# Can I increment a (possibly null) context variable in a change node?

**URL:** <https://discourse.nodered.org/t/can-i-increment-a-possibly-null-context-variable-in-a-change-node/87041>\
**Category:** General\
**Tags:** jsonata\
**Created:** [6 April 2024 14:12 UTC](https://discourse.nodered.org/t/can-i-increment-a-possibly-null-context-variable-in-a-change-node/87041 "2024-04-06T14:12:03Z")\
**Posts on this page:** 10\
**Page:** 1

<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:** [6 April 2024 14:12 UTC](https://discourse.nodered.org/t/can-i-increment-a-possibly-null-context-variable-in-a-change-node/87041/1 "2024-04-06T14:12:03Z")

</div>

I am trying to count the number of each node type in flows.json. Not difficult using a function but I would like to do it without writing javascript code.

So read the file and split the resulting array gives me one message per node with the node type in msg.payload.type

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/6/d/6d5711e83af3f0de9248503aae3fc367cb11fc2d.png)

How can I use a change node to increment a flow counter eg flow.messagecounts["function"] += 1 ?

I have tried this but I think because flow.messagecounts[msg.payload.type] is initially undefined, the increment never works?

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/f/1/f102d4f060b5f89451a59d75eef75dfffc2caf94.png)

```auto
[{"id":"e14a6f161d1bebd0","type":"inject","z":"db04fd79f3a93d19","g":"8b1229d76a3c7516","name":"go","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":110,"y":600,"wires":[["ac2c02ad65697585"]]},{"id":"c932017ee7d0618e","type":"file in","z":"db04fd79f3a93d19","g":"8b1229d76a3c7516","name":"flows.json","filename":".node-red/flows.json","filenameType":"str","format":"utf8","chunk":false,"sendError":false,"encoding":"none","allProps":false,"x":500,"y":600,"wires":[["d9dd00679dbd79dc"]]},{"id":"d9dd00679dbd79dc","type":"json","z":"db04fd79f3a93d19","g":"8b1229d76a3c7516","name":"","property":"payload","action":"","pretty":false,"x":630,"y":600,"wires":[["232a1f4fb159234b"]]},{"id":"232a1f4fb159234b","type":"split","z":"db04fd79f3a93d19","g":"8b1229d76a3c7516","name":"","splt":"\\n","spltType":"str","arraySplt":1,"arraySpltType":"len","stream":false,"addname":"","x":750,"y":600,"wires":[["18620a135bbb3d51"]]},{"id":"ac2c02ad65697585","type":"change","z":"db04fd79f3a93d19","g":"8b1229d76a3c7516","name":"delete flow.messagecounts","rules":[{"t":"delete","p":"messagecounts","pt":"flow"}],"action":"","property":"","from":"","to":"","reg":false,"x":300,"y":600,"wires":[["c932017ee7d0618e"]]},{"id":"18620a135bbb3d51","type":"change","z":"db04fd79f3a93d19","g":"8b1229d76a3c7516","name":"","rules":[{"t":"set","p":"increment","pt":"msg","to":"messagecounts[msg.payload.type]","tot":"flow"},{"t":"set","p":"increment","pt":"msg","to":"increment + 1","tot":"jsonata"},{"t":"set","p":"messagecounts[msg.payload.type]","pt":"flow","to":"increment","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":680,"y":660,"wires":[[]]}]

```

---

<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:** [6 April 2024 14:26 UTC](https://discourse.nodered.org/t/can-i-increment-a-possibly-null-context-variable-in-a-change-node/87041/2 "2024-04-06T14:26:46Z")

</div>

You could use a Switch to test for undefined, and a change node to set it to zero if so. Then increment it in the next node.

I expect someone will come up with a one-liner JSONata expression to count them.

---

<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:** [6 April 2024 14:49 UTC](https://discourse.nodered.org/t/can-i-increment-a-possibly-null-context-variable-in-a-change-node/87041/3 "2024-04-06T14:49:16Z")

</div>

You can use the $sum() trick

```auto
$sum([$$.increment]) + 1

```

---

<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:** [6 April 2024 15:42 UTC](https://discourse.nodered.org/t/can-i-increment-a-possibly-null-context-variable-in-a-change-node/87041/4 "2024-04-06T15:42:22Z")

</div>

I thought it might be possible to do the whole thing in JSONata, feeding in the flows file and ending up with the total node count for each type.

---

<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:** [6 April 2024 15:44 UTC](https://discourse.nodered.org/t/can-i-increment-a-possibly-null-context-variable-in-a-change-node/87041/5 "2024-04-06T15:44:19Z")

</div>

Yes I am sure it is, and save it all to context

---

<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:** [6 April 2024 19:01 UTC](https://discourse.nodered.org/t/can-i-increment-a-possibly-null-context-variable-in-a-change-node/87041/6 "2024-04-06T19:01:28Z")

</div>

> [@Colin](#):
>
> I thought it might be possible to do the whole thing in JSONata

That's a good thought! It's a json file so I am sure that would be possible.

But what I am actually doing is revising my personal recipe for embedding awk in a template node. The Node-red version is for comparison.  
I only thought of flows.json because it's there and has records spanning multiple lines.  
A magical sentence of jsonata will only reinforce the idea that awk is last century technology!

I'm having trouble with @Colin's switch node suggestion. something odd is going on. More investigation needed.

---

<div class="post-metadata">

**Author:** ![shrickus](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/shrickus/32/517_2.png) [@shrickus](https://discourse.nodered.org/u/shrickus)\
**Post date:** [6 April 2024 23:48 UTC](https://discourse.nodered.org/t/can-i-increment-a-possibly-null-context-variable-in-a-change-node/87041/7 "2024-04-06T23:48:24Z")

</div>

> [@jbudd](#):
>
> because flow.messagecounts[msg.payload.type] is initially undefined, the increment never works?

JSONata will indeed have trouble incrementing an undefined value... but you can protect your expression by using this ternary:

```
increment ? increment + 1 : 1

```

I know you don't want to write "function" code, but just for reference you can easily group/count the entire flow using this JSONata expression:

```
payload { type: $count($) }

```

---

<div class="post-metadata">

**Author:** ![shrickus](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/shrickus/32/517_2.png) [@shrickus](https://discourse.nodered.org/u/shrickus)\
**Post date:** [7 April 2024 00:02 UTC](https://discourse.nodered.org/t/can-i-increment-a-possibly-null-context-variable-in-a-change-node/87041/8 "2024-04-07T00:02:34Z")

</div>

I think this flow does the same thing, but using the `localhost:1880/admin/flows` url to get the flow JSON (instead of trying to locate, read, and parse the flows.json file).

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/d/f/df6cbd70ce0b0dfe8ea6bfae6a57c2236ad3d438.png)

```json
[{"id":"e14a6f161d1bebd0","type":"inject","z":"3729ccee2b11666e","name":"go","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":110,"y":100,"wires":[["95d9639c08ce944b"]]},{"id":"30d599f146373c72","type":"change","z":"3729ccee2b11666e","name":"flow: messagecounts","rules":[{"t":"set","p":"payload","pt":"msg","to":"payload { type: $count($) }","tot":"jsonata"},{"t":"set","p":"messagecounts","pt":"flow","to":"payload","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":480,"y":100,"wires":[["7e3ed51935ab040b"]]},{"id":"7e3ed51935ab040b","type":"debug","z":"3729ccee2b11666e","name":"type: count","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":670,"y":100,"wires":[]},{"id":"95d9639c08ce944b","type":"http request","z":"3729ccee2b11666e","name":"GET /admin/flows","method":"GET","ret":"obj","paytoqs":"ignore","url":"http://localhost:1880/admin/flows","tls":"","persist":false,"proxy":"","insecureHTTPParser":false,"authType":"","senderr":false,"headers":[],"x":270,"y":100,"wires":[["30d599f146373c72"]]}]

```

Since the entire flows.json is processed into a summary object, I just replace the whole flow variable.

---

<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:** [7 April 2024 00:57 UTC](https://discourse.nodered.org/t/can-i-increment-a-possibly-null-context-variable-in-a-change-node/87041/9 "2024-04-07T00:57:29Z")

</div>

> [@shrickus](#):
>
> `increment ? increment + 1 : 1`

That's the answer!  
So using that I have

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/0/0/002a58f4305381f43b28063d812cc1c432b844f0.png)

```auto
[{"id":"112a69ed01730f17","type":"inject","z":"3378d78bb37c052e","name":"go","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":90,"y":60,"wires":[["d51cde875f4b5a0a"]]},{"id":"b23439ad43e95d6e","type":"file in","z":"3378d78bb37c052e","name":"flows.json","filename":".node-red/flows.json","filenameType":"str","format":"utf8","chunk":false,"sendError":false,"encoding":"none","allProps":false,"x":480,"y":60,"wires":[["69782fbbccdc6b91"]]},{"id":"69782fbbccdc6b91","type":"json","z":"3378d78bb37c052e","name":"","property":"payload","action":"","pretty":false,"x":610,"y":60,"wires":[["cb63f29ecd2aaa90"]]},{"id":"cb63f29ecd2aaa90","type":"split","z":"3378d78bb37c052e","name":"","splt":"\\n","spltType":"str","arraySplt":1,"arraySpltType":"len","stream":false,"addname":"","x":730,"y":60,"wires":[["ba5835d2d3c68141"]]},{"id":"d51cde875f4b5a0a","type":"change","z":"3378d78bb37c052e","name":"delete flow.messagecounts","rules":[{"t":"set","p":"messagecounts","pt":"flow","to":"{\"ui_spacer\": 2}","tot":"json"}],"action":"","property":"","from":"","to":"","reg":false,"x":280,"y":60,"wires":[["b23439ad43e95d6e"]]},{"id":"ba5835d2d3c68141","type":"change","z":"3378d78bb37c052e","name":"Count each node type","rules":[{"t":"set","p":"increment","pt":"msg","to":"messagecounts[msg.payload.type]","tot":"flow"},{"t":"set","p":"increment","pt":"msg","to":"increment ? increment + 1 : 1","tot":"jsonata"},{"t":"set","p":"messagecounts[msg.payload.type]","pt":"flow","to":"increment","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":140,"y":120,"wires":[["9893a5a9708d4e00"]]},{"id":"9893a5a9708d4e00","type":"switch","z":"3378d78bb37c052e","name":"Last msg part?","property":"parts.count","propertyType":"msg","rules":[{"t":"eq","v":"parts.index + 1","vt":"jsonata"}],"checkall":"true","repair":false,"outputs":1,"x":340,"y":120,"wires":[["6c23c7047b383eb2"]]},{"id":"b121e56aabb8829a","type":"debug","z":"3378d78bb37c052e","name":"debug 482","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":750,"y":120,"wires":[]},{"id":"6c23c7047b383eb2","type":"change","z":"3378d78bb37c052e","name":"Final counts to payload","rules":[{"t":"set","p":"payload","pt":"msg","to":"messagecounts","tot":"flow"}],"action":"","property":"","from":"","to":"","reg":false,"x":550,"y":120,"wires":[["b121e56aabb8829a"]]}]

```

None of the other approaches worked for me, sorry guys 🤔  
I appreciate the suggestions though.

---

<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:** [21 April 2024 00:57 UTC](https://discourse.nodered.org/t/can-i-increment-a-possibly-null-context-variable-in-a-change-node/87041/10 "2024-04-21T00:57:46Z")

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