Top 10 Most Asked JavaScript Interview Questions in 2025 (With Simple Answers + Code Examples)

Javascript is evolving rapidly and so are the interview questions from companies. Whether you are a beginner aspiring to break into the industry and land your first job, or an expert who wants to refresh your memory on the essentials, this guide is here to help walk you through the top 10 most asked JavaScript questions in career interviews this year (2025) with concise explanations and code samples.

In this detailed guide, you'll find not only the most commonly asked JavaScript interview questions in 2025 but also clear, beginner-friendly explanations with real code examples. This post is tailored to help you stand out in technical interviews and understand the core JavaScript concepts that top companies are evaluating in 2025.

Let’s dive in 👇

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

In JavaScript, the three keywords for declaring variables are var, let, and const. Each of these keywords has distinct features regarding their scope, hoisting behavior, and mutability. Grasping these distinctions is essential for creating reliable and high-quality code.

A) var :

• Function-scoped
• Gets hoisted (initialized with undefined)
• Can be redeclared

function varExample() {
  var x = 5;
  if (true) {
    var x = 10; // Same function scope – overrides outer 'x'
    console.log("Inside if block:", x); // 10
  }
  console.log("Outside if block:", x); // 10
}

varExample();

B) let:

• Block-scoped
• Hoisted but not initialized (Temporal Dead Zone)
• Cannot be redeclared in the same scope

function letExample() {
  let y = 5;
  if (true) {
    let y = 10; // Block scoped – this is a different 'y'
    console.log("Inside if block:", y); // 10
  }
  console.log("Outside if block:", y); // 5
}

letExample();

C) const:

• Block-scoped
• Must be initialized during declaration
• Cannot be reassigned

function constExample() {
  const z = 5;
  if (true) {
    const z = 10; // Block scoped – different variable
    console.log("Inside if block:", z); // 10
  }
  console.log("Outside if block:", z); // 5
}

constExample();

2. What is a Closure in JavaScript?

When an inner function may access the variables of an outside function even after the outer function has returned, this is known as a closure.

Strong ideas like data encapsulation, currying, and even React hooks are built on closures.

function outer() {
  let name = "JavaScript";

  return function inner() {
    console.log(`Hello from ${name}`);
  };
}

const greet = outer();
greet(); // Hello from JavaScript

Here, the inner function "remembers" the variable name, even after outer function has finished executing.

3. Explain the 'this' keyword in JavaScript

The value of 'this' varies depending on how a function is invoked in JavaScript. The global object (browser window) is referred to in the global scope.

'this' is the object itself in object methods. However, 'this' is something that arrow functions inherit from their surrounding (lexical) scope rather than having it of their own.

A) In global scope:

console.log(this); // window (in browsers)

B) In a method:

const person = {
  name: 'Sid',
  greet: function() {
    console.log(`Hello, ${this.name}`);
  }
};

person.greet(); // Hello, Sid

C) In an arrow function:

Arrow functions do not have their own 'this'. They inherit it from their lexical parent.

const person = {
  name: 'Sid',
  greet: () => {
    console.log(`Hello, ${this.name}`);
  }
};

person.greet(); // Hello, undefined

4. What is the difference between '==' and '===' ?

In JavaScript, the '==' operator is referred to as loose equality. If required, it converts the types of the two values before comparing them. Conversely, '===', often known as strict equality, verifies the type and the value without converting either.

Unexpected outcomes can occasionally arise from using ==, particularly when comparing disparate data types like strings and numbers. Because of this, using '===' at all times is regarded as best practice in order to guarantee correct comparisons and prevent minor errors in your code.

'5' == 5    // true  (types are different, but values are loosely equal)
'5' === 5   // false (types are different, no conversion)
true == 1   // true
true === 1  // false

Pro Tip: Always use '===' to avoid unexpected bugs.

5. What are Promises and async/await?

A) Promises:

In JavaScript, 'promises' are used to manage asynchronous actions like reading files or making API requests. A promise is a value that could exist right now, in the future, or never at all.
It is in three different states: rejected, fulfilled, and pending.

fetch('https://api.example.com/data')
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.error(err));

Using .then() and .catch(), you can chain actions after an async operation succeeds or fails.

B) async/await:

'Async/await' is a syntactic sugar over Promises that makes asynchronous code appear and perform like synchronous code. It enhances mistake management and readability, particularly in intricate reasoning.

async function getData() {
  try {
    const res = await fetch('https://api.example.com/data');
    const data = await res.json();
    console.log(data);
  } catch (err) {
    console.error(err);
  }
}

Using await pauses execution until the Promise resolves, making the code cleaner and easier to debug.

6. What is Event Delegation?

Event delegation is a clever JavaScript method that attaches a single event listener to a common parent element rather of adding separate listeners to several child components. This listener keeps an eye out for things that come out of the kids.

Because it uses less memory and improves efficiency by lowering the number of event listeners, it's particularly helpful when working with huge lists or dynamically added elements.

document.getElementById('list').addEventListener('click', function(e) {
  if (e.target && e.target.nodeName === 'LI') {
    console.log('Clicked item:', e.target.textContent);
  }
});

7. What is the difference between null, undefined, and NaN?

Undefined, null, and NaN are three distinct special value types in JavaScript that each represent invalid or missing data in a unique way.

A variable that has been declared but not given a value is said to be undefined. It is the uninitialized variable's default value.

Null is a purposeful assignment that denotes the lack of any object or value. Developers frequently intentionally convey "nothing" with null.

"Not a Number" (NaN) is the result of performing an improper mathematical operation, such as dividing a sequence by a number.

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

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

let c = 'abc' / 2;
console.log(c); // NaN

8. What are Arrow Functions?

In ES6, arrow functions were included as a shorter syntax for function writing. When writing short, one-line functions, such as callbacks or basic calculations, they are quite helpful.

Arrow functions are perfect when you wish to keep this from the surrounding (lexical) scope because they don't have their own this, arguments, or super like ordinary functions do. They should not, however, be used as constructors or in methods that need dynamic binding.

For code that is clearer and easier to comprehend, use arrow functions, particularly in array methods like map(), filter(), and forEach().

// Regular function
function add(a, b) {
  return a + b;
}

// Arrow function (shorter version)
const addArrow = (a, b) => a + b;

console.log(add(2, 3));       // 5
console.log(addArrow(2, 3));  // 5

9. What is Hoisting?

Hoisting is JavaScript’s default behavior of moving functions and variable declarations to the top of their scope before code runs.

With var, variables are hoisted and initialized as undefined.
With let and const, they’re hoisted too, but stay in the Temporal Dead Zone, causing a ReferenceError if accessed before declaration.

// Variable Hoisting
console.log(x); // undefined (var is hoisted)
var x = 10;

console.log(y); // ReferenceError (let is hoisted but in Temporal Dead Zone)
let y = 20;

// Function Hoisting
greet(); // "Hello" (function declaration is fully hoisted)
function greet() {
  console.log("Hello");
}

// Function Expression Hoisting
sayHi(); // TypeError: sayHi is not a function
var sayHi = function () {
  console.log("Hi");
};

10. Debouncing vs Throttling

Debouncing and throttling are techniques for optimizing performance that help manage the frequency of function executions during events that occur at a high rate, such as typing, scrolling, or resizing.

Debounce: Triggers the function after a set delay, but only if no additional calls are made within that time frame. This is particularly beneficial for search inputs, resize events, and similar scenarios.

Throttle: Guarantees the function executes at specific intervals, no matter how often it is activated. This is especially useful for events like scroll or mouse movement.

// Debounce
function debounce(fn, delay) {
  let timer;
  return function (...args) {
    clearTimeout(timer);
    timer = setTimeout(() => fn.apply(this, args), delay);
  };
}

// Throttle
function throttle(fn, interval) {
  let last = 0;
  return function (...args) {
    const now = Date.now();
    if (now - last >= interval) {
      last = now;
      fn.apply(this, args);
    }
  };
}

🤝 Looking to Ace the Interview?

🧠 These 10 questions are only the beginning. However, getting a handle on them creates a solid base for confidently addressing React, Node.js, or complex system design inquiries.

💬 Share in the comments which question you find most challenging— or let me know if you're interested in a cheat sheet PDF version of this post!