I'm trying to do a slider with jQuery

  // Previous button is clicked
  $('span.prev').on('click', function() {
    // remove class to current item and add class oldActive to know this was the last slide shown
    $('.slide.active').removeClass('active').addClass('oldActive');
    // if oldActive was the last-child go to the first one
    if ( $('.oldActive').is(':first-child') ) {
      $('.slide').last().addClass('active');
    } else {
      // if oldActive was not the last-child, then add class to the previous one
      $('.oldActive').prev().addClass('active');
    }
    // Clear oldActive class
    $('.oldActive').removeClass('oldActive');
    // 
    $('.slide').fadeOut(speed);
    $('.slide.active').fadeIn(speed);
  });