var $ajaxScript = 'scripts/ajax.php';  //  controller-script that handles all ajax script request
/**
 *  ajax wrapper
 *  $_POST array of form posts
 */
function submitAjax($_POST){
    ajaxObj = $.ajax({url:$ajaxScript, data:$_POST, type:'POST', dataType:'json' });
    return ajaxObj;
}

$(function(){
    $("form.ajax").submit(function(e){
        e.preventDefault();
        submitAjax($(this).serialize())
        .done(function(data,statusText,jqXHR){
            $(this).prepend('<div class="notice"><p class="icon">'+data['msg']+'</p></div>');  //  *--  this triggers a "TypeError"
            console.log('done : '+statusText);  //  <--  this alone doesn't throw any error
        })
        .fail(function(){
            console.log('fail');
        })
        .always(function(){
            console.log('always');
        });
    });
});