alert on check selector jQuery

Please advise on why the alert is not working, I have tried several methods and it still wont alert when the checkbox is being checked.

$(document).ready(function(){
  $('.classname').on('change', function(){ 
    if(this.checked) 
    {
    alert("hello world");
    }
  });
});

$(document).ready(function() {
  $('.classname').change(function() {
    if ($(this).prop('checked')) {
        alert("hello World"); 
    }

  });
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<input type="checkbox" id="classname"> 1st method

<input type="checkbox" id="classname2"> 2nt method

Can anyone please tell me where I went wrong or what I missed?