test.js

// attach the callback
  window.addEventListener("documentViewerLoaded", function () {
    TXDocumentViewer.signatures.setSubmitCallback(handleSignedDocument);
});

  function handleSignedDocument(data) {

      var returnObject = JSON.parse(data);

      if (returnObject.success === true)
          alert("Thanks for signing!");
      else
          alert("Signing failed!");

  }