how can i parse JSON coming from a nrf24 Buffer? - node.js

Question

I am sending a valid JSON object constructed with ArduinoJSON to a RaspberryPi running node.js with the library https://github.com/natevw/node-nrf over a nrf24 radio link. The node.js server receives the data seemingly without problem. But for some reason I can't JSON.parse() the object (or buffer?) without getting a SyntaxError: Unexpected token in JSON at position ...
For some reason the node-nrf library receives data backwards, so i need to reverse the order of the bytes with Array.prototype.reverse.call(d), and then console.log(d.toString()) and everything seems fine. In this case, the console gets Got data: [{"key":"a1","value":150}]. At this point, the content of the buffer looks like : Buffer 5b 7b 22 6b 65 79 22 3a 22 61 31 22 2c 22 76 61 6c 75 65 22 3a 31 35 30 7d 5d 00 00 00 00 00 00. Those are the actual 32 bytes that the nrf24 buffer contains i guess.
But then, when the code gets to the JSON.parse() call, i get SyntaxError: Unexpected token in JSON at position 26. This is the position my object data actually ends in the buffer.
I've also experimented with .toJSON() and JSON.stringify() , but can't actually get a proper object to use ( ie. obj.key, obj.value). It's only returning undefined properties. It seems to me the parsing fails when it reaches the end of the object. I've also tried to match the buffer size with the actual size of the message just to see if the parsing would succeed to no avail !
I am probably very mixed up in concepts of buffers, streams, pipes and objects ... what am i doing wrong ?
I need ideas (or fixes!)
Code running on the receiving end in node.js:
var nrf = NRF24.connect(spiDev, cePin, irqPin);
nrf.printDetails();
nrf.channel(0x4c).transmitPower('PA_MIN').dataRate('1Mbps').crcBytes(2).autoRetransmit({count:15, delay:4000}).begin(function () {
var rx = nrf.openPipe('rx', pipes[0]);
rx.on('data', d => {
let obj = Array.prototype.reverse.call(d);
try {
console.log("Got data: ", d.toString());
console.log(obj);
obj = JSON.parse(obj);
console.log(obj);
} catch (err) {
console.error(err)
}
});
});
I don't think the problem is here in forming the JSON message. But for reference purposes, this is the code running on the Arduino:
#include <SPI.h>
#include <nRF24L01.h>
#include <RF24.h>
#include <ArduinoJson.h>
const uint64_t addresses[5] = {0x65646f4e32LL,0x65646f4e31LL} ;
RF24 radio(7,8);
char output[32];
void setup()
{
Serial.begin(115200);
radio.begin();
radio.setAutoAck(true);
radio.setDataRate(RF24_1MBPS);
radio.enableDynamicPayloads();
radio.setCRCLength(RF24_CRC_16);
radio.setChannel(0x4c);
radio.setPALevel(RF24_PA_MAX);
radio.openWritingPipe(addresses[0]);
}
void loop()
{
const int capacity = JSON_ARRAY_SIZE(2) + 2*JSON_OBJECT_SIZE(2);
StaticJsonBuffer<capacity> jb;
JsonArray& arr = jb.createArray();
JsonObject& obj1 = jb.createObject();
obj1["key"] = "a1";
obj1["value"] = analogRead(A1);
arr.add(obj1);
arr.printTo(output);
bool ok = radio.write(&output, sizeof(output));
arr.printTo(Serial);
Serial.print(ok);
delay(1000);
}

Answer 1

Most likely you have NUL characters at the end of the string. JSON.parse will refuse to parse it.
let obj = '[{"key":"a1","value":150}]\x00\x00\x00\x00\x00\x00';
JSON.parse(obj);
Uncaught SyntaxError: Unexpected token  in JSON at position 26
If you remove the NUL characters, parsing succeeds:
let obj = '[{"key":"a1","value":150}]\x00\x00\x00\x00\x00\x00';
obj = obj.replace(/\0/g, "");
JSON.parse(obj);

Answer 2

Parse 'buffer data' into 'string' like:
rx.on('data', d => {
try {
let obj = d.toString();
console.log(obj);
obj = JSON.parse(obj);
console.log(obj);
} catch (err) {
console.error(err)
}
});