# Dynamic Objectpath for extracting an element

**URL:** <https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951>\
**Category:** General\
**Created:** [13 September 2021 09:14 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951 "2021-09-13T09:14:39Z")\
**Posts on this page:** 18\
**Page:** 1

<div class="post-metadata">

**Author:** ![protonmw](https://avatars.discourse-cdn.com/v4/letter/p/b3f665/32.png) [@protonmw](https://discourse.nodered.org/u/protonmw)\
**Post date:** [13 September 2021 09:14 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/1 "2021-09-13T09:14:39Z")

</div>

Hi all!

I want to extract a message element by a known path, but dynamically (via dropwdown or inputstring).  
How can I do this?

Thanks for help on this!

---

<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:** [13 September 2021 09:30 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/2 "2021-09-13T09:30:30Z")

</div>

If the element path is in the variable `path` then you can use msg[path] to address that element in the message.

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<div class="post-metadata">

**Author:** ![protonmw](https://avatars.discourse-cdn.com/v4/letter/p/b3f665/32.png) [@protonmw](https://discourse.nodered.org/u/protonmw)\
**Post date:** [13 September 2021 09:33 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/3 "2021-09-13T09:33:05Z")

</div>

I'm not sure understanding you right, but the [path] should be user definable and not hardcoded.

![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/3/8/386d39a7d63d105c8fa26fd9dd8eebf214379b36.png)

I want the user to let him choose, wich parameter he wants to see visualized of a large dataset.

---

<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:** [13 September 2021 09:48 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/4 "2021-09-13T09:48:33Z")

</div>

`path` is a variable which you got from the input string. So if that is in the payload and your object is in `msg.JsonDatset__DataCycleModuleDataset` then you can use, in a Function node  
`msg.JsonDatset__DataCycleModuleDataset[msg.payload]`

---

<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:** [13 September 2021 09:56 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/5 "2021-09-13T09:56:13Z")

</div>

This should work:

> var userinput = "measureDataset.measureData[0].cellsensitivity"  
> msg.payload = msg.JsonDataset\_\_DataCycleModuleDataset[userinput]

But you will presumably have to devise a way to translate a simple user selection such as "Sensitivity" into "measureDataset.measureData[0].cellsensitivity"

---

<div class="post-metadata">

**Author:** ![protonmw](https://avatars.discourse-cdn.com/v4/letter/p/b3f665/32.png) [@protonmw](https://discourse.nodered.org/u/protonmw)\
**Post date:** [13 September 2021 10:29 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/6 "2021-09-13T10:29:37Z")

</div>

Sounds good, but is not working...

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/e/6/e69e66d66652a910543db3d5d211f1e8824a4f4c.png)

---

<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:** [13 September 2021 10:39 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/7 "2021-09-13T10:39:29Z")

</div>

Please export that flow so that we can try it ourselves. See this post how to post a flow - [How to share code or flow json](https://discourse.nodered.org/t/how-to-share-code-or-flow-json/506)

---

<div class="post-metadata">

**Author:** ![protonmw](https://avatars.discourse-cdn.com/v4/letter/p/b3f665/32.png) [@protonmw](https://discourse.nodered.org/u/protonmw)\
**Post date:** [13 September 2021 10:41 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/8 "2021-09-13T10:41:38Z")

</div>

```auto
[{"id":"7f526c4d.e81224","type":"debug","z":"193cbcd5.135b83","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":430,"y":520,"wires":[]},{"id":"c647637b.689c1","type":"inject","z":"193cbcd5.135b83","name":"","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{\"Timestamp __DataCycleModuleDataset\":\"2021-09-09T06:45:45.580Z\",\"ModuleIndex\":3,\"ModuleOk\":true,\"JsonDataset__ DataCycleModuleDataset\":{\"measureDataset\":{\"encodingMask\":1,\"ok\":true,\"supplyVoltage\":5.01016976,\"remark\":null,\"temperatureActual\":22,\"measureData\":[{\"bridgeNr\":1,\"cellOffset\":-3.93310546875,\"cellOffsetOk\":true,\"cellSensitivity\":1.7035264726429749,\"cellSensitivityOk\":true,\"pressures\":[{\"encodingMask\":3,\"nr\":1,\"pressureTarget\":0.5,\"pressureActual\":0.4987254,\"pressureRegister\":19032,\"temperatureRegister\":16144,\"ntcRegister\":0,\"chipTemperatureRegister\":15552,\"diodeRegister\":16148,\"outputValue\":0,\"outputValueNormed\":0,\"outputValueDiff\":0,\"outputValueDiffNormed\":0,\"pressureDataOk\":true,\"pressureActualOk\":true,\"chipTemperatureRegisterOk\":true,\"outputValueNormedOk\":null,\"outputValueDiffNormedOk\":null},{\"encodingMask\":3,\"nr\":1,\"pressureTarget\":1.8,\"pressureActual\":1.797337,\"pressureRegister\":11783,\"temperatureRegister\":16148,\"ntcRegister\":0,\"chipTemperatureRegister\":15556,\"diodeRegister\":16148,\"outputValue\":0,\"outputValueNormed\":0,\"outputValueDiff\":0,\"outputValueDiffNormed\":0,\"pressureDataOk\":true,\"pressureActualOk\":true,\"chipTemperatureRegisterOk\":true,\"outputValueNormedOk\":null,\"outputValueDiffNormedOk\":null}],\"pressuresMeasureOk\":true},{\"bridgeNr\":2,\"cellOffset\":1.71234130859375,\"cellOffsetOk\":true,\"cellSensitivity\":1.6407810500749416,\"cellSensitivityOk\":true,\"pressures\":[{\"encodingMask\":3,\"nr\":2,\"pressureTarget\":0.5,\"pressureActual\":0.4987254,\"pressureRegister\":10773,\"temperatureRegister\":16168,\"ntcRegister\":0,\"chipTemperatureRegister\":15552,\"diodeRegister\":16168,\"outputValue\":0,\"outputValueNormed\":0,\"outputValueDiff\":0,\"outputValueDiffNormed\":0,\"pressureDataOk\":true,\"pressureActualOk\":true,\"chipTemperatureRegisterOk\":true,\"outputValueNormedOk\":null,\"outputValueDiffNormedOk\":null},{\"encodingMask\":3,\"nr\":2,\"pressureTarget\":1.8,\"pressureActual\":1.797337,\"pressureRegister\":17755,\"temperatureRegister\":16168,\"ntcRegister\":0,\"chipTemperatureRegister\":15556,\"diodeRegister\":16168,\"outputValue\":0,\"outputValueNormed\":0,\"outputValueDiff\":0,\"outputValueDiffNormed\":0,\"pressureDataOk\":true,\"pressureActualOk\":true,\"chipTemperatureRegisterOk\":true,\"outputValueNormedOk\":null,\"outputValueDiffNormedOk\":null}],\"pressuresMeasureOk\":true}],\"measureOk\":true,\"supplyVoltageOk\":true},\"dataValidation\":{\"dataChangeId\":637667667711844900,\"dataCRC\":0},\"cycleTime\":25601.914,\"timestampEntry\":\"2021-09-09T06:45:45.5796704Z\",\"index\":3,\"objMessage\":{\"locale\":null,\"text\":null},\"message\":{\"locale\":null,\"text\":null},\"modulPartCycleQuality\":1}}","payloadType":"json","x":150,"y":520,"wires":[["7f526c4d.e81224","6581831c.51f63c"]]},{"id":"77c84f3f.8f4b3","type":"debug","z":"193cbcd5.135b83","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":410,"y":720,"wires":[]},{"id":"6581831c.51f63c","type":"function","z":"193cbcd5.135b83","name":"","func":"var userinput = \"JsonDataset__DataCycleModuleDataset.measureDataset.supplyVoltage\";\n\nmsg.payload = msg.payload[\"JsonDataset__DataCycleModuleDataset.measureDataset.supplyVoltage\"];\n\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":220,"y":720,"wires":[["77c84f3f.8f4b3"]]}]

```

---

<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:** [13 September 2021 10:48 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/9 "2021-09-13T10:48:01Z")

</div>

Ah, no, that won't work. You can't have multiple levels in the path variable. You can do `msg.payload["a"]["b"]` but you can't do `msg.payload["a.b"]` which is what you want to do (where the string is actually a variable but that doesn't make any difference). I am not sure how to solve that.

---

<div class="post-metadata">

**Author:** ![protonmw](https://avatars.discourse-cdn.com/v4/letter/p/b3f665/32.png) [@protonmw](https://discourse.nodered.org/u/protonmw)\
**Post date:** [13 September 2021 10:56 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/10 "2021-09-13T10:56:52Z")

</div>

Ok, damn. is there any chance to use JSONata? I mean, feed a jsonata searchstring by a message string?

---

<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:** [13 September 2021 11:02 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/11 "2021-09-13T11:02:14Z")

</div>

> [@protonmw](#):
>
> is there any chance to use JSONata

I don't know, but when I said I didn't know how to do it in JS, I meant that someone else surely does. I haven't found it so far but likely someone will interject with the solution.

---

<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:** [13 September 2021 11:06 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/12 "2021-09-13T11:06:28Z")

</div>

This technique seems to work

```auto
msg.payload = {a: {b: 7}}
const path = "a.b"
msg.payload = eval(`msg.payload.${path}`)
return msg;

```

---

<div class="post-metadata">

**Author:** ![protonmw](https://avatars.discourse-cdn.com/v4/letter/p/b3f665/32.png) [@protonmw](https://discourse.nodered.org/u/protonmw)\
**Post date:** [13 September 2021 11:13 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/13 "2021-09-13T11:13:35Z")

</div>

> [@Colin](#):
>
> `eval(`msg.payload.${path}`)`

Thanks a lot!! Yes it works.

Could you give a short explanation on this?

---

<div class="post-metadata">

**Author:** ![E1cid](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/e1cid/32/77971_2.png) [@E1cid](https://discourse.nodered.org/u/E1cid)\
**Post date:** [13 September 2021 11:18 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/14 "2021-09-13T11:18:47Z")

</div>

A JSONata example .

```auto
[{"id":"c647637b.689c1","type":"inject","z":"b779de97.b1b46","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"payload.JsonDataset __DataCycleModuleDataset.measureDataset.supplyVoltage","payload":"{\"Timestamp__ DataCycleModuleDataset\":\"2021-09-09T06:45:45.580Z\",\"ModuleIndex\":3,\"ModuleOk\":true,\"JsonDataset__DataCycleModuleDataset\":{\"measureDataset\":{\"encodingMask\":1,\"ok\":true,\"supplyVoltage\":5.01016976,\"remark\":null,\"temperatureActual\":22,\"measureData\":[{\"bridgeNr\":1,\"cellOffset\":-3.93310546875,\"cellOffsetOk\":true,\"cellSensitivity\":1.7035264726429749,\"cellSensitivityOk\":true,\"pressures\":[{\"encodingMask\":3,\"nr\":1,\"pressureTarget\":0.5,\"pressureActual\":0.4987254,\"pressureRegister\":19032,\"temperatureRegister\":16144,\"ntcRegister\":0,\"chipTemperatureRegister\":15552,\"diodeRegister\":16148,\"outputValue\":0,\"outputValueNormed\":0,\"outputValueDiff\":0,\"outputValueDiffNormed\":0,\"pressureDataOk\":true,\"pressureActualOk\":true,\"chipTemperatureRegisterOk\":true,\"outputValueNormedOk\":null,\"outputValueDiffNormedOk\":null},{\"encodingMask\":3,\"nr\":1,\"pressureTarget\":1.8,\"pressureActual\":1.797337,\"pressureRegister\":11783,\"temperatureRegister\":16148,\"ntcRegister\":0,\"chipTemperatureRegister\":15556,\"diodeRegister\":16148,\"outputValue\":0,\"outputValueNormed\":0,\"outputValueDiff\":0,\"outputValueDiffNormed\":0,\"pressureDataOk\":true,\"pressureActualOk\":true,\"chipTemperatureRegisterOk\":true,\"outputValueNormedOk\":null,\"outputValueDiffNormedOk\":null}],\"pressuresMeasureOk\":true},{\"bridgeNr\":2,\"cellOffset\":1.71234130859375,\"cellOffsetOk\":true,\"cellSensitivity\":1.6407810500749416,\"cellSensitivityOk\":true,\"pressures\":[{\"encodingMask\":3,\"nr\":2,\"pressureTarget\":0.5,\"pressureActual\":0.4987254,\"pressureRegister\":10773,\"temperatureRegister\":16168,\"ntcRegister\":0,\"chipTemperatureRegister\":15552,\"diodeRegister\":16168,\"outputValue\":0,\"outputValueNormed\":0,\"outputValueDiff\":0,\"outputValueDiffNormed\":0,\"pressureDataOk\":true,\"pressureActualOk\":true,\"chipTemperatureRegisterOk\":true,\"outputValueNormedOk\":null,\"outputValueDiffNormedOk\":null},{\"encodingMask\":3,\"nr\":2,\"pressureTarget\":1.8,\"pressureActual\":1.797337,\"pressureRegister\":17755,\"temperatureRegister\":16168,\"ntcRegister\":0,\"chipTemperatureRegister\":15556,\"diodeRegister\":16168,\"outputValue\":0,\"outputValueNormed\":0,\"outputValueDiff\":0,\"outputValueDiffNormed\":0,\"pressureDataOk\":true,\"pressureActualOk\":true,\"chipTemperatureRegisterOk\":true,\"outputValueNormedOk\":null,\"outputValueDiffNormedOk\":null}],\"pressuresMeasureOk\":true}],\"measureOk\":true,\"supplyVoltageOk\":true},\"dataValidation\":{\"dataChangeId\":637667667711844900,\"dataCRC\":0},\"cycleTime\":25601.914,\"timestampEntry\":\"2021-09-09T06:45:45.5796704Z\",\"index\":3,\"objMessage\":{\"locale\":null,\"text\":null},\"message\":{\"locale\":null,\"text\":null},\"modulPartCycleQuality\":1}}","payloadType":"json","x":170,"y":3520,"wires":[["4248b176.d0b738"]]},{"id":"4248b176.d0b738","type":"change","z":"b779de97.b1b46","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"\t$lookup(**,$split($$.topic,\".\")[-1])\t","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":350,"y":3620,"wires":[["77c84f3f.8f4b3"]]},{"id":"77c84f3f.8f4b3","type":"debug","z":"b779de97.b1b46","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":430,"y":3720,"wires":[]}]

```

Eval in the javascript example creates a string "msg.payload.JsonDataset\_\_DataCycleModuleDataset.measureDataset.supplyVoltage" Then executes it, that is why it is dangerous as it can be used to execute malious code.

---

<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:** [13 September 2021 11:21 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/15 "2021-09-13T11:21:59Z")

</div>

No. Don't use eval. It is fraught with danger.

Use `RED.util.getObjectProperty`

See similar thread here : [Alternatives to eval() - #2 by Steve-Mcl](https://discourse.nodered.org/t/alternatives-to-eval/50550/2)

---

<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:** [13 September 2021 11:29 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/16 "2021-09-13T11:29:50Z")

</div>

Oh rats! In the absence of the OP's real data I tested my suggestion with a much simpler dataset and didn't include any dots.

Yes the eval suggestion seems to work, but it is obviously fraught with danger (!).

```auto
var userinput = "measureDataset.supplyVoltage";
msg.payload = eval(`msg.payload.JsonDataset__DataCycleModuleDataset.${userinput}`);
return msg;

```

It's all beyond my javascript pay level.

---

<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:** [13 September 2021 12:40 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/17 "2021-09-13T12:40:30Z")

</div>

> [@Steve-Mcl](#):
>
> Don't use eval. It is fraught with danger.

I agree, it was all I could get to work at the time. I didn't know about getObjectProperty. It works well.

```auto
msg.payload = {a: {b: 7}}
const path = "a.b"
msg.payload = RED.util.getObjectProperty(msg.payload, path)
return msg;

```

---

<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:** [27 September 2021 12:40 UTC](https://discourse.nodered.org/t/dynamic-objectpath-for-extracting-an-element/50951/18 "2021-09-27T12:40:32Z")

</div>

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