AJAX – The XMLHttpRequest Object

Before sending data to the server, we have to explain three important properties of the XMLHttpRequest object.
The onreadystatechange Property

After a request to the server, we need a function that can receive the data that is returned by the server.

The onreadystatechange property stores your function that will process the response from a server. This is not a method, the function is stored in the property to be called automatically. The following code sets the onreadystatechange property and stores an empty function inside it:
xmlHttp.onreadystatechange=function()
{
// We are going to write some code here
}
The readyState property holds the status of the server’s response. Each time the readyState changes, the onreadystatechange function will be executed.

Here are the possible values for the readyState property:
o-The request is not initialized
1-The request has been set up
2-The request has been sent
3-The request is in process
4-The request is complete
xmlHttp.onreadystatechange=function()
{
if(xmlHttp.readyState==4)
{
// Get the data from the server’s response
}
}
The data sent back from the server can be retrieved with the responseText property.

In our code, we will set the value of our “time” input field equal to responseText:
xmlHttp.onreadystatechange=function()
{
if(xmlHttp.readyState==4)
{
document.myForm.time.value=xmlHttp.responseText;
}
}

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