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XML HttpRequest


All modern browsers have a built-in XMLHttpRequest object to
request data from a server.

The XMLHttpRequest Object
The XMLHttpRequest object can be used to request data from a web server.
The XMLHttpRequest object is a developers dream, because you can:

Update a web page without reloading the page
Request data from a server - after the page has loaded
Receive data from a server  - after the page has loaded
Send data to a server - in the background


XMLHttpRequest Example
When you type a character in the input field below, an XMLHttpRequest is sent to the server,
and some name suggestions are returned
(from the server):

Example

Start typing a name in the input field below:

Name:
  Suggestions:




The example above is explained in the AJAX chapters of this tutorial.


Sending an XMLHttpRequest
A common JavaScript syntax for using the XMLHttpRequest object looks much like this:

Example

var xhttp = new XMLHttpRequest();xhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
       // Typical action to be performed when the document is ready:      
document.getElementById("demo").innerHTML = xhttp.responseText;
    }};
xhttp.open("GET", "filename", true);
xhttp.send();

Try it Yourself »








Example Explained
The first line in the example above creates an XMLHttpRequest
object:


var xhttp = new XMLHttpRequest();


The onreadystatechange property specifies a function to be executed
every time the status of the XMLHttpRequest object changes:


xhttp.onreadystatechange = function()


When readyState property is 4 and the status
property is 200, the response is ready:


if (this.readyState == 4 && this.status == 200)


The responseText property returns the server response as a
text string.
The text string can be used to update a web page:


document.getElementById("demo").innerHTML = xhttp.responseText;


You will learn a lot more about the XMLHttpRequest object in the AJAX chapters of this tutorial.













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Reference: https://www.w3schools.com/xml/xml_http.asp