I am going to provide a general example here
in view file
$.post('<?php echo site_url("test/test"); ?>', {'id':1}, function(response){
if(response.success)
{
alert(response.message);
} else
{
alert('Something went wrong!!');
}
}, 'json');
in controller Test.php
function test()
{
$id = $this->input->post('id');
//do additional stuff
$result = 'i am coming right out of controller!! ';
echo json_encode(array('success' => true, 'message' => $result));
}