The XMLHttpRequest Object
The XMLHttpRequest object provides a way to communicate with a server after a web page has loaded.
What is the XMLHttpRequest Object?
The XMLHttpRequest object is the developers dream, because you can:
- Update a web page with new data without reloading the page
- Request data from a server after the page has loaded
- Receive data from a server after the page has loaded
- Send data to a server in the background
The XMLHttpRequest object is supported in all modern browsers.
Example: XML HTTP communication with a server while typing input
Creating an XMLHttpRequest Object
Creating an XMLHttpRequest object is done with one single line of JavaScript.
In all modern browsers (including IE7):
| xmlhttp=new XMLHttpRequest() |
In Internet Explorer 5 and 6:
| xmlhttp=new ActiveXObject("Microsoft.XMLHTTP") |
Example
<script type="text/javascript">
var xmlhttp;
function loadXMLDoc(url)
{
xmlhttp=null;
if (window.XMLHttpRequest)
{// code for all new browsers
xmlhttp=new XMLHttpRequest();
}
else if (window.ActiveXObject)
{// code for IE5 and IE6
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
if (xmlhttp!=null)
{
xmlhttp.onreadystatechange=state_Change;
xmlhttp.open("GET",url,true);
xmlhttp.send(null);
}
else
{
alert("Your browser does not support XMLHTTP.");
}
}
function state_Change()
{
if (xmlhttp.readyState==4)
{// 4 = "loaded"
if (xmlhttp.status==200)
{// 200 = OK
// ...our code here...
}
else
{
alert("Problem retrieving XML data");
}
}
}
</script>
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Try it yourself »
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Note: onreadystatechange is an event handler. The value (state_Change)
is the name of a function which is
triggered when the state of the XMLHttpRequest object changes. States run from 0 (uninitialized)
to 4 (complete). Only when the state = 4, we can execute our code.
Why Use Async=true?
Our examples use "true" in the third parameter of open().
This parameter specifies
whether the request should be handled asynchronously.
True means
that the script
continues to run after the send() method, without waiting for a response
from the server.
The onreadystatechange event complicates the code. But it is the safest way
if you want to prevent the code from stopping if you don't get a response from
the server.
By setting the parameter to "false", your can avoid the extra
onreadystatechange code. Use this if it's not important to execute the rest of
the code if the request fails.
Try it yourself using JavaScript
More Examples
Load a
textfile into a div element with XML HTTP
Make a
HEAD request with XML HTTP
Make a
specified HEAD request with XML HTTP
List data from an XML file with XML HTTP
XML / ASP
You can also open and send an XML document to an ASP page on
the server, analyze the request, and send back the result.
Example
<html>
<body>
<script type="text/javascript">
xmlHttp=null;
if (window.XMLHttpRequest)
{// code for IE7, Firefox, Opera, etc.
xmlHttp=new XMLHttpRequest();
}
else if (window.ActiveXObject)
{// code for IE6, IE5
xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
}
if (xmlHttp!=null)
{
xmlHttp.open("GET", "note.xml", false);
xmlHttp.send(null);
xmlDoc=xmlHttp.responseText;
xmlHttp.open("POST", "demo_dom_http.asp", false);
xmlHttp.send(xmlDoc);
document.write(xmlHttp.responseText);
}
else
{
alert("Your browser does not support XMLHTTP.");
}
</script>
</body>
</html>
|
The ASP page, written in VBScript:
<%
set xmldoc = Server.CreateObject("Microsoft.XMLDOM")
xmldoc.async=false
xmldoc.load(request)
for each x in xmldoc.documentElement.childNodes
if x.NodeName = "to" then name=x.text
next
response.write(name)
%>
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Try it yourself »
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You send the result back to the client using the response.write property.
Is the XMLHttpRequest Object a W3C Standard?
The XMLHttpRequest object is not specified in any W3C
recommendation.
However, the W3C DOM Level 3 "Load and Save" specification contains some
similar functionality, but these are not implemented in any browsers yet.
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