js

            $('body').on('submit','#contact-form', function(event){
                event.preventDefault();

                var form = $(this);

                $.ajax({
                   'method': 'post', // use post since you use $_POST in php file to process data
                   'action': '/' + $(form).attr('action'), // url to send a form data
                   'data': $(form).serialize(), // serialized form data
                }).done(function(response){
                    // to do something with response text
                    // change other elements e.g.
                    var e = $("#contact-sent");

                    e.css({
                        'display': 'block',
                        'color': (e[0].style.color == 'black') ? 'blue' : 'black'
                    });

                    $("#contact-sent").css('color','blue');

                });

            });