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Interview Questions and Answers
1. What is AJAX?
AJAX stands for Asynchronous JavaScript and XML. It is a set of web development techniques using many web technologies on the client side to create asynchronous web applications. With AJAX, web applications can send and retrieve data from a server asynchronously without interfering with the display and behavior of the existing page.
2. What are the key advantages of using AJAX?
Improved user experience, reduced server load, faster page rendering, partial page updates, seamless integration with RESTful APIs, and better performance by avoiding full page reloads.
3. What is the role of XMLHttpRequest (XHR) in AJAX?
XMLHttpRequest is a built-in browser object that enables asynchronous HTTP requests to servers. It allows JavaScript to send GET/POST requests and handle responses without reloading the page.
4. What is the difference between synchronous and asynchronous AJAX requests?
Synchronous requests block the browser until the response is received. Asynchronous requests allow the browser to continue processing while waiting for the server response, improving responsiveness.
5. What is jQuery AJAX?
jQuery provides simplified methods like $.ajax(), $.get(), $.post(), and $.load() to perform AJAX requests with less code and cross-browser compatibility.
6. How does the Fetch API differ from XMLHttpRequest?
Fetch API is a modern, promise-based alternative to XHR. It has a cleaner syntax, supports streams, and works natively with async/await. It does not send cookies by default and requires explicit handling for older browsers.
7. What is Axios?
Axios is a promise-based HTTP client for the browser and Node.js. It supports request/response interceptors, automatic JSON parsing, timeout configuration, and cancelable requests.
8. What are the benefits of using Axios over Fetch?
Automatic JSON transformation, built-in XSRF protection, request cancellation, progress tracking for uploads/downloads, and better error handling with response data in catch blocks.
9. What is JSONP and when is it used?
JSONP (JSON with Padding) is a technique to bypass same-origin policy by loading data via
12. Name some popular AJAX frameworks.
jQuery, Axios, Fetch API, Prototype, Dojo Toolkit, MooTools, Angular HTTP Client, Vue Resource (deprecated), React (with fetch/axios), and Ext JS.
13. What is the same-origin policy?
It is a security mechanism that restricts web pages from making requests to a different domain than the one that served the page, preventing unauthorized data access.
14. How do you handle errors in AJAX requests?
Use .catch() with promises, onerror events with XHR, or try/catch with async/await. Check HTTP status codes (4xx, 5xx), network errors, and timeout conditions.
15. What is the purpose of the async attribute in AJAX?
The async attribute in XMLHttpRequest determines whether the request is asynchronous (true, default) or synchronous (false). Asynchronous is preferred for better UX.
16. What are request and response interceptors in Axios?
Interceptors allow you to run code before a request is sent (e.g., add auth tokens) or before a response is handled (e.g., refresh token on 401). They are useful for logging, authentication, and error handling.
17. How does AJAX improve SEO?
AJAX itself does not improve SEO. Search engines may not execute JavaScript. Use server-side rendering, progressive enhancement, or prerendering services to ensure content is crawlable.
18. What is the difference between GET and POST in AJAX?
GET retrieves data and should be idempotent; data is sent via URL parameters. POST submits data to create/update resources; data is sent in the request body and is more secure for sensitive data.
19. What is caching in AJAX and how to prevent it?
Browsers cache GET requests. To prevent caching, add a unique query parameter like ?t=timestamp or set Cache-Control: no-cache headers on the server.
20. How do you abort an AJAX request?
Use AbortController with Fetch API (signal property) or XMLHttpRequest.abort(). Axios supports cancellation via CancelToken or AbortController.