Create Document Set using JSOM

gistfile1.js

var clientContext = null;
function createTestDocumentSet() {
  if(clientContext === null) {
    return false;
  }
        
  var item = null;
  var oList = clientContext.get_web().get_lists().getByTitle('Library Name');
  clientContext.load(oList);
  var itemCreateInfo = new SP.ListItemCreationInformation();
  itemCreateInfo.set_underlyingObjectType(SP.FileSystemObjectType.folder);
  itemCreateInfo.set_leafName('Documentset Name');
  
  item = oList.addItem(itemCreateInfo);
  item.set_item("ContentTypeId", "0x0120D520");
  item.set_item('Title', 'Documentset Name');
  
  item.update();
  clientContext.load(item);
  clientContext.executeQueryAsync(
    function() { console.log("document-set created!");} , 
    function(sender,args) { console.log("Doh... " + args.get_message()); }  
  );
}