window.onbeforeunload = function() {
  swal({
    title: 'Sweet!',
    text: 'Modal with a custom image.',
    imageUrl: 'https://unsplash.it/400/200',
    imageWidth: 400,
    imageHeight: 200,
    animation: false
  })
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://cdn.rawgit.com/t4t5/sweetalert/32bd141c/dist/sweetalert.min.js"></script>
<link href="https://cdn.rawgit.com/t4t5/sweetalert/32bd141c/dist/sweetalert.css" rel="stylesheet" />

<a href="https://www.google.com/">Visit google</a>