Top 50 JavaScript Interview Questions and Answers (2025)

Looking to ace your JavaScript interview? Whether you’re a college student, fresher, job seeker, or experienced developer, this guide is packed with the top 50 most frequently asked JavaScript interview questions with clear, human-like explanations and real-time examples. Let’s dive in! 🚀

Q1. What is JavaScript and how is it different from Java?

Answer:
JavaScript is a lightweight, interpreted programming language primarily used to add interactivity and dynamic content to websites. It runs on the browser (client-side) and is a core technology of the web along with HTML and CSS.

Difference from Java:

JavaScript	Java
Interpreted	Compiled
Used for frontend web development	Used for backend/server-side and applications
Runs in browser	Runs on JVM
Loosely typed	Strongly typed

Example:

Q2. What are the different data types in JavaScript?

Answer:
JavaScript supports the following primitive and reference data types:

Primitive Types:
- String
- Number
- Boolean
- Undefined
- Null
- BigInt
- Symbol
Reference Types:
- Objects
- Arrays
- Functions

Example:

Q3. What is the difference between `var`, `let`, and `const`?

Answer:

Keyword	Scope	Reassignable	Hoisted
`var`	Function	Yes	Yes (undefined)
`let`	Block	Yes	No
`const`	Block	No	No

Example:

var x = 5;
let y = 10;
const z = 15;

var x = 5;

let y = 10;

const z = 15;

Use let and const in modern JavaScript for better scoping and safety.

Q4. What are arrow functions in JavaScript?

Answer:
Arrow functions are a shorter syntax for writing functions introduced in ES6, and they don’t bind their own this.

Syntax:

Short version (for single return):

Q5. What is the difference between `==` and `===` in JavaScript?

Answer:

== (Loose Equality): Compares values after type coercion
=== (Strict Equality): Compares values and types

Example:

Always prefer === to avoid unexpected type conversions.

Q6. Explain event delegation in JavaScript.

Answer:
Event Delegation is a technique where a single event listener is added to a parent element to handle events for all of its child elements, using event bubbling.

Why use it?

Better performance
Fewer event listeners

Example:

document.getElementById("list").addEventListener("click", function(e) {
  if (e.target.tagName === "LI") {
    alert("Item clicked: " + e.target.innerText);
  }
});

document.getElementById("list").addEventListener("click", function(e) {

if (e.target.tagName === "LI") {

alert("Item clicked: " + e.target.innerText);

}

});

Q7. What is the difference between `null` and `undefined`?

Answer:

Value	Meaning
`undefined`	A variable is declared but not assigned a value
`null`	A deliberate assignment of “no value”

Example:

let a;
console.log(a); // undefined

let b = null;
console.log(b); // null

let a;

console.log(a); // undefined

let b = null;

console.log(b); // null

Q8. What are closures in JavaScript?

Answer:
A closure is when a function “remembers” the variables from its outer lexical scope even after that outer function has finished executing.

Example:

function outer() {
  let counter = 0;
  return function inner() {
    counter++;
    return counter;
  };
}

const count = outer();
console.log(count()); // 1
console.log(count()); // 2

function outer() {

let counter = 0;

return function inner() {

counter++;

return counter;

};

}

const count = outer();

console.log(count()); // 1

console.log(count()); // 2

Closures are used in data privacy and callback functions.

Q9. What is hoisting in JavaScript?

Answer:
Hoisting is JavaScript’s default behavior of moving declarations (not initializations) to the top of the current scope.

Example:

console.log(x); // undefined
var x = 10;

1 2	console.log(x); // undefined var x = 10;

Functions are fully hoisted, but variables declared with let and const are not initialized during hoisting.

Q10. Explain the difference between synchronous and asynchronous JavaScript.

Answer:

Type	Description
Synchronous	Code is executed line by line, blocking the next line until current finishes
Asynchronous	Code execution doesn’t wait; useful for tasks like API calls, timers, etc.

Example using setTimeout:

console.log("Start");

setTimeout(() => {
  console.log("Async Code");
}, 1000);

console.log("End");

console.log("Start");

setTimeout(() => {

console.log("Async Code");

}, 1000);

console.log("End");

Output:

Start
End
Async Code

Start

End

Async Code

Q11. What is the DOM in JavaScript?

Answer:
The DOM (Document Object Model) is a tree-like structure representing the contents of a web page. JavaScript uses the DOM to access and manipulate HTML and CSS dynamically.

Example:

DOM allows developers to:

Change content
Modify styles
Add/remove elements
Respond to events

Q12. What is the difference between `call()`, `apply()`, and `bind()`?

Answer:

Method	Description	Example
`call()`	Invokes function immediately with arguments passed individually	`fn.call(obj, arg1, arg2)`
`apply()`	Invokes function with arguments passed as an array	`fn.apply(obj, [arg1, arg2])`
`bind()`	Returns a new function with a bound context (does not invoke immediately)	`const newFn = fn.bind(obj)`

Example:

function greet(age) {
  console.log(`${this.name} is ${age} years old`);
}
const person = { name: "Brahmi" };

greet.call(person, 25);     // Brahmi is 25 years old
greet.apply(person, [30]);  // Brahmi is 30 years old
const boundGreet = greet.bind(person);
boundGreet(35);             // Brahmi is 35 years old

function greet(age) {

console.log(`${this.name} is ${age} years old`);

}

const person = { name: "Brahmi" };

greet.call(person, 25); // Brahmi is 25 years old

greet.apply(person, [30]); // Brahmi is 30 years old

const boundGreet = greet.bind(person);

boundGreet(35); // Brahmi is 35 years old

Q13. What is an IIFE (Immediately Invoked Function Expression)?

Answer:
An IIFE is a function that runs immediately after being defined. It’s often used to create a private scope.

Syntax:

(function() {
  console.log("This runs instantly!");
})();

(function() {

console.log("This runs instantly!");

})();

Useful for:

Avoiding polluting the global scope
Creating modular and self-contained code

Q14. What are template literals in JavaScript?

Answer:
Template literals (introduced in ES6) allow multi-line strings and embedded expressions using backticks ().

Example:

let name = "John";
let greeting = `Hello, ${name}!
Welcome to JavaScript world.`;
console.log(greeting);

let name = "John";

let greeting = `Hello, ${name}!

Welcome to JavaScript world.`;

console.log(greeting);

They simplify string concatenation and enhance readability.

Q15. What is the difference between `for...of` and `for...in` loops?

Answer:

Loop	Iterates Over	Use Case
`for...in`	Keys (property names) of an object	Objects
`for...of`	Values of an iterable (like arrays)	Arrays, strings, etc.

Example:

let arr = ['a', 'b', 'c'];

for (let i in arr) {
  console.log(i); // Index: 0, 1, 2
}

for (let val of arr) {
  console.log(val); // Values: a, b, c
}

let arr = ['a', 'b', 'c'];

for (let i in arr) {

console.log(i); // Index: 0, 1, 2

}

for (let val of arr) {

console.log(val); // Values: a, b, c

}

Q16. What are promises in JavaScript?

Answer:
A promise represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

States:

Pending
Fulfilled
Rejected

Example:

let myPromise = new Promise((resolve, reject) => {
  setTimeout(() => resolve("Success!"), 1000);
});

myPromise.then(result => console.log(result)); // Success!

let myPromise = new Promise((resolve, reject) => {

setTimeout(() => resolve("Success!"), 1000);

});

myPromise.then(result => console.log(result)); // Success!

Promises help write cleaner async code without deeply nested callbacks.

Q17. What is async/await in JavaScript?

Answer:
async/await is syntactic sugar over promises that allows writing asynchronous code as if it were synchronous.

Example:

async function fetchData() {
  let response = await fetch("https://api.example.com/data");
  let data = await response.json();
  console.log(data);
}
fetchData();

async function fetchData() {

let response = await fetch("https://api.example.com/data");

let data = await response.json();

console.log(data);

}

fetchData();

✅ Cleaner, more readable than .then() chains.

Q18. What is a callback function?

Answer:
A callback function is a function passed as an argument to another function and executed later.

Example:

function greetUser(name, callback) {
  console.log("Hi " + name);
  callback();
}

function sayBye() {
  console.log("Bye!");
}

greetUser("Emma", sayBye);
// Output:
// Hi Emma
// Bye!

function greetUser(name, callback) {

console.log("Hi " + name);

callback();

}

function sayBye() {

console.log("Bye!");

}

greetUser("Emma", sayBye);

// Output:

// Hi Emma

// Bye!

Used heavily in asynchronous programming (e.g., event handling, timers).

Q19. What is the difference between mutable and immutable data types in JavaScript?

Answer:

Type	Can be Changed?	Examples
Mutable	Yes	Arrays, Objects
Immutable	No	Strings, Numbers, Booleans

Example:

let arr = [1, 2];
arr.push(3); // Modified — mutable

let str = "hello";
str[0] = "H"; // No change — immutable

let arr = [1, 2];

arr.push(3); // Modified — mutable

let str = "hello";

str[0] = "H"; // No change — immutable

Understanding this helps avoid bugs related to unintended data mutations.

Q20. How is memory managed in JavaScript?

Answer:
JavaScript uses automatic garbage collection to manage memory. It tracks which values are accessible and frees memory used by unreferenced values.

Key Concepts:

Reachability: If a value is reachable, it’s kept in memory.
Garbage Collection: Unreachable objects are removed.

Example:

let obj = { name: "Bob" };
obj = null; // Original object becomes unreachable

1 2	let obj = { name: "Bob" }; obj = null; // Original object becomes unreachable

💡 Tip: Avoid memory leaks by:

Removing unused event listeners
Clearing intervals/timers
Nullifying unnecessary object references

Q21. What is event delegation in JavaScript?

Answer:
Event delegation is a technique where a parent element handles events of its child elements. Instead of attaching event listeners to every child, you attach one to the parent and use event.target to detect which child triggered the event.

Benefits:

Better performance
Less memory usage
Easier to handle dynamically added elements

Example:

document.getElementById("list").addEventListener("click", function(event) {
  if (event.target.tagName === "LI") {
    console.log("You clicked:", event.target.textContent);
  }
});

document.getElementById("list").addEventListener("click", function(event) {

if (event.target.tagName === "LI") {

console.log("You clicked:", event.target.textContent);

}

});

Q22. What is the difference between `==` and `===` in JavaScript?

Answer:

Operator	Description	Example
`==`	Loose equality (performs type coercion)	`5 == "5"` → `true`
`===`	Strict equality (no type coercion)	`5 === "5"` → `false`

Best Practice:
Always use === for strict and predictable comparisons.

Q23. What are arrow functions in JavaScript?

Answer:
Arrow functions (=>) are a shorter syntax for writing functions, introduced in ES6.

Syntax:

const add = (a, b) => a + b;

1	const add = (a, b) => a + b;

Key differences from regular functions:

No own this binding
Not suitable for methods or constructors
Cannot use arguments object

Example:

let greet = name => console.log(`Hello, ${name}`);
greet("Alex");

1 2	let greet = name => console.log(`Hello, ${name}`); greet("Alex");

Q24. What is a closure in JavaScript?

Answer:
A closure is created when a function remembers its lexical scope even when the function is executed outside that scope.

Use cases:

Data privacy
Function factories
Callback-based logic

Example:

function outer() {
  let counter = 0;
  return function inner() {
    counter++;
    return counter;
  };
}
const increment = outer();
console.log(increment()); // 1
console.log(increment()); // 2

function outer() {

let counter = 0;

return function inner() {

counter++;

return counter;

};

}

const increment = outer();

console.log(increment()); // 1

console.log(increment()); // 2

Also Read,

Top 50 HTML Interview Questions and Answers

Top 50 Common Interview Questions & How to Answer Them

Q25. What is hoisting in JavaScript?

Answer:
Hoisting is JavaScript’s default behavior of moving declarations to the top of the current scope before code execution.

Variables (var) and function declarations are hoisted.
let and const are hoisted but not initialized (TDZ – Temporal Dead Zone).

Example:

console.log(a); // undefined (due to hoisting)
var a = 5;

1 2	console.log(a); // undefined (due to hoisting) var a = 5;

Note: Prefer let and const to avoid hoisting issues.

Q26. What are higher-order functions in JavaScript?

Answer:
A higher-order function is a function that takes another function as an argument, returns a function, or both.

Example:

function greet(name) {
  return function(message) {
    console.log(`Hi ${name}, ${message}`);
  };
}
const greetJohn = greet("John");
greetJohn("Good morning!");

function greet(name) {

return function(message) {

console.log(`Hi ${name}, ${message}`);

};

}

const greetJohn = greet("John");

greetJohn("Good morning!");

Examples in JavaScript:

map()
filter()
reduce()

Q27. What is the event loop in JavaScript?

Answer:
The event loop is the mechanism that handles asynchronous operations like setTimeout, Promises, and I/O in a non-blocking way.

Flow:

Executes the call stack
Checks the message queue
Moves callbacks to the call stack when it’s empty

Example:

console.log("Start");
setTimeout(() => console.log("Timeout"), 0);
console.log("End");

// Output:
// Start
// End
// Timeout

console.log("Start");

setTimeout(() => console.log("Timeout"), 0);

console.log("End");

// Output:

// Start

// End

// Timeout

Q28. What is the difference between `null` and `undefined`?

Answer:

Term	Meaning
`undefined`	A variable that has been declared but not assigned a value
`null`	Represents the intentional absence of any object value

Example:

let a;
console.log(a); // undefined

let b = null;
console.log(b); // null

let a;

console.log(a); // undefined

let b = null;

console.log(b); // null

Q29. What are JavaScript data types?

Answer:
JavaScript has 8 data types:

Primitive types:

String
Number
Boolean
Null
Undefined
Symbol (ES6)
BigInt (ES11)

Non-primitive:
8. Object (includes arrays, functions, dates, etc.)

Example:

let name = "Alice"; // String
let age = 25;       // Number
let active = true;  // Boolean
let data = null;    // Null
let info;           // Undefined

let name = "Alice"; // String

let age = 25; // Number

let active = true; // Boolean

let data = null; // Null

let info; // Undefined

Q30. What is the difference between synchronous and asynchronous code in JavaScript?

Answer:

Type	Description
Synchronous	Executes line by line, blocking the thread
Asynchronous	Executes in the background, non-blocking

Synchronous Example:

Asynchronous Example:

Asynchronous code improves performance and responsiveness.

Q31. What are JavaScript Generators and how are they different from regular functions?

Answer:
Generators are special functions in JavaScript that can pause and resume execution using the function* syntax and yield keyword.

Key Differences:

Use function* to define
Use yield to pause execution
Return an iterator object

Example:

function* counter() {
  yield 1;
  yield 2;
  yield 3;
}

const gen = counter();
console.log(gen.next()); // { value: 1, done: false }
console.log(gen.next()); // { value: 2, done: false }

function* counter() {

yield 1;

yield 2;

yield 3;

}

const gen = counter();

console.log(gen.next()); // { value: 1, done: false }

console.log(gen.next()); // { value: 2, done: false }

Generators are ideal for lazy evaluations, handling asynchronous flows, and implementing iterators.

Q32. What is the event loop in JavaScript?

Answer:
The event loop is the core of JavaScript’s non-blocking concurrency model, allowing async operations like callbacks, promises, and DOM events.

Working Process:

Executes the call stack (main thread)
When async code is encountered (e.g., setTimeout, fetch), it’s sent to the Web APIs
After completion, the callback is pushed to the task queue
The event loop checks if the call stack is empty, then pushes queued tasks to it

Example:

console.log("Start");
setTimeout(() => console.log("Timeout"), 0);
console.log("End");

// Output: Start → End → Timeout

console.log("Start");

setTimeout(() => console.log("Timeout"), 0);

console.log("End");

// Output: Start → End → Timeout

Q33. What are WeakMap and WeakSet in JavaScript?

Answer:

WeakMap: A collection of key-value pairs where keys must be objects
WeakSet: A collection of objects only, without duplicates

Advantages:

Keys are weakly held, meaning they are eligible for garbage collection
Used to prevent memory leaks

Example:

They don’t support iteration and don’t prevent memory cleanup.

Q34. Explain the concept of reflow and repaint in the browser rendering process.

Answer:
When DOM changes happen, the browser may trigger:

Repaint: Changes only appearance (e.g., color, visibility)
Reflow (Layout): Changes that affect layout or position, like element size or DOM structure

Reflow is more expensive than repaint.
Minimizing DOM manipulations and batching changes improves performance.

Tip: Use classList.add() instead of directly changing multiple styles.

Q35. What are the different types of memory in JavaScript?

Answer:
JavaScript uses:

Stack Memory
- Stores primitive values and function execution context
- Fast access
Heap Memory
- Stores objects, arrays, and functions
- Dynamically allocated and larger

Example:

let num = 5; // Stack
let obj = { age: 25 }; // Heap

1 2	let num = 5; // Stack let obj = { age: 25 }; // Heap

Understanding memory helps debug leaks and optimize performance.

Q36. What are Service Workers in JavaScript?

Answer:
A Service Worker is a background script that runs independently of the web page, enabling:

Offline experiences
Background sync
Push notifications
Caching resources

Key Features:

Runs in a separate thread
Cannot access the DOM directly
Acts like a proxy between the app and the network

Basic Registration:

if ("serviceWorker" in navigator) {
  navigator.serviceWorker.register("/sw.js");
}

if ("serviceWorker" in navigator) {

navigator.serviceWorker.register("/sw.js");

}

Q37. How does optional chaining work in JavaScript (`?.`)?

Answer:
Optional chaining (?.) allows you to safely access deeply nested properties without checking each level.

Example:

Helps prevent runtime errors from accessing undefined or null properties.

Q38. What is the difference between microtasks and macrotasks in JavaScript?

Answer:

Type	Examples	Priority
Microtasks	`Promise.then()`, `queueMicrotask()`	High priority (executed after current task)
Macrotasks	`setTimeout`, `setInterval`, DOM events	Lower than microtasks

Execution Order:

console.log("Start");

setTimeout(() => console.log("Macrotask"), 0);
Promise.resolve().then(() => console.log("Microtask"));

console.log("End");

// Output: Start → End → Microtask → Macrotask

console.log("Start");

setTimeout(() => console.log("Macrotask"), 0);

Promise.resolve().then(() => console.log("Microtask"));

console.log("End");

// Output: Start → End → Microtask → Macrotask

Q39. What is function memoization and how do you implement it in JavaScript?

Answer:
Memoization is an optimization technique to cache previously computed results of function calls.

Use case: Improve performance of recursive or heavy computations.

Example:

function memoize(fn) {
  const cache = {};
  return function (n) {
    if (cache[n]) return cache[n];
    return (cache[n] = fn(n));
  };
}

const factorial = memoize(function (x) {
  if (x <= 1) return 1;
  return x * factorial(x - 1);
});

console.log(factorial(5)); // 120

function memoize(fn) {

const cache = {};

return function (n) {

if (cache[n]) return cache[n];

return (cache[n] = fn(n));

};

}

const factorial = memoize(function (x) {

if (x <= 1) return 1;

return x * factorial(x - 1);

});

console.log(factorial(5)); // 120

Q40. What is the Temporal API in JavaScript?

Answer:
The Temporal API is a new date and time handling API (introduced to replace Date) with better precision, immutability, and time zone support.

Key Features:

Precise time calculations
Time zones support
Immutable objects
Safer than Date

Example:

const now = Temporal.Now.instant();
console.log(now.toString()); // 2025-05-29T...

const date = Temporal.PlainDate.from("2025-12-25");
console.log(date.add({ days: 5 }).toString()); // 2025-12-30

const now = Temporal.Now.instant();

console.log(now.toString()); // 2025-05-29T...

const date = Temporal.PlainDate.from("2025-12-25");

console.log(date.add({ days: 5 }).toString()); // 2025-12-30

Note: Still in proposal stage in some environments—check browser compatibility.

Q41. What is the difference between `Object.freeze()` and `const` in JavaScript?

Answer:
Both are used for immutability, but they serve different purposes:

const prevents reassignment of a variable.
Object.freeze() makes an object immutable, i.e., its properties cannot be changed, added, or deleted.

Example:

const obj = { name: "Alex" };
obj.name = "Sam"; // valid

Object.freeze(obj);
obj.name = "Bob"; // no effect in strict mode

const obj = { name: "Alex" };

obj.name = "Sam"; // valid

Object.freeze(obj);

obj.name = "Bob"; // no effect in strict mode

Object.freeze() is shallow—nested objects are still mutable unless also frozen.

Q42. What is tail call optimization in JavaScript?

Answer:
Tail Call Optimization (TCO) is a technique where the last action of a function is a call to another function, allowing the JS engine to reuse stack frames, preventing stack overflow.

Example (with tail call):

function factorial(n, acc = 1) {
  if (n === 0) return acc;
  return factorial(n - 1, acc * n);
}

function factorial(n, acc = 1) {

if (n === 0) return acc;

return factorial(n - 1, acc * n);

}

⚠️ Supported in strict mode in some engines (e.g., Safari).

Q43. What is the difference between `==` and `Object.is()` in JavaScript?

Answer:

Comparison	Behavior
`==`	Performs type coercion
`Object.is()`	No coercion, uses SameValue algorithm

Special Case:

NaN === NaN         // false
Object.is(NaN, NaN) // true

+0 === -0           // true
Object.is(+0, -0)   // false

NaN === NaN // false

Object.is(NaN, NaN) // true

+0 === -0 // true

Object.is(+0, -0) // false

Use Object.is() when you want exact identity checks.

Q44. What is the difference between `call`, `apply`, and `bind` methods?

Answer:

Method	Description	Example
`call()`	Calls function with `this` and individual arguments	`fn.call(thisArg, arg1, arg2)`
`apply()`	Calls function with `this` and array of arguments	`fn.apply(thisArg, [args])`
`bind()`	Returns a new function with `this` bound	`let newFn = fn.bind(thisArg)`

Example:

function greet(msg) {
  return `${msg}, ${this.name}`;
}

const user = { name: "Ravi" };
console.log(greet.call(user, "Hello"));      // Hello, Ravi
console.log(greet.apply(user, ["Hi"]));      // Hi, Ravi
const greetRavi = greet.bind(user);
console.log(greetRavi("Welcome"));           // Welcome, Ravi

function greet(msg) {

return `${msg}, ${this.name}`;

}

const user = { name: "Ravi" };

console.log(greet.call(user, "Hello")); // Hello, Ravi

console.log(greet.apply(user, ["Hi"])); // Hi, Ravi

const greetRavi = greet.bind(user);

console.log(greetRavi("Welcome")); // Welcome, Ravi

Q45. How can you deep clone an object in JavaScript?

Answer:
Deep cloning means copying all nested objects and arrays.

Common Methods:

StructuredClone (modern)

const newObj = structuredClone(original);

1	const newObj = structuredClone(original);

JSON method (basic)

const newObj = JSON.parse(JSON.stringify(original));

1	const newObj = JSON.parse(JSON.stringify(original));

⚠️ Doesn’t handle undefined, functions, or circular references.

Manual recursion or libraries (e.g., Lodash’s cloneDeep) for complex cases.

Q46. What is the Temporal Dead Zone (TDZ) in JavaScript?

Answer:
The TDZ is the time between the declaration of a variable with let/const and its initialization, during which accessing it results in a ReferenceError.

Example:

console.log(a); // ReferenceError
let a = 10;

1 2	console.log(a); // ReferenceError let a = 10;

Variables declared with var do not have TDZ, which can lead to undefined.

Q47. What is the difference between `.forEach()`, `.map()`, `.filter()`, and `.reduce()`?

Answer:

Method	Purpose	Returns
`.forEach()`	Iterates for side effects	`undefined`
`.map()`	Transforms array	New array
`.filter()`	Filters items	New array
`.reduce()`	Reduces to a value	Single value

Example:

const nums = [1, 2, 3];

nums.forEach(n => console.log(n));        // 1 2 3
console.log(nums.map(n => n * 2));        // [2, 4, 6]
console.log(nums.filter(n => n > 1));     // [2, 3]
console.log(nums.reduce((a, b) => a + b));// 6

const nums = [1, 2, 3];

nums.forEach(n => console.log(n)); // 1 2 3

console.log(nums.map(n => n * 2)); // [2, 4, 6]

console.log(nums.filter(n => n > 1)); // [2, 3]

console.log(nums.reduce((a, b) => a + b));// 6

Q48. What is a throttling function and how is it different from debouncing?

Answer:

Concept	Description	Use Case
Debounce	Executes after delay of inactivity	Search input
Throttle	Executes at fixed intervals	Scroll event

Throttle Example:

function throttle(func, limit) {
  let inThrottle;
  return function() {
    if (!inThrottle) {
      func();
      inThrottle = true;
      setTimeout(() => (inThrottle = false), limit);
    }
  };
}

function throttle(func, limit) {

let inThrottle;

return function() {

if (!inThrottle) {

func();

inThrottle = true;

setTimeout(() => (inThrottle = false), limit);

}

};

}

Q49. What is the use of `Symbol` in JavaScript?

Answer:
Symbol is a primitive data type introduced in ES6 for creating unique identifiers, often used for object keys to avoid naming collisions.

Example:

Symbols are not enumerable and provide encapsulation.

Q50. What is the difference between synchronous and asynchronous code in JavaScript?

Answer:

Type	Behavior	Example
Synchronous	Runs in blocking order	`for`, `if`, DOM
Asynchronous	Non-blocking, waits for callbacks	`setTimeout`, `fetch`, `Promise`

Async Example:

console.log("1");
setTimeout(() => console.log("2"), 0);
console.log("3");

// Output: 1, 3, 2

console.log("1");

setTimeout(() => console.log("2"), 0);

console.log("3");

// Output: 1, 3, 2

Async programming enables better performance and responsiveness in apps.

🏁 Conclusion

Mastering these top 50 JavaScript interview questions can significantly boost your confidence and chances of success. Whether you’re a student, fresher, or experienced developer, understanding these concepts with real examples will help you tackle any interview with ease. Keep practicing, stay curious, and you’ll crack your dream job in no time!

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Top 50 JavaScript Interview Questions and Answers (2025)

Q1. What is JavaScript and how is it different from Java?

Q2. What are the different data types in JavaScript?

Q3. What is the difference between var, let, and const?

Q4. What are arrow functions in JavaScript?

Q5. What is the difference between == and === in JavaScript?

Q6. Explain event delegation in JavaScript.

Q7. What is the difference between null and undefined?

Q8. What are closures in JavaScript?

Q9. What is hoisting in JavaScript?

Q10. Explain the difference between synchronous and asynchronous JavaScript.

Q11. What is the DOM in JavaScript?

Q12. What is the difference between call(), apply(), and bind()?

Q13. What is an IIFE (Immediately Invoked Function Expression)?

Q14. What are template literals in JavaScript?

Q15. What is the difference between for...of and for...in loops?

Q16. What are promises in JavaScript?

Q17. What is async/await in JavaScript?

Q18. What is a callback function?

Q19. What is the difference between mutable and immutable data types in JavaScript?

Q20. How is memory managed in JavaScript?

Q21. What is event delegation in JavaScript?

Q22. What is the difference between == and === in JavaScript?

Q23. What are arrow functions in JavaScript?

Q24. What is a closure in JavaScript?

Q25. What is hoisting in JavaScript?

Q26. What are higher-order functions in JavaScript?

Q27. What is the event loop in JavaScript?

Q28. What is the difference between null and undefined?

Q29. What are JavaScript data types?

Q30. What is the difference between synchronous and asynchronous code in JavaScript?

Q31. What are JavaScript Generators and how are they different from regular functions?

Q32. What is the event loop in JavaScript?

Q33. What are WeakMap and WeakSet in JavaScript?

Q34. Explain the concept of reflow and repaint in the browser rendering process.

Q35. What are the different types of memory in JavaScript?

Q36. What are Service Workers in JavaScript?

Q37. How does optional chaining work in JavaScript (?.)?

Q38. What is the difference between microtasks and macrotasks in JavaScript?

Q39. What is function memoization and how do you implement it in JavaScript?

Q40. What is the Temporal API in JavaScript?

Q41. What is the difference between Object.freeze() and const in JavaScript?

Q42. What is tail call optimization in JavaScript?

Q43. What is the difference between == and Object.is() in JavaScript?

Q44. What is the difference between call, apply, and bind methods?

Q45. How can you deep clone an object in JavaScript?

Q46. What is the Temporal Dead Zone (TDZ) in JavaScript?

Q47. What is the difference between .forEach(), .map(), .filter(), and .reduce()?

Q48. What is a throttling function and how is it different from debouncing?

Q49. What is the use of Symbol in JavaScript?

Q50. What is the difference between synchronous and asynchronous code in JavaScript?

🏁 Conclusion

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Q49. What is the use of `Symbol` in JavaScript?