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AJAX - The XMLHttpRequest Object


The keystone of AJAX is the XMLHttpRequest object.

Create an XMLHttpRequest object
Define a callback function
Open the XMLHttpRequest object
Send a Request to a server


The XMLHttpRequest Object
All modern browsers support the XMLHttpRequest object.
The XMLHttpRequest object can be used to exchange data with a web server behind the
scenes. This means that it is possible to update parts of a web page, without
reloading the whole page.

Create an XMLHttpRequest Object
All modern browsers (Chrome, Firefox, IE, Edge, Safari, Opera) have a built-in XMLHttpRequest object.
Syntax for creating an XMLHttpRequest object:

variable = new XMLHttpRequest();

Define a Callback Function
A callback function is a function passed as a parameter to another function.
In this case, the callback function should contain the code to execute when the
response is ready.

xhttp.onload = function() {
  // What to do when the response is ready
}


Send a Request
To send a request to a server, you can use the open() and send() methods of the
XMLHttpRequest object:

xhttp.open("GET", "ajax_info.txt");
xhttp.send();

Example

// Create an XMLHttpRequest object
const xhttp = new XMLHttpRequest();
// Define a callback function
xhttp.onload = function() {
  // Here you can use the Data
}
// Send a request
xhttp.open("GET", "ajax_info.txt");
xhttp.send();

Try it Yourself »


Access Across Domains
For security reasons, modern browsers do not allow access across domains.
This means that both the web page and the XML file it tries to load, must be located on the same server.
The examples on W3Schools all open XML files located on the W3Schools domain.
If you want to use the example above on one of your own web pages,
the XML files you load must be located on your own server.







XMLHttpRequest Object Methods


Method
Description


new XMLHttpRequest()
Creates a new XMLHttpRequest object


abort()
Cancels the current request


getAllResponseHeaders()
Returns header information


getResponseHeader()
Returns specific header information


open(method, url, async, user, psw)
Specifies the requestmethod: the request type GET or POST
url: the file location
async: true (asynchronous) or false (synchronous)user: optional user namepsw: optional password


send()
Sends the request to the serverUsed for GET requests


send(string)
Sends the request to the server.Used for POST requests


setRequestHeader()
Adds a label/value pair to the header to be sent



XMLHttpRequest Object Properties


Property
Description


onload
Defines a function to be called when the request is received (loaded)


onreadystatechange
Defines a function to be called when the readyState property changes


readyState
Holds the status of the XMLHttpRequest.
0: request not initialized
1: server connection established
2: request received
3: processing request
4: request finished and response is ready


responseText
Returns the response data as a string


responseXML
Returns the response data as XML data


status
Returns the status-number of a request200: "OK"403: "Forbidden"404: "Not Found"
For a complete list go to the Http
Messages Reference


statusText
Returns the status-text (e.g. "OK" or "Not Found")



The onload Property
With the XMLHttpRequest object you can define a callback function to be executed when
the request receives an answer.
The function is defined in the onload property of the XMLHttpRequest object:

Example

xhttp.onload = function() {
  document.getElementById("demo").innerHTML = this.responseText;
}
xhttp.open("GET", "ajax_info.txt");
xhttp.send();
Try it Yourself »


Multiple Callback Functions
If you have more than one AJAX task in a website, you should create one function for
executing the XMLHttpRequest object, and one callback function for each
AJAX task.
The function call should contain the URL and what function to call when the
response is ready.

Example

loadDoc("url-1", myFunction1);

loadDoc("url-2", myFunction2);
function loadDoc(url, cFunction) {
  const xhttp = new XMLHttpRequest();
  xhttp.onload = function() {cFunction(this);}
  xhttp.open("GET", url);
  xhttp.send();
}
function myFunction1(xhttp) {
  // action goes here
}

function myFunction2(xhttp) {
  // action goes here
}



The onreadystatechange Property
The readyState property holds the status of the XMLHttpRequest.
The onreadystatechange property defines a callback function to be executed when the readyState changes.
The status property and the statusText properties hold the status of the XMLHttpRequest object.


Property
Description


onreadystatechange
Defines a function to be called when the readyState property changes


readyState
Holds the status of the XMLHttpRequest.
0: request not initialized
1: server connection established
2: request received
3: processing request
4: request finished and response is ready


status
200: "OK"403: "Forbidden"
404: "Page not found"For a complete list go to the
Http Messages Reference


statusText
Returns the status-text (e.g. "OK" or "Not Found")


The onreadystatechange function is called every time the readyState changes.
When readyState is 4 and status is 200, the response is ready:

Example

function loadDoc() {
  const xhttp = new XMLHttpRequest();
  xhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
      document.getElementById("demo").innerHTML =
      this.responseText;
    }
  };
  xhttp.open("GET", "ajax_info.txt");
  xhttp.send();
}

Try it Yourself »


The onreadystatechange event is triggered four times (1-4), one time for each change in the readyState.













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Reference: https://www.w3schools.com/js/js_ajax_http.asp