Get all Folders in a Library using jsom


function GetFolders()
{
var context = new SP.ClientContext.get_current();
var web = context.get_web();
var list = web.get_lists().getByTitle("Project Documents");
var query = SP.CamlQuery.createAllFoldersQuery();
allItems = list.getItems(query);
context.load(allItems);
context.executeQueryAsync(Function.createDelegate(this, this.success), Function.createDelegate(this, this.failed));
}
function success()
{
var FolderNames = "";
var ListEnumerator = this.allItems.getEnumerator();
while(ListEnumerator.moveNext())
{
var currentItem = ListEnumerator.get_current();
var _contentType = currentItem.get_fileSystemObjectType();
if(currentItem.get_fileSystemObjectType() == 1)
{
FolderNames  += currentItem.get_item('Title')+ '\n';
}
}
alert(FolderNames);
}
function failed(sender, args)
{
alert("failed. Message:" + args.get_message());
}
GetFolders();