Noob Node-RED question: how to access a value in JSON tree?

[joennuh]

Hi all.

I’m completely new to Node RED and although it seems to be a nice system I’m struggling to use a specific variable/value in the JSON tree.

I have a flow that fetches the changed device status of a Sonoff S26R2 and this is the output:

"{"endpoint":{"serial_number":"****************","third_serial_number":"*************"},"payload":{"power":{"powerState":"off"}}}"

How can I get that powerState value to use in a mssage for e.g. debug, PushBullet message, etc.?

I tried to write these at such nodes:

msg.payload.power.powerState
msg.payload.power[].powerState
msg.payload["power"].powerState
msg.payload["power"]["powerState"]

But all leads to the output value undefined.

Can anyone explain me what I did wrong and how to get the powerState value?

[mezeibela63]

If you use node

the syntax is

in the next node, succes. I am new in node-red also!

[joennuh]

I don’t have the time to check directly, but I believe I use a noce called like “Control device” or so. But your answer gives me a new clue to try out! payload[0] could be the solution.

[joennuh]

I saw a remark that the output of the eWeLink CUBE add-on in Node-RED doesn’t output a Javascript object. It does output a string. See Not getting any events in Node-Red - #7 by csongor.varga And that appearantly is the culprit.

It turns out easy to solve: just use the node that converts a JSON string to Javascript objects and then use the rest of the code.

Below is a visual representation of how I solved it. Maybe this explanation can help others to get started. I hope eWeLink will improve the output of their nodes to get objects instead of strings. In that way the JSON conversion won’t be necessary anymore.

1. The flow

• At the left you see the “event - state” node. The device involved here is called “Maanlampjes” (litterally translated too “Little moon lights”, as they look like the half of a moon).
• Then you see the JSON converter.
• Then it splits up to a debug node and a switch node.
• The first debug node just outputs all payload data that the JSON converter converted.
• The switch checks whether the device has been switched on. Appearantly the iHost doesn’t send an “off” state, it only sends “on” or no “powerState” at all.
• Then the “on” part of the switch goes to a debug node which only outputs a text “Switched on”.
• The “not on” part of the switch goes to a debug node which only outputs a text “Switched off”.

2. JSON converter settings

• You can convert to a string and Javascript object, but I only need the Javascript object and so at Action I choose Always convert to JavaScript Object.
• I set Property to msg.payload.
• The Name can be anything but I gave it a recognizable name.
• The output will be just msg.payload again, but then converted into a Javascript Object.

3. Switch settings

• At Name I gave it a recognizable name.
• This time I only want to check for an on state at the JSON data at powerState. So at Property I filled in: msg.payload.power.powerState.
• At the first rule I choose ==, then String and then I filled in on.
• Then I added a second rule with the + add button below the list.
• Then I choose !=, then String and again filled in on.
• At the bottom I left the dropdown at checking all rules and left the recreate message sequences unticked.

4. Debug settings for on state

This is just for the on state, but the off state is only 2 letters difference.

• Output has been set to Expression and the contents is just the string Switched on. I felt more comfortable to open the extended editor by clickking on the ... button, but I believe that is not necessary.
• At To I only ticked debug window.
• At Name I again filled in a recognizable name.

[edmasec]

payload.payload.power.powerState

[joennuh]

See my reply above. Without conversion from string to object it is not possible to call the value in your proposed way.