gistfile1.js

$(document).ready(function() {
  $('form').submit(function(e) {
    e.preventDefault();

    var fullname = $('input[name=full_name]').val();
    var answer = $('textarea[name=answer]').val();
    var email = $('input[name=email]').val();

    var ready = 0;

    if ( ! fullname) {
      $('div.full_name').addClass('error');
      $('.fullname-error').show();
    } else {
      $('div.full_name').removeClass('error');
      $('.fullname-error').hide();
      ready+1;
    }

    if ( ! answer) {
      $('div.answer').addClass('error');
      $('.answer-error').show();
    } else {
      $('div.answer').removeClass('error');
      $('.answer-error').hide();
      ready+1;
    }

    if ( ! email) {
      $('div.email').addClass('error');
      $('.email-error').show();
    } else {
      $('div.email').removeClass('error');
      $('.email-error').hide();
      ready+1;
    }

    if ($('input[name=agree]').is(':checked')) {
      $('div.agree').removeClass('error');
      $('.agree-error').hide();
      ready+1;
    } else {
      $('div.agree').addClass('error');
      $('.agree-error').show();
    }

    if (ready = 4) return true;
  });
});