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AJAX - The onreadystatechangeEvent

The onreadystatechange event

When a request to a server is sent, we want to perform some actions based on the response.

The onreadystatechange event is triggered every time the readyState changes.

The readyState property holds the s tatus of the XMLHttpRequest.

Three important prope rties of the XMLHttpRequest ob ject:

Property Description

onreadystate change Stores a function (or the name of a function) to be called automatically each time the

readyState property changes

readyState Holds the status of the XMLHttpRequest. Changes from 0 to 4:0: request no t initialized

1: server connection es tablished

2: request received

3: processing request

4: request finished and response is ready

status 200: "OK"

404: Page not found

In the onreadystatechange event, we specify what will happen when the server response is ready to be

processed.

When readyState is 4 and s tatus is 200, the response is ready:

Example

xmlhttp.onreadystatechange=function()

{

if (xmlhttp.readyState==4 && xmlhttp.status==200)

{

document.getElementById("myDiv").innerHTML=xmlhttp.responseText;

}

}

Try it yourself

Note:The onreadysta techange event is triggered five times (0-4), one time for each change in readyState.

Using a Callback Function

A callback function is a function pass ed as a parameter to ano ther function.

If you have more than one AJAX task on your webs ite, you should create ONE standard function for creating the

XMLHttpRequest object, and call this for each AJAX task.

The function call should contain the URL and what to do on onreadystatechange (which is probably different foreach call):

Example

function myFunction()

{

loadXMLDoc("ajax_info.txt",function()

{

if (xmlhttp.readyState==4 && xmlhttp.status==200)

{

document.getElementById("myDiv").innerHTML=xmlhttp.responseText;

}

});

}

Try it yourself

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