Interview Questions and Answers

Ques 1. Explain the concept of closures in JavaScript.

Closures allow a function to access variables from its outer scope even after the function has finished executing.

Example:

function outer() {
  let x = 10;
  function inner() {
    console.log(x);
  }
  return inner;
}
const closureExample = outer();
closureExample();

Ques 2. What is AJAX, and how does it work?

AJAX (Asynchronous JavaScript and XML) allows web pages to be updated asynchronously by exchanging small amounts of data with the server behind the scenes.

Example:

var xhr = new XMLHttpRequest();
xhr.open('GET', 'https://api.example.com/data', true);
xhr.onreadystatechange = function() {
  if (xhr.readyState == 4 && xhr.status == 200) {
    console.log(xhr.responseText);
  }
};
xhr.send();

Ques 3. Explain the concept of event delegation in JavaScript.

Event delegation involves using a single event listener to manage all related events for a specific type, reducing the number of event listeners.

Example:

document.getElementById('parentElement').addEventListener('click', function(event) {
  if (event.target.tagName === 'BUTTON') {
    console.log('Button clicked!');
  }
});

Ques 4. How can you optimize the performance of a website?

Performance optimization techniques include minimizing HTTP requests, using asynchronous loading, optimizing images, and employing caching strategies.

Ques 5. Explain the concept of 'AJAX Long Polling' in web development.

AJAX Long Polling is a technique where the server holds an HTTP request open until new data is available, allowing real-time updates.

Ques 6. Explain the concept of 'WebSockets' and how they differ from HTTP.

WebSockets provide a full-duplex communication channel over a single, long-lived connection, enabling real-time communication between the client and server, unlike the request-response nature of HTTP.

Ques 7. Explain the concept of 'Cross-site Scripting (XSS)' and how to prevent it.

XSS is a security vulnerability where attackers inject malicious scripts into web pages. To prevent it, developers should sanitize user input, use proper encoding, and implement Content Security Policy (CSP).

Ques 8. What is the purpose of the 'SQL Injection' attack and how can it be prevented?

SQL Injection is an attack where malicious SQL statements are inserted into user inputs, leading to unauthorized access or data manipulation. Prevention involves using parameterized queries and input validation.

Ques 9. Explain the concept of 'Cross-Origin Embedder Policy' (COEP) in web security.

COEP is a security policy that controls whether a document can be embedded in another document across different origins, enhancing security by preventing unauthorized embedding.

Ques 10. Explain the concept of 'Debouncing' in JavaScript.

Debouncing is a technique used to ensure that time-consuming tasks do not fire so often, making it more efficient for tasks like handling input events.

Example:

function debounce(func, delay) {
  let timeoutId;
  return function() {
    clearTimeout(timeoutId);
    timeoutId = setTimeout(() => func.apply(this, arguments), delay);
  };
}