Looking for Assistance with Grasping the Concepts of Ajax Function

Question

Looking for Assistance with Grasping the Concepts of Ajax Function

After completing my first Ajax function using online tutorials, such as Google and w3schools, I've successfully made it work. However, I'm struggling to grasp the logic behind it and would greatly appreciate an explanation!

Below is my complete code:

function loadPlayer(id)
{
    var xmlhttp;

    if (window.XMLHttpRequest)
        xmlhttp=new XMLHttpRequest();
    else if(window.ActiveXObject)
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");

    xmlhttp.onreadystatechange=function() 
    {
        if (xmlhttp.readyState==4 && xmlhttp.status==200)
            document.getElementById("newPlayer").innerHTML=xmlhttp.responseText;
    };

    xmlhttp.open("GET","http://localhost:8084/gmustudent/players?id=" + id,true);
    xmlhttp.send();
}

My main concern revolves around the order in which I've written this code and how each statement is executed. Specifically, I am puzzled by the fact that I'm placing the response text in the newPlayer div within the onreadystatechange function before actually retrieving the data asynchronously from the URL.

I'm perplexed because I don't understand how you can insert the response text into the div without having retrieved it yet. Although it's functioning, the reason eludes me. Therefore, a simple explanation breaking down what exactly is happening here - particularly regarding the sequence of statements and their actions - would be incredibly helpful. Thank you in advance for your assistance!

javascript ajax xmlhttprequest onreadystatechange

Answer 1

Answer №1

Check this out:

xmlhttp.onreadystatechange=function() 
    {
        if (xmlhttp.readyState==4 && xmlhttp.status==200)
            document.getElementById("newPlayer").innerHTML=xmlhttp.responseText;
    };

This function serves as an event handler, triggering when a specific event occurs. In this case, it activates when the state of the request changes. The process follows these steps:

The event handler is attached in the provided code snippet
An actual request is made
The onreadystatechange event is fired multiple times as the request progresses through different states
Once the request is completed successfully (as per the conditions within the if block), the response text is added to the designated div element.

Essentially, the event callback is set up initially (#1), and then the crucial code executes at a later stage (#4).

Answer 2

Check this out:

xmlhttp.onreadystatechange=function() 
    {
        if (xmlhttp.readyState==4 && xmlhttp.status==200)
            document.getElementById("newPlayer").innerHTML=xmlhttp.responseText;
    };

This function serves as an event handler, triggering when a specific event occurs. In this case, it activates when the state of the request changes. The process follows these steps:

The event handler is attached in the provided code snippet
An actual request is made
The onreadystatechange event is fired multiple times as the request progresses through different states
Once the request is completed successfully (as per the conditions within the if block), the response text is added to the designated div element.

Essentially, the event callback is set up initially (#1), and then the crucial code executes at a later stage (#4).

Answer 3

Answer №2

Your code has been annotated with comments to explain what each part is doing.

function loadPlayer(id)
{
    var xmlhttp;

    // The code determines the best request object based on the browser being used.
    if (window.XMLHttpRequest)
        xmlhttp=new XMLHttpRequest();
    else if(window.ActiveXObject)
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");

    // This function will run when the state changes on the ajax request.
    xmlhttp.onreadystatechange=function() 
    {
        // Check if the request is finished before using the response.
        if (xmlhttp.readyState==4 && xmlhttp.status==200)
            // Update the element with the response text.
            document.getElementById("newPlayer").innerHTML=xmlhttp.responseText;
    };

    xmlhttp.open("GET","http://localhost:8084/gmustudent/players?id=" + id,true);
    // Send the request.
    xmlhttp.send();
}

To see the onreadystatechange function in action, you can add a console.log(xmlhttp.readyState); statement at the beginning of the function.

Answer 4

Your code has been annotated with comments to explain what each part is doing.

function loadPlayer(id)
{
    var xmlhttp;

    // The code determines the best request object based on the browser being used.
    if (window.XMLHttpRequest)
        xmlhttp=new XMLHttpRequest();
    else if(window.ActiveXObject)
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");

    // This function will run when the state changes on the ajax request.
    xmlhttp.onreadystatechange=function() 
    {
        // Check if the request is finished before using the response.
        if (xmlhttp.readyState==4 && xmlhttp.status==200)
            // Update the element with the response text.
            document.getElementById("newPlayer").innerHTML=xmlhttp.responseText;
    };

    xmlhttp.open("GET","http://localhost:8084/gmustudent/players?id=" + id,true);
    // Send the request.
    xmlhttp.send();
}

To see the onreadystatechange function in action, you can add a console.log(xmlhttp.readyState); statement at the beginning of the function.

Answer 5

Answer №3

When you set a function to the .onreadystatechange property of an XMLHttpRequest Object, you're defining what's known as a callback function. As the name suggests, this function will be invoked at a later time, possibly triggered by another process or part of the application.

In this scenario, the callback function is triggered by various Ajax events, such as when the request is being prepared, data is received, and when the request is complete. This is why it checks for conditions like the readyState being equal to 4 and the status being 200.

Therefore, the sequence of code placement isn't crucial here; the function isn't executed immediately but rather referenced and invoked at a later stage.

Answer 6

When you set a function to the .onreadystatechange property of an XMLHttpRequest Object, you're defining what's known as a callback function. As the name suggests, this function will be invoked at a later time, possibly triggered by another process or part of the application.

In this scenario, the callback function is triggered by various Ajax events, such as when the request is being prepared, data is received, and when the request is complete. This is why it checks for conditions like the readyState being equal to 4 and the status being 200.

Therefore, the sequence of code placement isn't crucial here; the function isn't executed immediately but rather referenced and invoked at a later stage.

Looking for Assistance with Grasping the Concepts of Ajax Function

Answer №1

Answer №2

Answer №3

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