[Solved] How to get Value

If I understand you correctly, you don't want the value, but the number of selected items.
Try this:

Code:

$('#demo').click(function() {            
    var sel = $('#selectList:selected').length;
    alert(sel);        
});

Thanks you so much Wouter60,

Problem solve already...

I add "option" to get selected value.

Code:

jQuery( document ).ready(function($) {
       //code
    $('#demo').click(function() {             
    var sel = $('#selectList option:selected').length;
       alert(sel);    
    }
            
    )    
   });