How does it works?
Ajax uses a programming model with display and events. These events are user actions, they call functions associated to elements of the web page.
Interactivity is achieved with forms and buttons. DOM allows to link elements of the page with actions and also to extract data from XML files provided by the server.
To get data on the server, XMLHttpRequest provides two methods:
- open: create a connection.
- send: send a request to the server.
Data furnished by the server will be found in the attributes of the XMLHttpRequest object:
- responseXml for an XML file or
- responseText for a plain text.
Take note that a new XMLHttpRequest object has to be created for each new file to load.
We have to wait for the data to be available to process it, and in this purpose, the state of availability of data is given by the readyState attribute of XMLHttpRequest.
States of readyState follow (only the last one is really useful):
Interactivity is achieved with forms and buttons. DOM allows to link elements of the page with actions and also to extract data from XML files provided by the server.
To get data on the server, XMLHttpRequest provides two methods:
- open: create a connection.
- send: send a request to the server.
Data furnished by the server will be found in the attributes of the XMLHttpRequest object:
- responseXml for an XML file or
- responseText for a plain text.
Take note that a new XMLHttpRequest object has to be created for each new file to load.
We have to wait for the data to be available to process it, and in this purpose, the state of availability of data is given by the readyState attribute of XMLHttpRequest.
States of readyState follow (only the last one is really useful):
Ajax and DHTML
Dynamic HTML has same purpose and is a set of standards:
- HTML,
- CSS,
- JavaScript.
DHTML allows to change the display of page from user commands or from text typed by user.
Ajax allows also to send requests asynchronously and load data from the server. It is DHTML plus the XHR object.
- HTML,
- CSS,
- JavaScript.
DHTML allows to change the display of page from user commands or from text typed by user.
Ajax allows also to send requests asynchronously and load data from the server. It is DHTML plus the XHR object.
Allows to interact with the servers, thanks to its methods and attributes.
Attributes
Attributes
| readyState | the code successively changes value from 0 to 4 that means for "ready". |
| Status | 200 is OK 404 if the page is not found. |
| responseText | holds loaded data as a string of characters. |
| responseXml | holds an XML loaded file, DOM's method allows to extract data. |
| onreadystatechange | property that takes a function as value that is invoked when the readystatechange event is dispatched. |
Methods
| open(mode, url, boolean) | mode: type of request, GET or POST url: the location of the file, with a path. boolean: true (asynchronous) / false (synchronous). optionally, a login and a password may be added to arguments. |
| send("string") | null for a GET command. |
This is just a classical instance of class, but two options must be tried, for browser compatibility.
if (window.XMLHttpRequest) // Object of the current windows
{
xhr = new XMLHttpRequest(); // Firefox, Safari, ...
}
else
if (window.ActiveXObject) // ActiveX version
{
xhr = new ActiveXObject("Microsoft.XMLHTTP"); // Internet Explorer
}
Or exceptions may be used instead:
try {
xhr = new ActiveXObject("Microsoft.XMLHTTP"); // Trying Internet Explorer
}
catch(e) // Failed
{
xhr = new XMLHttpRequest(); // Other browsers.
}
The response and further processing are included in a function and the return of the function will be assigned to the onreadystatechange attribute of the object previously created.
xhr.onreadystatechange = function() { // instructions to process the response };
if (xhr.readyState == 4)
{
// Received, OK
} else
{
// Wait...
}
Two methods of XMLHttpRequest are used:
- open: command GET or POST, URL of the document, true for asynchronous.
- send: with POST only, the data to send to the server.
- open: command GET or POST, URL of the document, true for asynchronous.
- send: with POST only, the data to send to the server.
The request below read a document on the server.
xhr.open('GET', 'http://www.xul.fr/somefile.xml', true);
xhr.send(null);
<html>
<head>
<script>
function submitForm()
{
var xhr;
try { xhr = new ActiveXObject('Msxml2.XMLHTTP'); }
catch (e)
{
try { xhr = new ActiveXObject('Microsoft.XMLHTTP'); }
catch (e2)
{
try { xhr = new XMLHttpRequest(); }
catch (e3) { xhr = false; }
}
}
xhr.onreadystatechange = function()
{
if(xhr.readyState == 4)
{
if(xhr.status == 200)
document.ajax.dyn="Received:" + xhr.responseText;
else
document.ajax.dyn="Error code " + xhr.status;
}
};
xhr.open(GET, "data.txt", true);
xhr.send(null);
}
</script>
</head>
<body>
<FORM method="POST" name="ajax" action="">
<INPUT type="BUTTON" value="Submit" ONCLICK="submitForm()">
<INPUT type="text" name="dyn" value="">
</FORM>
</body>
</html>
Syntax of form using Ajax
View a demo of the example in action.
Download the source.
Comments on the code:
new ActiveXObject(Microsoft.XMLHTTP)
This constructor is for Internet Explorer.
new XMLHttpRequest()
This constructor is for any other browser including Firefox.
http.onreadystatechange
An anonymous function is assigned to the event indicator.
http.readyState == 4
The 4 state means for the response is ready and sent by the server.
http.status == 200
This status means ok, otherwise some error code is returned, 404 for example.
http.open( "POST", "data.xml", true);
POST or GET
URL of the script to execute.
true for asynchronous (false for synchronous).
http.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
This is for POST only.
http.send(document.getElementById("TYPEDTEXT").value);
Send data to the server. Data comes from the "TYPEDTEXT" variable filled throught the form by the user.
This constructor is for Internet Explorer.
new XMLHttpRequest()
This constructor is for any other browser including Firefox.
http.onreadystatechange
An anonymous function is assigned to the event indicator.
http.readyState == 4
The 4 state means for the response is ready and sent by the server.
http.status == 200
This status means ok, otherwise some error code is returned, 404 for example.
http.open( "POST", "data.xml", true);
POST or GET
URL of the script to execute.
true for asynchronous (false for synchronous).
http.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
This is for POST only.
http.send(document.getElementById("TYPEDTEXT").value);
Send data to the server. Data comes from the "TYPEDTEXT" variable filled throught the form by the user.
Get from XML
To get data from an XML file, we have just to replace this line:
document.ajax.dyn="Received:" + xhr.responseText;
by this code:
var doc = xhr.responseXML; // Assign the XML file to a var
var element = doc.getElementsByTagName('root').item(0); // Read the first element
document.ajax.dyn.value= element.firstChild.data; // Assign the content to the form
View a demo of the get from XML example in action.
Write to body
In this demo, the text read is put into the body of the page, and not into a textfield.
The code below replaces the textfield form object and the second part replaces the assignment into the JavaScript function.
The code below replaces the textfield form object and the second part replaces the assignment into the JavaScript function.
<div id="zone">
... some text to replace ...
</div>
document.getElementById("zone").innerHTML = "Received:" + xhr.responseText;
Post a text
In this demo, a text is sent to the server and is written into a file. The call to the "open" method changes, the argument is POST, the url is the name of a file or script that receives the data sent, and that must process it. And the "send" method has now a value as argument that is a string of parameters.
xhr.open("POST", "ajax-post-text.php", true);
xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
xhr.send(data);
The parameter of the send method is in the format of the HTML POST method. When several values are sent, they are separated by the ampersand symbol:
var data = "file=" + url + "&content=" + content;
The "file" parameter is the name of a file created to store the content. The filename must be checked by the server to prevent any other file to be modified.
Using an external file
It is simpler to include a JavaScript file. This line will be included into the head section of the HTML page:
<script src="ajax.js" type="text/javascript"></script>
And the function is called with this statement:
var xhr = createXHR();
View the script in the ajax.js file.
You need for some wrapper. A short list of frameworks is provided below.
Your JavaScript program, integrated into a web page, sends request to the server to load files for rebuilding of pages. The received documents are processed with Dom's methods or XML parsers and the data are used to update the pages.
Your JavaScript program, integrated into a web page, sends request to the server to load files for rebuilding of pages. The received documents are processed with Dom's methods or XML parsers and the data are used to update the pages.
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