How to escape special chars in JSON

if print() work, then JSON.parse() should work too

Nope... because cmd is {"txt":"012345\nabcde\n\u00E4\u00F6\u00FC\u00DF"}

So what you're doing is telling Espruino to do : JSON.parse({"txt":"012345\nabcde\n\u00E4\u00F6\u00FC\u00DF"})

The JSON is already parsed by the command-line. There's no need to parse it at all - what you get is the actual JSON object you wanted in the first place!