title: Uncaught TypeError: Cannot read property 'then' of undefined using Sweet Alert
body: <p>I am using <a href="http://limonte.github.io/sweetalert2/" rel="noreferrer">SweetAlert2</a> and I am getting the following error</p>
  
  <blockquote>
    <p>Uncaught TypeError: Cannot read property 'then' of undefined</p>
  </blockquote>
  
  <p>When I use the exact same code as suggested in SweetAlert page.</p>
  
  <pre><code>swal({
  title: 'Are you sure?',
    text: "You won't be able to revert this!",
    type: 'warning',
    showCancelButton: true,
    confirmButtonColor: '#3085d6',
    cancelButtonColor: '#d33',
    confirmButtonText: 'Yes, delete it!',
    cancelButtonText: 'No, cancel!',
    confirmButtonClass: 'btn btn-success',
    cancelButtonClass: 'btn btn-danger',
    buttonsStyling: false,
    closeOnConfirm: false,
    closeOnCancel: false
  }).then(function(isConfirm) {
    if (isConfirm === true) {
      swal(
        'Deleted!',
        'Your file has been deleted.',
        'success'
      );
    } else if (isConfirm === false) {
      swal(
        'Cancelled',
        'Your imaginary file is safe :)',
        'error'
      );
    } else {
      // Esc, close button or outside click
      // isConfirm is undefined
    }
  })
  </code></pre>
  
  <p>I have the code at the end of the page. I am using it in a Ruby on Rails app. I have tried returning a value as suggested in other posts, but it does not fix the problem</p>