# Retrieve a object comming from MQTT

**URL:** <https://discourse.nodered.org/t/retrieve-a-object-comming-from-mqtt/64387>\
**Category:** General\
**Created:** [26 June 2022 11:46 UTC](https://discourse.nodered.org/t/retrieve-a-object-comming-from-mqtt/64387 "2022-06-26T11:46:49Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![HenrikDK1971](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/henrikdk1971/32/63844_2.png) [@HenrikDK1971](https://discourse.nodered.org/u/HenrikDK1971)\
**Post date:** [26 June 2022 11:46 UTC](https://discourse.nodered.org/t/retrieve-a-object-comming-from-mqtt/64387/1 "2022-06-26T11:46:49Z")

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Hi  
I'm dead sure it's because I'm newbie, I hope someone will help me out here - here is my problem:

I'm trying to calculate where if have power to start other things in the house, this is done by looking at the value: (payload.data.PO - payload.data.P) comming on MQTT, this part works.

My problem is in the node: "Calculate power diff" here I keep getting the error: TypeError: Cannot read properties of undefined (reading 'data')

My code is:

```auto
var value = msg.payload["power"] 
msg.payload = value.data.PO - value.data.P 
return msg;

```

You can see the values from the picture below:

 ![1](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/7/b/7bb7411c20b9080099e35847606e57ee20aded2d.jpeg)

Thank you for your help

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**Author:** ![Steve-Mcl](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/steve-mcl/32/4826_2.png) [@Steve-Mcl](https://discourse.nodered.org/u/Steve-Mcl)\
**Post date:** [26 June 2022 11:49 UTC](https://discourse.nodered.org/t/retrieve-a-object-comming-from-mqtt/64387/2 "2022-06-26T11:49:40Z")

</div>

There’s a great page in the docs ([Working with messages : Node-RED](https://nodered.org/docs/user-guide/messages)) that will explain how to use the debug panel to find the right path to any data item.

Pay particular attention to the part about the buttons that appear under your mouse pointer when you over hover a debug message property in the sidebar.

![BX00Cy7yHi](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/b/d/bd1f333a9e043b061b39917c328b0094d3e52d30.gif)

* * *

in short `var value = msg.payload["power"] ` means `value` is now pointing to `msg.payload.power` and `value.data.PO` is pointing to `msg.payload.power.data.PO`

![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/4/5/456d283d5742b8889620374b438e12f0189e4ad6.png)

But again - use the tools provided to avoid simple mistakes (use the "Copy Path" button)

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<div class="post-metadata">

**Author:** ![HenrikDK1971](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/henrikdk1971/32/63844_2.png) [@HenrikDK1971](https://discourse.nodered.org/u/HenrikDK1971)\
**Post date:** [26 June 2022 12:02 UTC](https://discourse.nodered.org/t/retrieve-a-object-comming-from-mqtt/64387/3 "2022-06-26T12:02:43Z")

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Thank you, but that is what I have done:/  
Hence I need help!!

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<div class="post-metadata">

**Author:** ![Steve-Mcl](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/steve-mcl/32/4826_2.png) [@Steve-Mcl](https://discourse.nodered.org/u/Steve-Mcl)\
**Post date:** [26 June 2022 12:06 UTC](https://discourse.nodered.org/t/retrieve-a-object-comming-from-mqtt/64387/4 "2022-06-26T12:06:16Z")

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> [@HenrikDK1971](#):
>
> Thank you, but that is what I have done:/

Clearly you have not (or clicked something else further off screen) - where did `msg.payload["power"]` come from - its not in that image.

Try again - hover over the item of interest (e.g. `PO`) and click the "copy path" item. When you paste it in a function you need to suffix it with `msg.` e.g. `msg.payload.x.y.x`

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**Author:** ![HenrikDK1971](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/henrikdk1971/32/63844_2.png) [@HenrikDK1971](https://discourse.nodered.org/u/HenrikDK1971)\
**Post date:** [26 June 2022 12:40 UTC](https://discourse.nodered.org/t/retrieve-a-object-comming-from-mqtt/64387/5 "2022-06-26T12:40:15Z")

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Hi  
Thank you, you are absolutly correct,- I've owe you a beer!

/Henrik

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**Author:** ![system](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/1X/d073cd938eafa2e558d7c2cd59003b3ef4963033.png) [@system](https://discourse.nodered.org/u/system)\
**Post date:** [10 July 2022 12:41 UTC](https://discourse.nodered.org/t/retrieve-a-object-comming-from-mqtt/64387/6 "2022-07-10T12:41:15Z")

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