var checkEl = document.getElementById('check');
var nextEl = document.getElementById('next');
var questionsEl = document.getElementsByClassName('question');
// if ( questionsEl[1].style.display == 'none' )
//  questionsEl[1].style.display = 'block';
var result = [];

var i = 0;
nextEl.addEventListener('click', changeQuestion());
function changeQuestion() {
        questionsEl[i].style.display = 'none';
        questionsEl[i + 1].style.display = 'block';
        i++;
}

checkEl.addEventListener('click', checkAnswers);
function checkAnswers() {
    for (var i = 0; i < questionsEl.length - 1; i++) {
        var answers = questionsEl[i].querySelectorAll('input[type=radio]');
        result.push( checkOneAnswer(answers) );
    }
    printResult();
    result = [];
}

function checkOneAnswer(answers)
{
    var result = null;
    var noAnswer = true;
    for (var i = 0; i < answers.length; i++)
    {
        var isTrue = (answers[i].getAttribute('data-true') != null);
        if (answers[i].checked)
        {
            noAnswer = false;
            if (isTrue)
                result = true;
            else
                result = false;
        } else
        {
            if (isTrue)
                result = false;
        }
    }
    if (noAnswer)
        result = null;
    return result;
}