在Nette PHP框架中不使用模板返回字符串

What is the best way to return a json string or a simple text on screen in nette php framework without loading the template?

You can use method sendJson($data) in your presenter.

function renderDefault()
{
    $data = ['hello' => 'world'];
    $this->sendJson($data);
}

Will output {"hello":"world"} with application/json Content-Type