# Jsonata: return value from context array

**URL:** https://discourse.nodered.org/t/jsonata-return-value-from-context-array/5469
**Category:** General
**Created:** [4 December 2018 09:20 UTC](https://discourse.nodered.org/t/jsonata-return-value-from-context-array/5469 "2018-12-04T09:20:54Z")
**Posts on this page:** 3
**Page:** 1

<div class="post-metadata">

### Author: ![psmtvdw](https://avatars.discourse-cdn.com/v4/letter/p/e480ec/32.png) [@psmtvdw](https://discourse.nodered.org/u/psmtvdw)
#### Post date: [4 December 2018 09:20 UTC](https://discourse.nodered.org/t/jsonata-return-value-from-context-array/5469/1 "2018-12-04T09:20:54Z")

</div>

Hi,

I'm looking for the right jsonata expression to do the following:

I have a context variable containing a json array with the following data:  
{  
"Products": [  
{  
"ProductName": "Red Bowler Hat ",  
"ProductID": 858383  
},  
{  
"ProductName": "Green Bowler Hat",  
"ProductID": 858384  
}  
]  
}

The flow I want to create is as follows:  
My inject node has a topic with the "ProductID" of the product.  
Desired result: a string composed of the "ProductID" + "ProductName"

I am now trying to use a change node with a jsonata expression to search the "ProductName" in the context array, with the "ProductID" from the topic as an argument.

So for start I use this basic expression if I use the json object as input:  
Products[ProductID=858383].ProductName

to use a context variable I use the following expression::  
$flowContext('Products.Products[0].ProductName')

But now I want to replace the fixed array index between the square brackets [0] by the ProductID from the topic of the message.  
Something like this:  
$flowContext('Products.Products[ProductID=topic].ProductName')

This last expression is unfortunately not working. I tried a few things but until now with no succes.  
Does anyone have suggestions?

thanks a lot  
Pascal

my node-red flow:  
[{"id":"bfe7a70c.dbcfc8","type":"change","z":"4c075864.0ddb58","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"$flowContext('Products.Products[0].ProductName')\t\t","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":700,"y":440,"wires":[["6a972a5b.cdfd04"]]},{"id":"f2047df2.0117a","type":"inject","z":"4c075864.0ddb58","name":"","topic":"858383","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":410,"y":440,"wires":[["bfe7a70c.dbcfc8"]]},{"id":"6a972a5b.cdfd04","type":"debug","z":"4c075864.0ddb58","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","x":930,"y":440,"wires":[]},{"id":"6997b923.a9e778","type":"change","z":"4c075864.0ddb58","name":"","rules":[{"t":"set","p":"Products","pt":"flow","to":"{"Products":[{"ProductName":"Red Bowler Hat","ProductID":858383},{"ProductName":"Green Bowler Hat","ProductID":858384}]}","tot":"json"}],"action":"","property":"","from":"","to":"","reg":false,"x":710,"y":400,"wires":[[]]},{"id":"d477c511.1d3ca8","type":"inject","z":"4c075864.0ddb58","name":"","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":460,"y":400,"wires":[["6997b923.a9e778"]]}]

---

<div class="post-metadata">

### Author: ![knolleary](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/knolleary/32/3_2.png) [@knolleary](https://discourse.nodered.org/u/knolleary)
#### Post date: [4 December 2018 09:42 UTC](https://discourse.nodered.org/t/jsonata-return-value-from-context-array/5469/2 "2018-12-04T09:42:15Z")

</div>

Hi @psmtvdw

you cannot use a JSONata expression as the key passed to the `flowContext` function.

You would have to get the full object from context and _then_ use a JSONata expression to pull out the piece you want:

```auto
$flowContext('Products').Products[ProductID=$$.topic].ProductName

```

This assumes the product ID you want to lookup is in `msg.topic` and that it is a number type - not a String. JSONata is strict about types.

For future reference, please read this post about sharing code/flows in the forum: [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: ![psmtvdw](https://avatars.discourse-cdn.com/v4/letter/p/e480ec/32.png) [@psmtvdw](https://discourse.nodered.org/u/psmtvdw)
#### Post date: [5 December 2018 07:31 UTC](https://discourse.nodered.org/t/jsonata-return-value-from-context-array/5469/3 "2018-12-05T07:31:04Z")

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> [@knolleary](#):
>
> _then_

Hi,

This works fine. thanks!  
topic is a string by default. So I used $number() function in my jsonata expression

```auto
$flowContext('Products').Products[ProductID=$$.$number(topic)].ProductName	

```

the example flow:

```auto
[{"id":"bfe7a70c.dbcfc8","type":"change","z":"4c075864.0ddb58","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"$flowContext('Products').Products[ProductID=$$.$number(topic)].ProductName\t","tot":"jsonata"},{"t":"set","p":"payload","pt":"msg","to":"topic & \"_\" & payload","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":760,"y":440,"wires":[["6a972a5b.cdfd04"]]},{"id":"f2047df2.0117a","type":"inject","z":"4c075864.0ddb58","name":"","topic":"858383","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":270,"y":440,"wires":[["bfe7a70c.dbcfc8"]]},{"id":"6a972a5b.cdfd04","type":"debug","z":"4c075864.0ddb58","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":1010,"y":440,"wires":[]}]

```