Enum ParagraphHeading

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

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

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

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