Hello Readers,

Step 1 : First we write the ajax code in default.php file inside module (Ex: mod_mymodule) tmpl/default.php folder.
 
 Code Example :

$("#id").change(function()
{
var id=$(this).val();

    $.ajax
    ({
    type: "POST",
    url: "<?php echo JRoute::_('index.php?option=com_ajax&module=my_module&method=getdata&format=raw')?>",
    data: {'id':id}
        success: function(res)
        {
        alert(res);
        } 
    });
});

Here,

1. option=com_ajax
2. [module|plugin]=name
3. format=[json|debug|raw]

In the above code, use the option = com_ajax, name of the module and it's format in raw|json|debug form.

Step 2 : helper file in joomla module (mod_mymodule) where we write the getdata method or function to post the ajax request.

Code Example:

public static function getdataAjax()
{
    $id=JRequest::getVar('id');

    //databse query
}

In the above code, write the getdata method or function as a getdataAjax() (beacuse all method's must end in Ajax if we use com_ajax as a option in Joomla).