How to escape special chars in JSON

I don't understand. I ran this on a Pixl, talking to a Puck.js, and it works great using print:

function bleSendJS(_id, _cmd) {
  print('_cmd:',_cmd);
  NRF.requestDevice({ filters: [{ id: id }] }).then(function(device) {
      return require("ble_uart").connect(device);
  }).then(function(uart) {
    uart.on('data', function(d) { print("Got:" + JSON.stringify(d)); });
    uart.write(`\x03\x10print(${_cmd})\n`);
    setTimeout(function() {
        uart.disconnect();
        console.log("Disconnected");
    }, 1E3);
  });
}

var aJS = JSON.stringify({txt : "012345\nabcde\näöüß"}),    
id =  "c8:c1:f1:47:c7:79 random";
bleSendJS(id,aJS);

Reports:

Got:"{ \r\n  \"txt\": \"012345"
Got:"\\nabcde\\n\\xE4\\xF6\\xF"
Got:"C\\xDF\"\r\n }\r\n>"

Using var aJS = E.toJS({txt : "012345\nabcde\näöüß"}) instead does exactly the same.

Is your transfer function really:

transfer = function (saJS5) {
        var aJS5;
        try {
            aJS5 = eval('('+saJS5+')');
        } catch(e){
            console.log("error in eval:",e);
        }
}

If so then the issue is that in transfer you're trying to evaluate something that isn't a string. It's already a properly decoded JSON object.

So...

transfer = function (saJS5) {
        var aJS5;
        try {
            aJS5 = eval('('+saJS5+')');
        } catch(e){
            console.log("error in eval:",e);
        }
}

// What you think you're doing
transfer("{txt:\"012345\\nabcde\\n\\xE4\\xF6\\xFC\\xDF\"}") // ok

// what you were doing just by adding '' around a string
transfer('{"txt":"012345\nabcde\n\u00E4\u00F6\u00FC\u00DF"}') // error

// what you're actually doing
transfer({"txt":"012345\nabcde\n\u00E4\u00F6\u00FC\u00DF"}) // error