title: Uncaught TypeError: Cannot read property 'classList' of undefined
body: <p>I'm trying to do simple slider for my site on js, but i have problem. </p>
  
  <p>I have variable "i", and i can't use it.</p>
  
  <p>You can see at my code.</p>
  
  <p>What is solution of this problem?</p>
  
  <pre><code>    var slider = [];
      var radio = [];
  
  for (var i = 1; document.querySelector('.radio-' + i) !== null; i++) { //this variable
  
  radio[i] = document.querySelector('.radio-' + i);
  radio[i].addEventListener('click', function () {
      for (var u = 1; document.querySelector('.slider' + u) !== null; u++) {
          slider[u] = document.querySelector('.slider' + u);
  
          if (slider[u].classList.contains('slider-show')) {
              slider[u].classList.remove('slider-show');
          }
      }
      slider[i].classList.add('slider-show'); //want to use here
  });
  }
  </code></pre>