How to escape special chars in JSON

Ok, let's look at JSON.

// receiving device
transfer = function (sJS) {
        try {
            print(JSON.parse(sJS));
        } catch(e){
            print(e);
       }
}

// sender device
bleSendJS = function(_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\x10transfer('${_cmd}')\n`);
        uart.write("\x03\x10transfer('"+_cmd+"')\n");
        setTimeout(function() {
            uart.disconnect();
            console.log("Disconnected");
        }, 1E3);
    });
};

var aJS = JSON.stringify({txt : "012345\nabcde\näöüß"}),
        aJSe = JSON.stringify({txt : "012345\nabcde\näöüß"}).replace(/\\n/g, '\\\\n').replace(/\\u/g, '\\\\u');
    id = "ed:4f:05:32:73:ad random";

bleSendJS(id,aJS);
/* output
_cmd: {"txt":"012345\nabcde\n\u00E4\u00F6\u00FC\u00DF"}
Got:"<- Serial1\r\n>"
Got:"SyntaxError: SyntaxE"
Got:"rror: Expecting a va"
Got:"lid value, got UNFIN"
Got:"ISHED STRING\r\n>"
Disconnected
*/

setTimeout(()=>{bleSendJS(id,aJSe);},2E4);
/* output
_cmd: {"txt":"012345\\nabcde\\n\\u00E4\\u00F6\\u00FC\\u00DF"}
Got:"<- Serial1\r\n>"
Got:"{ \r\n  \"txt\": \"012345"
Got:"\\nabcde\\n\\xE4\\xF6\\xF"
Got:"C\\xDF\"\r\n }\r\n>"
Disconnected

*/

Hope you have an idea why the extra escaping is needed?