AJAX - AJAX with Fetch API vs XMLHttpRequest (Deep Comparison)

AJAX (Asynchronous JavaScript and XML) traditionally relied on XMLHttpRequest (XHR) to communicate with servers. However, modern web development increasingly uses the Fetch API, which provides a more powerful, flexible, and cleaner way to handle HTTP requests. Understanding the differences between these two approaches is important for writing efficient and maintainable web applications.

1. Overview of XMLHttpRequest

XMLHttpRequest is an older JavaScript API that allows web pages to send HTTP requests to servers and receive responses without reloading the page.

Key Characteristics:

  • Event-based (uses callbacks like onreadystatechange)

  • Supports synchronous and asynchronous requests (though synchronous is deprecated)

  • Verbose syntax

  • Works in all older browsers

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();

Limitations:

  • Callback-heavy code leads to complexity

  • Difficult to chain multiple requests

  • Poor error handling structure

  • No native support for promises

2. Overview of Fetch API

The Fetch API is a modern replacement for XMLHttpRequest. It is built on Promises, making asynchronous code cleaner and easier to manage.

Key Characteristics:

  • Promise-based

  • Cleaner and more readable syntax

  • Supports modern JavaScript features like async/await

  • Better handling of streams and responses

Example:

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

Or using async/await:

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

3. Syntax and Readability Comparison

XMLHttpRequest requires multiple steps:

  • Create object

  • Open request

  • Define callback

  • Send request

Fetch simplifies this:

  • Single function call

  • Direct promise handling

  • Cleaner chaining

Conclusion: Fetch is significantly more readable and concise.

4. Asynchronous Handling

XMLHttpRequest:

  • Uses event listeners or callbacks

  • Can become complex in nested operations (callback hell)

Fetch:

  • Uses Promises

  • Supports async/await

  • Easier to manage multiple sequential or parallel requests

Conclusion: Fetch provides a more structured and modern approach.

5. Error Handling

XMLHttpRequest:

  • Must manually check status codes

  • Errors are not handled uniformly

Fetch:

  • Only rejects promise on network failure

  • HTTP errors (like 404, 500) must still be handled manually

Example:

fetch(url)
    .then(response => {
        if (!response.ok) {
            throw new Error("HTTP error " + response.status);
        }
        return response.json();
    })

Conclusion: Fetch improves structure but still requires explicit HTTP error handling.

6. Response Handling

XMLHttpRequest:

  • Uses properties like responseText or responseXML

  • Manual parsing required

Fetch:

  • Provides built-in methods:

    • response.json()

    • response.text()

    • response.blob()

    • response.formData()

Conclusion: Fetch simplifies response parsing significantly.

7. Streaming and Advanced Capabilities

Fetch supports streaming responses, allowing processing of data as it arrives. XMLHttpRequest has limited support for this.

This is useful for:

  • Large data downloads

  • Real-time updates

  • Progressive rendering

Conclusion: Fetch is more powerful for modern web applications.

8. Request Cancellation

XMLHttpRequest:

  • Uses xhr.abort()

Fetch:

  • Uses AbortController

Example:

const controller = new AbortController();

fetch(url, { signal: controller.signal });

controller.abort();

Conclusion: Both support cancellation, but Fetch uses a more modern approach.

9. Browser Compatibility

XMLHttpRequest:

  • Supported in all browsers, including very old ones

Fetch:

  • Supported in modern browsers

  • Requires polyfills for older browsers like Internet Explorer

Conclusion: XMLHttpRequest is better for legacy support.

10. Performance Considerations

  • Both APIs use the same underlying HTTP mechanism

  • Performance differences are minimal in most cases

  • Fetch may perform better in modern environments due to optimizations and streaming

Conclusion: Performance is similar, but Fetch is more efficient in modern use cases.

11. When to Use Which

Use Fetch API when:

  • Building modern applications

  • Using async/await

  • Handling JSON APIs

  • Working with streams or advanced features

Use XMLHttpRequest when:

  • Supporting very old browsers

  • Maintaining legacy codebases

  • Needing specific low-level control not yet supported by Fetch

Final Summary

XMLHttpRequest laid the foundation for AJAX, but it is now considered outdated for most modern development. The Fetch API provides a cleaner, more flexible, and more powerful way to perform asynchronous HTTP requests. While XMLHttpRequest still has relevance in legacy environments, Fetch is the preferred choice for modern web applications due to its simplicity, scalability, and integration with modern JavaScript features.