The "truth" parameter within the xmlHttpRequest .open() function

Question

The "truth" parameter within the xmlHttpRequest .open() function

After referencing MDN, it mentioned that by default, the JavaScript function will keep running until the server response is received.

This is essentially what AJAX is all about - Asynchronous JavaScript and XML.

Even though I have some experience with AJAX, I found myself a bit puzzled after reading that explanation. It made me question whether I truly understand the concept of AJAX. I do know that AJAX allows communication with the server without refreshing the page, meaning everything happens seamlessly in the background.

However, the scenario described in the reference got me thinking. For instance, if I have a piece of JavaScript code like this:

//true, hence the function runs while waiting for server response
var xmlResponse;
var url = "http://example.com/file.xml";
xml_req.open("GET", url, true); 

xml_req.onreadystatechange = function() {
     if(xml_req.readyState == 4 && xml_req.status == 200) {
        if(xml_req.responseText != null)
             xmlResponse = xml_req.responseXML; //server response may not yet arrive
        else {
             alert("failed");
             return false;
        }
     };
xml_req.send(null);

Based on this, it seems like xmlResponse could potentially be undefined since the server is still busy fetching the data. Can someone please clarify how the execution flow works in AJAX technology? Your insights are greatly appreciated.

javascript ajax asynchronous xmlhttprequest

Answer 1

Answer №1

If you want more details, check out my article here, but here's the gist of it.

When you set it to true, you're essentially making an asynchronous request. This means that the code won't pause while the http request is ongoing. On the other hand, a synchronous call will lock up the browser, preventing anything else from running, which can lead to issues. Asynchronous is preferred for this reason.

The XHR object keeps us informed about its activities, sending updates through the onreadystatechange event. By registering a function, we can monitor its progress, as this event occurs 4 times, each with a different state:

0 = uninitialized
1 = loading
2 = loaded
3 = interactive
4 = complete

We can access the data once the readystate reaches 4.

In the provided code snippet, it's checking for the complete state, ensuring that the status is 200 [ok]:

if(xml_req.readyState == 4 && xml_req.status == 200){

Attempting to use the xmlResponse value elsewhere in the code before it's returned will result in it being undefined. For instance:

ml_req.send(null);
alert(xmlResponse );

Check out one of the earliest articles on the XMLHttpRequest for further information. Here's a link to the Apple Article on xmlhttpreq.

Answer 2

If you want more details, check out my article here, but here's the gist of it.

When you set it to true, you're essentially making an asynchronous request. This means that the code won't pause while the http request is ongoing. On the other hand, a synchronous call will lock up the browser, preventing anything else from running, which can lead to issues. Asynchronous is preferred for this reason.

The XHR object keeps us informed about its activities, sending updates through the onreadystatechange event. By registering a function, we can monitor its progress, as this event occurs 4 times, each with a different state:

0 = uninitialized
1 = loading
2 = loaded
3 = interactive
4 = complete

We can access the data once the readystate reaches 4.

In the provided code snippet, it's checking for the complete state, ensuring that the status is 200 [ok]:

if(xml_req.readyState == 4 && xml_req.status == 200){

Attempting to use the xmlResponse value elsewhere in the code before it's returned will result in it being undefined. For instance:

ml_req.send(null);
alert(xmlResponse );

Check out one of the earliest articles on the XMLHttpRequest for further information. Here's a link to the Apple Article on xmlhttpreq.

Answer 3

Answer №2

It is crucial to realize that the onreadystatechange handler does not get executed immediately. In fact, it runs more than once. A better way to understand this concept is by breaking down the code into separate functions:

function initiateRequest(url)
{
    var xhr = new XMLHttpRequest();
    xhr.open("GET", url, true);
    xhr.onreadystatechange = handleResponse;
    xhr.send();
}
function handleResponse(e)
{
    if (this.readyState == 4)
    {
        // Server response is fully received with xhr.readyState == 4
        if (this.status == 200)
        {
            // Response status is good with xhr.status == 200
            var response = this.responseXML;
            ...
        }
    }
}

Initially, the initiateRequest function is called and exits. Subsequently, when a response is received from the server, the handleResponse function is invoked. At each instance, we verify if the complete response has been received before proceeding with further processing.

Answer 4

It is crucial to realize that the onreadystatechange handler does not get executed immediately. In fact, it runs more than once. A better way to understand this concept is by breaking down the code into separate functions:

function initiateRequest(url)
{
    var xhr = new XMLHttpRequest();
    xhr.open("GET", url, true);
    xhr.onreadystatechange = handleResponse;
    xhr.send();
}
function handleResponse(e)
{
    if (this.readyState == 4)
    {
        // Server response is fully received with xhr.readyState == 4
        if (this.status == 200)
        {
            // Response status is good with xhr.status == 200
            var response = this.responseXML;
            ...
        }
    }
}

Initially, the initiateRequest function is called and exits. Subsequently, when a response is received from the server, the handleResponse function is invoked. At each instance, we verify if the complete response has been received before proceeding with further processing.

The "truth" parameter within the xmlHttpRequest .open() function

Answer №1

Answer №2

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