Top JavaScript Interview Questions and Answers

In today's rapidly evolving tech landscape, JavaScript remains a pivotal programming language that drives modern web development. Whether you are preparing for an interview or looking to deepen your understanding of JavaScript, it's important to be well-versed in the most frequently asked JavaScript interview questions. This guide covers essential questions to help you stay competitive.

What is JavaScript?

JavaScript is a high-level, interpreted programming language used for creating dynamic web applications. It enables developers to implement complex features such as interactive forms, real-time content updates, and multimedia control.

What is the difference between JavaScript and Java?

Despite their similar names, Java and JavaScript are distinct languages. Java is a compiled, object-oriented programming language used for backend systems, whereas JavaScript is an interpreted scripting language primarily used for frontend web development.

Explain Hoisting in JavaScript.

Hoisting is JavaScript’s default behavior of moving declarations to the top of the current scope. This means that variable and function declarations can be used before they are declared in the code.

For example:

console.log(x); // undefined
var x = 5;        

Due to hoisting, the var declaration of x is moved to the top of the scope, though its value assignment remains in place.

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

• var: Function-scoped, and can be redeclared and updated.
• let: Block-scoped, cannot be redeclared in the same scope, but can be updated.
• const: Block-scoped, cannot be redeclared or updated.

var a = 10; // function-scoped
let b = 20; // block-scoped
const c = 30; // block-scoped and constant        

What are JavaScript closures?

A closure is a function that remembers the variables from its scope at the time it was created. This enables it to access those variables even when it is invoked outside of that scope.

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

In this example, inner retains access to count from the outer function, even after outer has completed execution.

What is event delegation in JavaScript?

Event delegation allows you to handle events at a higher level in the DOM hierarchy than the event’s target element. This is particularly useful when dealing with dynamic content.

Example:

document.querySelector('ul').addEventListener('click', function (event) {
  if (event.target.tagName === 'LI') {
    console.log('List item clicked');
  }
});        

In this case, the click event is captured by the ul element, but it's delegated to the li elements inside.

Explain the concept of Promises in JavaScript.

A Promise is an object representing the eventual completion or failure of an asynchronous operation. It allows you to write cleaner asynchronous code without getting into "callback hell."

A promise can have three states:

1. Pending: Initial state, neither fulfilled nor rejected.
2. Fulfilled: Operation completed successfully.
3. Rejected: Operation failed.

Example:

let promise = new Promise(function(resolve, reject) {
  let success = true;
  if (success) resolve("Operation succeeded");
  else reject("Operation failed");
});

promise.then(function(result) {
  console.log(result);
}).catch(function(error) {
  console.log(error);
});        

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

• ==: Compares values after converting both sides to a common type (type coercion).
• ===: Compares both value and type, making it a stricter comparison.

Example:

console.log(5 == '5');  // true
console.log(5 === '5'); // false        

What is the "this" keyword in JavaScript?

The this keyword in JavaScript refers to the object that is currently executing the function. Its value depends on the context in which the function is invoked.

Example:

const obj = {
  name: 'Alamin',
  getName: function() {
    return this.name;
  }
};
console.log(obj.getName()); // 'Alamin'        

In this case, this.name refers to the name property of obj.

What is the use of the call, apply, and bind methods?

• call(): Invokes a function with a specified this value and arguments provided one by one.
• apply(): Similar to call(), but arguments are provided as an array.
• bind(): Returns a new function where this is bound to the specified value.

function greet(greeting, punctuation) {
  console.log(greeting + ' ' + this.name + punctuation);
}

const person = { name: 'Alamin' };

greet.call(person, 'Hello', '!');   // Hello Alamin!
greet.apply(person, ['Hi', '?']);   // Hi Alamin?
const greetPerson = greet.bind(person);
greetPerson('Hey', '.');            // Hey Alamin.        

Explain async/await in JavaScript.

async/await is syntactic sugar over promises, making asynchronous code appear synchronous, and is easier to understand.

Example:

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

The await keyword pauses the execution of the function until the Promise resolves or rejects, making the code easier to follow.

What are Arrow Functions in JavaScript?

Arrow functions provide a concise syntax for writing functions and also differ from regular functions in the way they handle the this keyword.

Example:

const add = (a, b) => a + b;
console.log(add(5, 10)); // 15        

Arrow functions do not bind their own this value and are typically used in scenarios like event handling or array methods.

What are Higher-Order Functions in JavaScript?

A Higher-Order Function is a function that either takes one or more functions as arguments or returns a function as its result. Common examples include map(), filter(), and reduce().

Example:

const numbers = [1, 2, 3, 4];
const doubled = numbers.map((n) => n * 2);
console.log(doubled); // [2, 4, 6, 8]        

Explain the difference between synchronous and asynchronous code.

• Synchronous code executes line by line, blocking further execution until the current task is complete.
• Asynchronous code allows the program to move on to the next task before the previous one has completed. Asynchronous behavior is achieved using callbacks, promises, and async/await.

Example of asynchronous code:

setTimeout(() => {
  console.log('This runs after 1 second');
}, 1000);
console.log('This runs immediately');        

What is DOM manipulation in JavaScript?

DOM (Document Object Model) manipulation refers to the ability to dynamically modify the structure and content of a webpage. JavaScript provides various methods to interact with and modify the DOM.

Example:

document.getElementById('myDiv').innerHTML = 'Hello World';        

In this example, the content of the div with id="myDiv" is updated with the text 'Hello World'.