Enum ParagraphHeading

const body =
    DocumentApp.getActiveDocument().getActiveTab().asDocumentTab().getBody();

// Append a paragraph, with heading 1.
const par1 = body.appendParagraph('Title');
par1.setHeading(DocumentApp.ParagraphHeading.HEADING1);

// Append a paragraph, with heading 2.
const par2 = body.appendParagraph('SubTitle');
par2.setHeading(DocumentApp.ParagraphHeading.HEADING2);

// Append a paragraph, with normal heading.
const par3 = body.appendParagraph('Text');
par3.setHeading(DocumentApp.ParagraphHeading.NORMAL);