Přidání/odebrání třídy

<span id="compare_isa_indicator" class=""><?php if(isset($_SESSION['compare_isa'])){echo count($_SESSION['compare_isa']);}else{echo 0;} ?>/3</span>

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<script>  
$(document).ready(function(){    
  function hasClass($element, $class) {
    return (' ' + $element.className + ' ').indexOf(' ' + $class + ' ') > -1;
  }
  
  $(".btm_compare").click(function(){     
    var $this_id_isa = (this.id); 
    
    var $selected_id = $('.selected');
    var $indicator = $('#compare_isa_indicator');
    
    var $el_btn = document.getElementById($this_id_isa);
    var $el_indicator = document.getElementById($indicator);
    
    $.ajax({
      url: 'ajax__compare.php',
      type: "GET",
      data: {id_isa: $this_id_isa},
      success: function(data) {
        $indicator.html(data);
        
        if (hasClass($el_btn, 'add') === true){          
          if($selected_id.length < 3){
            $el_btn.classList.add("button_red");
            $el_btn.classList.add("selected");
            $el_btn.classList.remove("button_blue");
            $el_btn.classList.remove("add");
            $el_btn.innerHTML = "Selected";
          } 
                  
          if($selected_id.length == 0){
            $el_indicator.classList.add("active");
          }
          
        } else if (hasClass($el_btn, 'selected') === true){ 
          $el_btn.classList.add("button_blue");
          $el_btn.classList.add("add");
          $el_btn.classList.remove("button_red");
          $el_btn.classList.remove("selected");
          $el_btn.innerHTML = "Compare";
                  
          if($selected_id.length == 1){
            $el_indicator.classList.remove("active");
          }
        }        
      }
    });     
  });  
});
</script>