# Is this a bug - flow.set()

**URL:** <https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537>\
**Category:** General\
**Created:** [21 October 2024 08:59 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537 "2024-10-21T08:59:29Z")\
**Posts on this page:** 13\
**Page:** 1

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**Author:** ![Tilerdon](https://avatars.discourse-cdn.com/v4/letter/t/ce73a5/32.png) [@Tilerdon](https://discourse.nodered.org/u/Tilerdon)\
**Post date:** [21 October 2024 08:59 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/1 "2024-10-21T08:59:29Z")

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I have a network of ESP32 based devices which monitor the moisture levels in my Houseplants. Each device identifies itself to NodeRed via an MQTT message with the IP Address of the device as the Topic. I want to be able to know if any device has stopped working and am therefore storing the last time they polled in Context using the IP address as the id. UNfortunately this doesn't work. It can be exemplified by the following code.

`[{"id":"894faba19e37b2c5","type":"function","z":"6569e3df316afd50","name":"function 18","func":"let currentTime = new Date().getTime();\nlet ip = \"192.120.1.23\";\nflow.set(ip,currentTime);\n\nreturn msg;","outputs":1,"timeout":0,"noerr":0,"initialize":"","finalize":"","libs":[],"x":310,"y":620,"wires":[[]]}]`

and this is the result in Flow

 ![Screenshot 2024-10-21 095725](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/6/0/60a1bb03de81a36c053f127104d391e02b260b59.png)

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**Author:** ![jbudd](https://avatars.discourse-cdn.com/v4/letter/j/5f8ce5/32.png) [@jbudd](https://discourse.nodered.org/u/jbudd)\
**Post date:** [21 October 2024 09:26 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/2 "2024-10-21T09:26:58Z")

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It certainly doesn't give a very pretty result.

A work-around might be  
`flow.set(ip.replace(/\./g, '_'),currentTime);`

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [21 October 2024 09:33 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/3 "2024-10-21T09:33:27Z")

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The reason that is happening is because we are allowed to specify a javascript object description as the id. For example, one could write `flow.set("a.b", 42)` and the `a.b` would be treated as meaning property `b` of object `a`. So by using "192.120.1.23" you are creating a multi-dimensional array named "192", where you are setting element 120 (which implies filling in empty values 0 to 119) to another array where element 1 contains the object "23" which has been set to the current time.

I am not sure if there is a solution. I will think about it.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [21 October 2024 09:39 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/4 "2024-10-21T09:39:45Z")

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This seems to work

```auto
flow.set(`"${ip}"`,currentTime)

```

By the way, you don't generally need semi-colons on the line ends in javascript any more.

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<div class="post-metadata">

**Author:** ![Tilerdon](https://avatars.discourse-cdn.com/v4/letter/t/ce73a5/32.png) [@Tilerdon](https://discourse.nodered.org/u/Tilerdon)\
**Post date:** [21 October 2024 16:36 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/5 "2024-10-21T16:36:32Z")

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Thanks for the suggestions.

My own workaround was to just use the last segment of the ip address and concatenate that to "IP". All my addresses are on the local network so I think that will work as well.

I'm 76 years old and started off life 52 years ago as a COBOL programmer where the semi-colon was absolutely sacrosanct. But now most of my non Javascript code is Arduino IDE where once again it rules so I do it out of habit now.

I thought that the original problem might have something to do with the Javascript object and I guess it is a problem when you have typeless variables. Its odd though to get data and data structure confused.

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<div class="post-metadata">

**Author:** ![jbudd](https://avatars.discourse-cdn.com/v4/letter/j/5f8ce5/32.png) [@jbudd](https://discourse.nodered.org/u/jbudd)\
**Post date:** [21 October 2024 16:54 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/6 "2024-10-21T16:54:41Z")

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Incidentally you can achieve an alert when one of your ESPs goes offline by using a single trigger node, no code at all. 😆

Each ESP is using a unique msg.topic , it's IP address.  
Send all the messages to a trigger node set to send nothing, wait for a certain time and then send the last message received:  
 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/optimized/3X/9/4/94a7c6936476588dbf1ba0d5388c576e91818bc8_2_517x50.png)

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/3/1/3147d9994675948fb8fbf6be0950374ef7c3f54d.png)

The message sent on will be the last message from the ESP which has stopped sending, including the device IP.

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**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [21 October 2024 19:43 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/7 "2024-10-21T19:43:57Z")

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Um,

Why not use the LWT of MQTT?

LWT is sent if a device goes offline.

So you have a `MQTT-IN` node and it listens to the LWT topic.

Then if one of the devices goes offline, you get a message and it (the message) will tell you which device it is - bonus. 😉

I know this is an older version of MQTT, but for the sake of sharing.

 ![Screenshot from 2024-10-22 06-42-49](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/d/5/d526b95583c5e1ceddc03594cec60e9bd6f12ec7.png)

If you are interested, I can post some arduino code that sets the LWT topic.

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<div class="post-metadata">

**Author:** ![Tilerdon](https://avatars.discourse-cdn.com/v4/letter/t/ce73a5/32.png) [@Tilerdon](https://discourse.nodered.org/u/Tilerdon)\
**Post date:** [21 October 2024 19:46 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/8 "2024-10-21T19:46:41Z")

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Thank you for that suggestion I will give it a try. Although often I like to use tried and tested methods that are independent of the platform I'm using. Because my moisture sensors are battery powered I'm putting them into deep sleep between samples and the sample interval is 30 minutes. When the device wakes it polls NodeRed for its calibration data and returns the latest readings from the sensor. After handling the the data returned, the function node checks the time stamp for all the other sensors and sends me an email for any that require attention. I'm still working out in my own mind how often the e-mail should be resent if the issue is not resolved.

Sometimes my elderly brain finds the specification of some inbuilt functions too hard to decipher and it is easier to write a bit of code myself. I'm totally self taught in the last 3 years on everything I have learned about IoT, including javascript, Arduino, NodeRed, HomeAssistant , MQTT and the building of the devices that the routines written using these apps monitor and control

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<div class="post-metadata">

**Author:** ![Tilerdon](https://avatars.discourse-cdn.com/v4/letter/t/ce73a5/32.png) [@Tilerdon](https://discourse.nodered.org/u/Tilerdon)\
**Post date:** [22 October 2024 08:49 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/9 "2024-10-22T08:49:57Z")

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I'm not sure whether this is relevant as I am not using the NodeRed MQTT broker but thank you for your suggestion and please correct me if I'm wrong. You might also like to read my response to @jbudd 's suggestion above

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<div class="post-metadata">

**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [22 October 2024 08:57 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/10 "2024-10-22T08:57:00Z")

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Yeah, he and I replied at about the same time.

His way is also nice and does work with given nodes.

You set the LWT on the device. Not the broker.

I showed you my config to save me typing.

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<div class="post-metadata">

**Author:** ![jbudd](https://avatars.discourse-cdn.com/v4/letter/j/5f8ce5/32.png) [@jbudd](https://discourse.nodered.org/u/jbudd)\
**Post date:** [22 October 2024 09:00 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/11 "2024-10-22T09:00:30Z")

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It seems that lots of folks use Node-red primarily as an environment for writing and running snippets of javascript and indeed it is very approachable for that (I have no idea how to write or run javascript outside of Node-red!)

Don't neglect all the other nodes though, it is very a capable language without having to write [much] code.

I think @Trying_to_learn's suggestion is considerably more elegant than mine.

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<div class="post-metadata">

**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [22 October 2024 09:11 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/12 "2024-10-22T09:11:28Z")

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To expand on my `LWT` suggestion:

an exert from my code for my Arduino.

```auto
byte willQoS = 0;
boolean willRetain = false;
const char* willTopic = "EOM";
const char* willMessage = "GPS comms FAILURE";
byte bsQoS = 0;
boolean bsRetain = false;
const char* bsTopic = "SOM";
const char* bsMessage = "GPS comms UP";

```

I use the topic `EOM` and `SOM`.  
(don't ask. I goofed it should be EOL and SOL)

So I _Publish_ the message `GPS comms FAILURE` to the `EOM` topic.

I put a `MQTT-IN` node with the broker you use.  
Set the `topic` to `EOM` - in my case - and then stick a `debug` node on it.

If the comms fails between my device MQTT publishes the declared message with the topic of `EOM`.

So......  
You look for a message coming out of the `MQTT-IN` node set with topic `EOM`.  
The message would tell you which device has failed.

With me?

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<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/1X/d073cd938eafa2e558d7c2cd59003b3ef4963033.png) [@system](https://discourse.nodered.org/u/system)\
**Post date:** [20 January 2025 09:12 UTC](https://discourse.nodered.org/t/is-this-a-bug-flow-set/92537/13 "2025-01-20T09:12:09Z")

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