JavaScript Interview Questions and Answers for Beginners & Experts

JavaScript Interview Questions – Explore common JS interview questions with answers, covering concepts like closures, promises, hoisting, ES6, async/await, and more.

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Preparing for JavaScript interview questions? Focus on core concepts like closures, hoisting, prototypal inheritance, and event loops. Expect technical challenges on ES6+, asynchronous programming, and debugging. Practice coding problems on arrays, objects, and functions. Mastering real-world scenarios and best practices will boost your confidence. Stay updated with latest JavaScript trends

JavaScript Interview Questions and Answers

What is JavaScript?
JavaScript is a versatile, high-level programming language primarily used for web development. It enables dynamic content, interactivity, and client-side scripting in web applications.

What are the key features of JavaScript?
JavaScript is lightweight, interpreted, event-driven, and supports object-oriented and functional programming paradigms. It enables asynchronous operations and works seamlessly with HTML and CSS.

What is the difference between var, let, and const?
var has function scope, let has block scope, and const is block-scoped but cannot be reassigned. var allows redeclaration, while let and const do not.

What is hoisting in JavaScript?
Hoisting is JavaScript's default behavior of moving function and variable declarations to the top of their scope before execution.

What are closures in JavaScript?
A closure is a function that remembers the scope in which it was created, even after that scope has finished executing. It allows access to outer function variables from an inner function.

What is the difference between == and === in JavaScript?
== checks for value equality with type coercion, whereas === checks for both value and type equality.

What is an Immediately Invoked Function Expression (IIFE)?
An IIFE is a function that executes immediately after being defined, preventing variable pollution in the global scope.

What are template literals in JavaScript?
Template literals use backticks () and allow embedding expressions with ${}`. They support multi-line strings and string interpolation.

What is the event loop in JavaScript?
The event loop handles asynchronous operations in JavaScript. It continuously checks the message queue and executes pending callbacks in the call stack.

What is the difference between null and undefined?
null is an intentional absence of value, whereas undefined means a variable has been declared but not assigned a value.

What is prototypal inheritance in JavaScript?
Prototypal inheritance allows objects to inherit properties and methods from other objects using prototypes instead of classical class-based inheritance.

What are promises in JavaScript?
A promise represents a value that may be available now, later, or never. It has three states: pending, resolved (fulfilled), and rejected.

What is async/await in JavaScript?
async/await is a modern way to handle asynchronous operations, making the code more readable by avoiding callback hell and promise chaining.

What are callbacks in JavaScript?
A callback is a function passed as an argument to another function, which is then executed after the completion of that function.

What is the difference between synchronous and asynchronous programming?
Synchronous execution runs code line-by-line, blocking further execution until the current task completes. Asynchronous execution allows other tasks to run while waiting for operations to complete.

What is the difference between let and const?
Both are block-scoped, but let allows reassignment while const does not.

What are arrow functions in JavaScript?
Arrow functions provide a concise syntax for defining functions and do not have their own this context.

What is the difference between function declaration and function expression?
A function declaration is hoisted and can be called before its definition, whereas a function expression is assigned to a variable and is not hoisted.

What is the difference between map, filter, and reduce?
map() transforms an array by applying a function to each element. filter() returns a new array with elements that satisfy a condition. reduce() aggregates values into a single result.

What is a higher-order function?
A higher-order function is a function that takes another function as an argument or returns a function.

What is the difference between call, apply, and bind?
call() and apply() invoke a function with a specified this value, but apply() accepts arguments as an array. bind() returns a new function with a fixed this value.

What is debouncing and throttling?
Debouncing delays function execution until after a specified time has elapsed. Throttling ensures a function executes at most once in a given period.

What is the difference between shallow and deep copy?
A shallow copy creates a new object but copies references to nested objects, whereas a deep copy duplicates all object properties, creating a completely independent copy.

What is localStorage and sessionStorage?
Both store key-value pairs in the browser, but localStorage persists across sessions, whereas sessionStorage is cleared when the session ends.

What is the difference between REST and GraphQL?
REST uses multiple endpoints with fixed structures, while GraphQL allows flexible queries to fetch only required data in a single request.

What is JSON and how is it used?
JSON (JavaScript Object Notation) is a lightweight data-interchange format. It is used for data exchange between clients and servers.

What is destructuring in JavaScript?
Destructuring allows extracting values from arrays or objects into separate variables using a concise syntax.

What is a spread operator (...) in JavaScript?
The spread operator expands elements of an iterable into individual elements, useful for copying arrays or merging objects.

What is the difference between setTimeout and setInterval?
setTimeout executes a function after a delay, whereas setInterval repeatedly executes a function at specified intervals.

What is event delegation?
Event delegation is a technique where a single event listener is attached to a parent element to handle events on dynamically added child elements.

What are WeakMap and WeakSet?
WeakMap and WeakSet hold weak references to objects, meaning they do not prevent garbage collection.

What is the purpose of the super keyword?
super is used in classes to call methods from a parent class.

What is the difference between for...in and for...of loops?
for...in iterates over object properties, whereas for...of iterates over iterable values like arrays.

What is a generator function in JavaScript?
A generator function returns an iterator that can yield multiple values using the yield keyword.

What are modules in JavaScript?
Modules allow code to be split into separate files for better maintainability, using import and export statements.

What is a polyfill in JavaScript?
A polyfill is a piece of code that replicates modern JavaScript features in older browsers.

What is the use of the typeof operator?
typeof determines the data type of a given variable.

What is NaN in JavaScript?
NaN (Not-a-Number) is a special value representing an invalid number.

What is the difference between slice and splice?
slice() returns a portion of an array without modifying the original array. splice() modifies the array by adding or removing elements.

What is the difference between Object.freeze() and Object.seal()?
Object.freeze() prevents modifications and additions to an object, while Object.seal() allows modifications but prevents new properties.

What are symbols in JavaScript?
Symbols are unique and immutable primitive values used as object keys.

What is the purpose of the instanceof operator?
instanceof checks whether an object is an instance of a particular class or constructor.

What is the difference between window, document, and navigator objects?
window represents the browser window, document represents the webpage content, and navigator provides browser-related information.

What is memoization in JavaScript?
Memoization is an optimization technique that caches function results to avoid redundant calculations.

What is a pure function?
A pure function always produces the same output for the same input and has no side effects.

 

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