Have shared an approach which will add the benefit of firing when a label associated with a checkbox is clicked. 
$(document).ready(function() { 
        //set initial state. 
        $('#textbox1').val(this.checked); 
        $('#checkbox1').change(function() { 
          if(this.checked) { 
              var returnVal = confirm("Are you sure?"); 
              $(this).prop("checked", returnVal); 
    } 
    $('#textbox1').val(this.checked); 
    });
});