From what I see in the docs, when you use the render api and pass a third argument for a callback, calls to render should be async, not sync, yet when I do this client-side:
nunjucks.render("template.html", data, function(err, res) {
$("#resultDiv").html(res);
});
It is still doing a sync xhr request. (To be clear, it renders just fine, but it isn't an async call.)