How to escape special chars in JSON

I just tried:

var aJS5 = {txt : "012345\nabcde\näöüß"};
var saJS5 = E.toJS(aJS5);  
//"{txt:\"012345\\nabcde\\n\\xE4\\xF6\\xFC\\xDF\"}"
var sa2JS5 =  E.toJS(saJS5);
// "{txt:\"012345\\nabcde\\n\\xE4\\xF6\\xFC\\xDF\"}"  // same
var COMMAND = `transfer(${sa2JS5})\n`;
// note I took the first 2 control chars out
eval(COMMAND); // this work as-is

// Exact code copied from your example that you say doesn't work
transfer("{txt:\"012345\\nabcde\\n\\xE4\\xF6\\xFC\\xDF\"}") // works fine

>print(COMMAND);
transfer("{txt:\"012345\\nabcde\\n\\xE4\\xF6\\xFC\\xDF\"}")
=undefined
//copy and paste
>transfer("{txt:\"012345\\nabcde\\n\\xE4\\xF6\\xFC\\xDF\"}") // also works fine

in the emulator

So I don't really see what the problem is here?

I guess there is maybe some problem with how you are sending this? It seems to me from your replace of \ with \\ that somehow you're trying to evaluate/parse the string one more times than was intended