Using SweetAlert in T3

I found this interesting SweetAlert in http://lipis.github.io/bootstrap-sweetalert/ gave it a shot with this code:

  

document.querySelector('.sweet-12').onclick = function(){

    swal({

      title: "Are you sure?",

      text: "You will not be able to recover this imaginary file!",

      type: "success",

      showCancelButton: true,

      confirmButtonClass: 'btn-success',

      confirmButtonText: 'Success!'

    });

  };

as i tested it via/Chrome, It has an error which said "Uncaught TypeError: Cannot set property 'onclick' of null". I don't get it....I guess JS is not compatible with T3 or joomla?