How Can I parse out JSON in a Node-RED Function

问题

I am working with Node-RED in Bluemix for IoT.

How can I parse out the individual pieces of information (like the cmdmsg and the tempr) in a function node so I can use it in other nodes in the flow? I'm getting an error when I try (see below)

I am receiving the JSON complete message object (from an IoT in Node) that I see in my "debug" node when I set it to look at the complete message object. see the object below.

It appears to me that the JSON is formatted correctly.

I tried putting the following in the function node, but I'm getting an error that says "TypeError: Cannot read property 'tempr' of undefined"

Here is what the function parameter is: return {payload:msg.payload.d.tempr};

and here is the message object

{
   "topic": "iot-2/type/Arduino-tempsensor/id/FFFFFFFFFFFF/evt/status/fmt/json", 
   "payload": "{\n\"d\": {\n\"myName\": \"Arduino CF\",\n\"cmdmsg\": \"Weekly\",\n\"tempr\": -3,\n}\n}", 
   "deviceId": "FFFFFFFFFFFF", 
   "deviceType": "Arduino-tempsensor", 
   "eventType": "status", 
   "format": "json", 
   "_msgid": "ffffffff.55555" 
}

note: I obfuscated the device ID (mac address) and msgid

Any ideas on how to parse the data out and why I'm getting an error?

回答1:

Sorry, but your JSON Payload is completely messed up, it should look like this: {"d": {"myName": "Arduino CF","cmdmsg": "Weekly","tempr": -3}} You shouldn't see any \or \nin the payload, they look like escape characters from the client side. I also believe that the last comma after the tempr value shouldn't be there for valid JSON.

I am not an Arduino expert but I have experimented with a Raspberry Pi and the Mosquitto client, this is how I can successfully send an event to IoTF: mosquitto_pub -h <org>.messaging.internetofthings.ibmcloud.com -p 1883 -u "use-token-auth" -P "<token>" -i d:<org>:raspi:raspi2 -t iot-2/evt/message/fmt/json -m {"d":{"text":"Hello World"}}

If the paylod is correct JSON your statement return {payload:msg.payload.d.tempr}; will work.

Have you seen this: http://www.ibm.com/developerworks/cloud/library/cl-bluemix-arduino-iot2/

回答2:

The JSON string you are showing should be converted to a Javascript object before you try to access its fields. To do that is as simple as wiring the arduino output to a "JSON" node, which does the conversion for you (or throws an error if the string is not valid JSON).

Wire the output of the JSON node to a debug node, if you want to see the structure of the msg object. You can also wire it to a "change" node, if you simply want to replace the msg.payload with the temperature value, for instance. You don't need any custom javascript code in a function node to do simple changes like that.

Here is a sample flow that you can import... the arduino output string is simulated by pasting your payload into a "template" node:

[{"id":"1a79abfe.b8abb4","type":"inject","z":"58c8eb7a.5496c4","name":"send output","topic":"iot-2/type/Arduino-tempsensor/id/FFFFFFFFFFFF/evt/status/fmt/json","payload":"true","payloadType":"bool","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":170,"y":2740,"wires":[["9fc678fb.ae18e8"]]},{"id":"69e91778.e0c6e8","type":"json","z":"58c8eb7a.5496c4","name":"","property":"payload","action":"","pretty":false,"x":390,"y":2800,"wires":[["d066800f.60a9b","cf991eb1.f2a1a"]]},{"id":"9d8d7da2.2a7da","type":"debug","z":"58c8eb7a.5496c4","name":"msg string","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":610,"y":2740,"wires":[]},{"id":"9fc678fb.ae18e8","type":"template","z":"58c8eb7a.5496c4","name":"arduino string","field":"payload","fieldType":"msg","format":"json","syntax":"plain","template":"{\n \"d\": {\n \"myName\": \"Arduino CF\",\n \"cmdmsg\": \"Weekly\",\n \"tempr\": -3\n }\n}","output":"str","x":360,"y":2740,"wires":[["69e91778.e0c6e8","9d8d7da2.2a7da"]]},{"id":"d066800f.60a9b","type":"change","z":"58c8eb7a.5496c4","name":"extract tempr","rules":[{"t":"set","p":"payload","pt":"msg","to":"payload.d.tempr","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":450,"y":2860,"wires":[["af730d72.2995a"]]},{"id":"af730d72.2995a","type":"debug","z":"58c8eb7a.5496c4","name":"tempr","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","x":630,"y":2860,"wires":[]},{"id":"cf991eb1.f2a1a","type":"debug","z":"58c8eb7a.5496c4","name":"msg object","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":610,"y":2800,"wires":[]}]

As was noted, your original JSON string was not valid, because of the trailing comma -- but it is ok to have double-quotes and newlines as long as they are escaped with a "\". When the string is parsed, they are stripped out anyway.

来源：https://stackoverflow.com/questions/32983841/how-can-i-parse-out-json-in-a-node-red-function