window.onload = function () {

  var form = document.getElementById('theForm');

  form.button.onclick = function (){

    for(var i=0; i < form.elements.length; i++){

      if(form.elements[i].value === '' && form.elements[i].hasAttribute('required')){

        alert('There are some required fields!');

        return false;

      }

    }

  }; 

};