Enum ElementType

const documentTab =
    DocumentApp.getActiveDocument().getActiveTab().asDocumentTab();
const firstChild = documentTab.getBody().getChild(0);
if (firstChild.getType() === DocumentApp.ElementType.PARAGRAPH) {
  // It's a paragraph, apply a paragraph heading.
  firstChild.asParagraph().setHeading(DocumentApp.ParagraphHeading.HEADING1);
}