How to convert string to JSON?

Elastic Stack Logstash
7

made

filter 
{
	if [detail][data][response][body]
	{
		ruby
		{
			code => "
			if event.get('[detail][data][response][body]').is_a? String
				event.set('[detail][data][response][body]', {'[detail][data][response][body][givenstring]' => event.get('[detail][data][response][body]')})
			end"
		}
	}
}

didnt work, again infinite restart