Purpose of AJAX
Read data from a web server - after the page has loaded
Update a web page without reloading the page
Send data to a web server - in the background
<!DOCTYPE html>
<html>
<body>
<div id="demo">
<h2>Let AJAX change this text</h2>
<button type="button" onclick="loadDoc()">Change
Content</button>
</div>
</body>
</html>
function loadDoc() {
const xhttp = new XMLHttpRequest();
xhttp.onload = function() {
document.getElementById("demo").innerHTML = this.responseText;
}
xhttp.open("GET", "ajax_info.txt", true);
xhttp.send();
}
what is AJAX?
AJAX = Asynchronous JavaScript And XML.
AJAX is not a programming language.
AJAX just uses a combination of:
A browser built-in XMLHttpRequest object (to request data from a web
server)
JavaScript and HTML DOM (to display or use the data).
How AJAX Works
1. An event occurs in a web page (the page is loaded, a button is clicked)
2. An XMLHttpRequest object is created by JavaScript
3. The XMLHttpRequest object sends a request to a web server
4. The server processes the request
5. The server sends a response back to the web page
6. The response is read by JavaScript
7. Proper action (like page update) is performed by JavaScript
The keystone of AJAX is the XMLHttpRequest object.
1. Create an XMLHttpRequest object
2. Define a callback function
3. Open the XMLHttpRequest object
4. 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();
<!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>
XMLHttpRequest Object Methods
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
XMLHttpRequest Object Properties
Property Description
onload Defines a function to be called when the request is
received (loaded)
onreadystatechang Defines a function to be called when the readyState
e 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"
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:
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
onreadystatechang Defines a function to be called when the readyState
e 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:
<!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>
