jQuery2

// Submit the form via Ajax

function submitForm() {
  var contactForm = $(this);

  // Are all the fields filled in?

  if ( !$('#senderName').val() || !$('#senderEmail').val() || !$('#message').val() ) {

    // No; display a warning message and return to the form
    $('#incompleteMessage').fadeIn().delay(messageDelay).fadeOut();
    contactForm.fadeOut().delay(messageDelay).fadeIn();

  } else {

    // Yes; submit the form to the PHP script via Ajax

    $('#sendingMessage').fadeIn();
    contactForm.fadeOut();

    $.ajax( {
      url: contactForm.attr( 'action' ) + "?ajax=true",
      type: contactForm.attr( 'method' ),
      data: contactForm.serialize(),
      success: submitFinished
    } );
  }

  // Prevent the default form submission occurring
  return false;