIFE (Immediately Invoked Function Expression) is a JavaScript function that runs as soon as it is defined. The signature of it would be as below

Example:

34. How do you decode or encode a URL in JavaScript?

encodeURI() function is used to encode an URL. This function requires a URL string as a parameter and return that encoded string. decodeURI() function is used to decode an URL. This function requires an encoded URL string as parameter and return that decoded string.

Note: Note: If you want to encode characters such as / ? : @ & = + $ # then you need to use encodeURIComponent()

35. What is memoization?

Memoization is an optimization technique that speeds up function execution by storing the results of expensive function calls and returning the cached result when the same inputs occur again.

Example:

36. What are classes in ES6?

ES6 classes provide a simpler and clearer syntax to create objects and deal with inheritance in JavaScript. They are primarily syntactic sugar over JavaScript’s existing prototype-based inheritance.

Example:

Whereas ES6 classes can be defined as an alternative

37. What are closures?

A closure is the combination of a function bundled(enclosed) together with its lexical environment within which that function was declared. i.e, It is an inner function that has access to the outer or enclosing function’s variables, functions and other data even after the outer function has finished its execution. The closure has three scope chains.

• Own scope where variables defined between its curly brackets
• Outer function’s variables
• Global variables

Example:

Explanation: As per the above code, the inner function(i.e, greetingInfo) has access to the variables in the outer function scope(i.e, Welcome) even after the outer function has returned.

38. What are modules?

Modules refer to small units of independent, reusable code and also act as the foundation of many JavaScript design patterns. Most of the JavaScript modules export an object literal, a function, or a constructor

39. Why do you need modules?

Below are the list of benefits using modules in JavaScript ecosystem

• Maintainability
• Reusability
• Namespacing

40. What is scope in JavaScript?

Scope is the accessibility of variables, functions, and objects in some particular part of your code during runtime. In other words, scope determines the visibility of variables and other resources in areas of your code.

41. What is a Cookie?

A cookie is a piece of data that is stored on your computer to be accessed by your browser. Cookies are saved as key/value pairs. For example, you can create a cookie named username as below:

42. Why do you need a Cookie?

Cookies are used to remember information about the user profile(such as username). It basically involves two steps:

• When a user visits a web page, the user profile can be stored in a cookie.
• Next time the user visits the page, the cookie remembers the user profile.

43. How do you delete a cookie?

You can delete a cookie by setting the expiry date as a passed date. You don’t need to specify a cookie value in this case

Example:

44. How do you convert a string to a number in JavaScript?

In JavaScript, you can convert a string to a number using several methods. The most common ones are

• Number()
• parseInt()
• parseFloat()
• unary plus operator (+)

45. What are template literals and how are they used?

Template literals are a feature in JavaScript that allow for easier string interpolation and multi-line strings. They are enclosed by backticks (`) instead of single or double quotes. You can embed expressions within template literals using ${expression} syntax.

46. What is the spread operator and how is it used?

The spread operator, represented by three dots (…), is used in JavaScript to expand iterable objects like arrays or strings into individual elements. It can also be used to spread object properties. For example, you can use it to combine arrays, copy arrays, or pass array elements as arguments to a function.

47. What is the purpose of the break and continue statements?

The break statement is used to exit a loop or switch statement prematurely, while the continue statement skips the current iteration of a loop and proceeds to the next iteration. For example, in a for loop, break will stop the loop entirely, and continue will skip to the next iteration.

Example:

48. What is the ternary operator and how is it used?

The ternary operator is a shorthand for an if-else statement and is used to execute one of two expressions based on a condition. It’s called “ternary” because it involves three operands: a condition, an expression to execute if the condition is true, and an expression to execute if the condition is false.

Syntax:

Example:

49. What is the purpose of the switch statement?

The switch statement is used to execute one block of code among many based on the value of an expression. It is an alternative to using multiple if…else if statements. The switch statement evaluates an expression, matches the expression’s value to a case label, and executes the associated block of code. If no case matches, the default block is executed.

50. What is the difference between a parameter and an argument?

A parameter is a variable in the declaration of a function, while an argument is the actual value passed to the function when it is called. For example, in the function function add(a, b) { return a + b; }, a and b are parameters. When you call add(2, 3), 2 and 3 are arguments.