Creating XMLHttpRequest in AJAX

I. Introduction to XMLHttpRequest

XMLHttpRequest is a built-in JavaScript object that allows web developers to create asynchronous requests to servers. This means you can send and receive data from a web server without needing to reload the entire page. The ability to perform operations asynchronously is crucial for a smooth user experience in modern web applications.

In the realm of AJAX (Asynchronous JavaScript and XML), XMLHttpRequest plays a significant role. AJAX refers to a combination of technologies that enable these asynchronous data requests. By leveraging this technique, developers can enhance their web applications, making them more interactive and responsive.

II. Creating XMLHttpRequest Object

Creating an instance of XMLHttpRequest is the first step to making AJAX calls. Here’s how you do it:

A. Browser Compatibility

While XMLHttpRequest is widely supported in most browsers, it’s important to understand the best practices for compatibility. Below is a simple check for creating the object depending on the browser:

let xhr;

if (window.XMLHttpRequest) {
    // code for modern browsers
    xhr = new XMLHttpRequest();
} else {
    // code for older IE browsers
    xhr = new ActiveXObject("Microsoft.XMLHTTP");
}

B. Code examples for different browsers

Browser	XMLHttpRequest Creation
Modern Browsers (Chrome, Firefox, etc.)	new XMLHttpRequest()
Internet Explorer (IE 6 and below)	new ActiveXObject(“Microsoft.XMLHTTP”)

III. Using XMLHttpRequest

Once the XMLHttpRequest object is created, you can start making requests and handling responses. Here are the essential methods and events to understand:

A. The open() method

The open() method initializes a request. It requires at least 2 parameters:

• method: The HTTP method (GET, POST, etc.)
• url: The endpoint where the request is sent

1. Parameters explained

xhr.open("GET", "https://api.example.com/data", true);

The third parameter is a boolean that indicates whether the request should be asynchronous. In most cases, this parameter should be set to true.

B. The send() method

The send() method sends the request to the server. If you’re sending data, like when using the POST method, you would include data as an argument:

xhr.send(null); // For GET requests
xhr.send(JSON.stringify(data)); // For POST requests

C. The onreadystatechange event

The onreadystatechange event is triggered every time the readyState of the request changes. You will typically set up a function to handle these state changes:

xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status === 200) {
        console.log(xhr.responseText);
    }
};

IV. Handling the Response

After sending a request, it’s crucial to handle the response correctly. Here’s what to keep in mind:

A. The readyState property

The readyState property indicates the current state of the request. It can have the following values:

Value	State Description
0	UNSENT: The request has not been opened yet.
1	OPENED: The request has been opened.
2	HEADERS_RECEIVED: The headers have been received.
3	LOADING: The response is being loaded.
4	DONE: The request is complete.

B. The status property

The status property contains the HTTP status code returned by the server. Common status codes include:

Status Code	Description
200	OK: The request was successful.
404	NOT FOUND: The requested resource could not be found.
500	INTERNAL SERVER ERROR: The server encountered an error.

C. Reading the response text

The responseText property holds the data returned from the server, and it can be accessed when the readyState is 4:

if (xhr.readyState === 4 && xhr.status === 200) {
    console.log(xhr.responseText);
}

V. Error Handling

Handling errors appropriately is crucial in AJAX development. Let’s look into common statuses and how to manage them:

A. Status codes explanation

Each HTTP status code indicates the outcome of your request:

• 200: Success.
• 400: Bad Request; check your request payload.
• 401: Unauthorized; authentication required.
• 403: Forbidden; you do not have permission to access the resource.
• 404: Not Found; invalid URL.
• 500: Internal Server Error; something went wrong on the server.

B. Handling network errors

Network errors can occur due to various reasons such as unstable internet connection or server downtime. To handle such errors, you can check if the request was unsuccessful and log the error:

xhr.onerror = function() {
    console.error("Network Error");
};

VI. Summary

In this article, we covered the foundational aspects of working with XMLHttpRequest in AJAX, including creating the object, making requests, handling responses, and implementing error handling.

A. Key takeaways

• The XMLHttpRequest object allows asynchronous communication with servers.
• The open() and send() methods are vital for sending requests.
• Understanding readyState and status properties is key for handling responses.
• Error handling should be implemented to enhance user experience.

B. Further resources for learning AJAX and XMLHttpRequest

To continue your learning journey, consider exploring additional resources, including documentation and tutorials available online. Practice makes perfect, so try building your own AJAX-powered applications!

FAQ

1. What is AJAX?

AJAX stands for Asynchronous JavaScript and XML, and it’s a set of web development techniques for creating asynchronous web applications.

2. How does XMLHttpRequest differ from Fetch API?

While both can make HTTP requests, the Fetch API is a modern alternative to XMLHttpRequest that provides a more powerful and flexible feature set, such as Promises.

3. Can XMLHttpRequest be used with JSON?

Yes, XMLHttpRequest can be used to send and receive JSON data by converting it between JavaScript objects and JSON strings.

4. Is XMLHttpRequest deprecated?

No, XMLHttpRequest is not deprecated, but the Fetch API is recommended for new projects due to its flexibility and simplicity.