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AJAX XMLhttp

All modern browsers support the XMLHttpRequest object.

The XMLHttpRequest object exchanges data with a web server behind the scenes. This refers to the possibility of updating 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 must consist of 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, use the open() and send() methods of the XMLHttpRequest object:

xhttp.open(“GET”, “ajax_info.txt”);

xhttp.send();

Example

<!DOCTYPE html>

<html>

<body>

<h2>The XMLHttpRequest Object</h2>

<div id=”demo”>

<p>Let AJAX change this text.</p>

<button type=”button” onclick=”loadDoc()”>Change Content</button>

</div>

<script>

function loadDoc() {

const xhttp = new XMLHttpRequest();

 xhttp.onload = function() {    document.getElementById(“demo”).innerHTML = this.responseText;

  }

 xhttp.open(“GET”, “ajax_info.txt”);

  xhttp.send();

}

</script>

</body>

</html>

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.

Use the example above on one of your web pages, the XML files you load must be located on your 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 request

method: the request type GET or POST
url: the file location
async: true (asynchronous) or false (synchronous)
user: optional user name
psw: optional password

send()

Sends the request to the server
Used 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 recieved (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 request
200: “OK”
403: “Forbidden”
404: “Not Found” 

statusText

Returns the status-text (e.g. “OK” or “Not Found”)

The onload Property

With the XMLHttpRequest object, dsefine a callback function to be executed when the request receives an answer.

The function is specified in the onload property of the XMLHttpRequest object:

Example

<!DOCTYPE html>

<html>

<body>

<div id=”demo”>

<h2>The XMLHttpRequest Object</h2>

<button type=”button” onclick=”loadDoc()”>Change Content</button>

</div>

<script>

function loadDoc() {

const xhttp = new XMLHttpRequest();

 xhttp.onload = function() {

 document.getElementById(“demo”).innerHTML =

    this.responseText;

  }

  xhttp.open(“GET”, “ajax_info.txt”);

  xhttp.send();

}

</script>

</body>

</html>

Multiple Callback Functions

If you have more than one AJAX task on 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 has the status of the XMLHttpRequest.

The onreadystatechange property specifies a callback function to be executed when the readyState changes.

The status property and the statusText properties have 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”

statusText

Returns the status text (e.g. “OK” or “Not Found”)

The onreadystatechange function is called the readyState changes.

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

Example

<!DOCTYPE html>

<html>

<body>

<div id=”demo”>

<h2>The XMLHttpRequest Object</h2>

<button type=”button” onclick=”loadDoc()”>Change Content</button>

</div>

<script>

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();

}

</script>

</body>

</html>


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