codeigniter check username when logging in using ajax

gistfile1.phtml

<?php
function ajax_lookUpUsername(){
    $username = $this->input->post('username');
    $this->db->where('username', $username);
    $query = $this->db->get('accounts');
    if ($query->num_rows() > 0){
       echo 0;
    } else {
       echo 1;
    }
}
?>
 
 
<script>
//and here's your simple onclick javascript function:
function lookUpUsername(name){
    $.post( 
        'http://yourwebsite/controller/ajax_lookUpUsername',
         { username: name },
         function(response) {  
            if (response == 1) {
               alert('username available');
            } else {
               alert('username taken');
            }
         }  
    );
}
</script>