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