Enum TabType

const tab = DocumentApp.getActiveDocument().getActiveTab();
// Use getType() to determine the tab's type before casting.
if (tab.getType() === DocumentApp.TabType.DOCUMENT_TAB) {
  // It's a document tab, write some text to it.
  tab.asDocumentTab().setText('Hello World!');
} else {
  // There are currently no types other than DOCUMENT_TAB.
}