JSON trouble parsing MQTT.subscribe result

ParaDogma

I'm trying to flip a switch based on a value I'm reading from my MQTT broker in Shelly script. My starting point was the "mqtt_discovery.js" example and I actually managed to get data read.

However, the data from the broker (mosqitto) is a JSON string - and that's where my problems start. While I managed to parse manually created JSON strings using JSON.parse(), I keep getting "Uncaught SyntaxError: Expecting a valid value, got EOF".

Could anybody please help me wrangle the (malformed?) JSON string?

Script:

JavaScript

// configuration object
let CONFIG = {
  topic:     "victron-multiplus/N/c0619ab08f1e/system/0/Ac/Grid/L1/Power",
  threshold: 100,
};

// callback function for the subscriber
function MQTTCmdListener(topic, msg) {
  print(msg);
  var msgObj = JSON.parse(msg);
  print(msgObj.value);
};

function subMQTT() {
  MQTT.subscribe(CONFIG.topic, MQTTCmdListener);
};

subMQTT();

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Value in MQTT:

plain: {"value": 193.2}
string from Debug Log: "{\"value\": 193.2}"

Weird effect - if I use JSON.stringify() on the value, I get:

plain: "{\"value\": 193.2}"
string from Debug Log: "\"{\\\"value\\\": 187.6}\""

Error in debug log:

Code

< {"ts":1700520635.55< {"ts":1700520985.856, "level":2, "data":"shelly_scripts_rpc.:348 Sending \"script_1.js\""}
< {"ts":1700520985.869, "level":-1, "data":""}
< {"ts":1700520985.888, "level":-1, "data":"Uncaught SyntaxError: Expecting a valid value, got EOF"}
< {"ts":1700520985.891, "level":-1, "data":" at line 1 col 1"}
< {"ts":1700520985.892, "level":-1, "data":""}
< {"ts":1700520985.894, "level":-1, "data":"^"}
< {"ts":1700520985.898, "level":-1, "data":"in function called from system"}
< {"ts":1700520985.899, "level":-1, "data":""}
< {"ts":1700520985.905, "level":1, "data":"shelly_user_script.:397 syntax_error: Error in EjsCall"}
< {"ts":1700520985.935, "level":2, "data":"shelly_notification:163 Status change of script:1: {\"id\":1,\"errors\":[\"syntax_error\"],\"running\":false}"}

_[Deleted]_

Hello ParaDogma ,

The first _msg is an empty string. To fix this error, a string and length check are required, here this should work:

Code

//configuration object
let config= {
    topic: "victron-multiplus/N/c0619ab08f1e/system/0/Ac/Grid/L1/Power",
    threshold: 100,
};

// callback function for the subscriber
function MQTTCmdListener(_topic, _msg) {
    print('before parse: ',_msg);
    if(typeof _msg !== 'string' || !_msg.length) return;
    var msgObj= JSON.parse(_msg);
    if(typeof msgObj !== 'object') return;
    print('after parse',msgObj.value);
};

function subMQTT() {
  MQTT.subscribe(config.topic, MQTTCmdListener);
};

print('Working');
subMQTT();

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