Mastering AJAX: Most Asked Interview Questions

AJAX, short for Asynchronous JavaScript and XML, is a crucial technology in modern web development. It allows web pages to update content dynamically without refreshing the page, providing a smoother user experience. Because of its importance, AJAX is frequently a topic in web development interviews, both for frontend and full-stack roles. In this guide, we’ll cover the most asked AJAX interview questions, along with clear explanations and examples.

1. What is AJAX?

Answer:
AJAX is a technique that allows a web page to send and receive data from a server asynchronously, without reloading the entire page. Although it’s called "AJAX", it’s not limited to XML; JSON is commonly used today because it’s lightweight and easier to work with.

Example:

const xhr = new XMLHttpRequest();
xhr.open('GET', 'data.json', true);
xhr.onload = function() {
    if(xhr.status === 200) {
        console.log(xhr.responseText);
    }
};
xhr.send();

2. How does AJAX work?

Answer:
AJAX works using JavaScript’s XMLHttpRequest object (or the modern fetch API) to send requests to the server. The server processes the request and returns data (JSON, XML, HTML, or text), which JavaScript then uses to update the page dynamically.

Steps:

1. User performs an action (like clicking a button).

2. JavaScript creates an AJAX request.

3. Request is sent to the server asynchronously.

4. Server processes the request and returns data.

5. JavaScript updates the DOM with new data.

3. Difference between AJAX and traditional requests

Feature	AJAX	Traditional HTTP Request
Page reload	No	Yes
Asynchronous	Yes	No
Speed	Faster	Slower
User experience	Smooth	Interruptive

4. What are the advantages of using AJAX?

• Improved user experience: No full-page reloads.

• Reduced server load: Only necessary data is sent and received.

• Faster updates: Partial page updates make applications feel responsive.

• Asynchronous behavior: Multiple requests can run simultaneously.

5. How do you make an AJAX request using JavaScript?

Answer (Using XMLHttpRequest):

const xhr = new XMLHttpRequest();
xhr.open('GET', 'users.json', true);
xhr.onreadystatechange = function() {
    if(xhr.readyState === 4 && xhr.status === 200) {
        const data = JSON.parse(xhr.responseText);
        console.log(data);
    }
};
xhr.send();

Answer (Using Fetch API):

fetch('users.json')
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error(error));

6. What are readyState and status in AJAX?

• readyState: Represents the state of the request. Values range from 0 to 4:

  • 0: Request not initialized

  • 1: Server connection established

  • 2: Request received

  • 3: Processing request

  • 4: Request finished and response ready

• status: HTTP status code returned by the server (e.g., 200 for success, 404 for not found).

7. Difference between GET and POST in AJAX

Method	Use Case	Data Limit	Security
GET	Retrieve data	Limited (URL length)	Less secure
POST	Send/submit data	No limit	More secure

8. Can AJAX work with JSON, XML, or HTML?

Answer:
Yes. AJAX can handle multiple data formats:

• JSON: Lightweight, easy to parse in JavaScript (JSON.parse())

• XML: Older standard, uses XMLHttpRequest.responseXML

• HTML/Text: Can update the DOM directly

9. What are common errors in AJAX requests?

• CORS (Cross-Origin Resource Sharing) errors

• Network errors (server unreachable)

• Parsing errors (invalid JSON/XML)

• Incorrect URL or method

10. Difference between synchronous and asynchronous AJAX

Feature	Synchronous	Asynchronous
Page behavior	Blocks page until request completes	Page remains interactive
User experience	Poor	Better
Syntax	xhr.open('GET', url, false);	xhr.open('GET', url, true);

Note: Always prefer asynchronous requests in modern web apps.

11. How is AJAX used in real-world applications?

• Live search suggestions (Google search autocomplete)

• Chat applications (Facebook Messenger, WhatsApp Web)

• Form validation without page reloads

• Dynamic content loading (infinite scroll, news feeds)

12. AJAX Best Practices

• Always use asynchronous requests.

• Handle errors gracefully using onerror or .catch().

• Use JSON instead of XML for modern apps.

• Avoid sending sensitive data via GET.

• Minimize requests to reduce server load.

 Conclusion

AJAX interview questions is an essential skill for web developers and often come

up in interviews. By understanding how AJAX works, its advantages, request methods, and common interview questions, you can confidently handle AJAX-related queries in both theory and coding challenges.

Practice creating dynamic web applications with AJAX, like live search, forms, and dashboards, to master this technology and boost your chances in interviews.