How to send a `null` from a `trigger` node?

I am wanting to inject a message of null to wipe my graphs/gauges.

But to make it a bit nicer I first want to send a 0 value to ...... (what ever) and then send the null to have them indicate they were told to turn off.

That is where the trigger node comes into the picture.

How do I get it to send null as the second message?

Ok, that may be nice to know, but I now realise that isn't exactly what I was wanting.

I am wanting the message payload to be [] - as I now see that [] is different to null.

Select JSON and send [ ]

As for sending null, you could a function node after the inject that sets msg.payload = null;

Probably possible with a change node too.

May even be possible with jsonata on the inject? Is JSON expression available on the inject node? I forget!

1 Like

Yes it is.

This is the flow on which I am working.

It is a mess, but that's because of the trigger node not playing the game.

[{"id":"d894ce7c.7fca88","type":"ui_gauge","z":"b74f6bef.ade128","name":"","group":"a3add0d4.f3ce7","order":4,"width":"4","height":"3","gtype":"gage","title":"gauge","label":"units","format":"{{value}}","min":0,"max":10,"colors":["#00b500","#e6e600","#ca3838"],"seg1":"","seg2":"","x":770,"y":3380,"wires":[]},{"id":"d6d252ac.e4a67","type":"ui_chart","z":"b74f6bef.ade128","name":"","group":"a3add0d4.f3ce7","order":5,"width":"4","height":"3","label":"chart","chartType":"line","legend":"false","xformat":"HH:mm:ss","interpolate":"linear","nodata":"No comms","dot":false,"ymin":"","ymax":"","removeOlder":1,"removeOlderPoints":"","removeOlderUnit":"3600","cutout":0,"useOneColor":false,"colors":["#1f77b4","#aec7e8","#ff7f0e","#2ca02c","#98df8a","#d62728","#ff9896","#9467bd","#c5b0d5"],"useOldStyle":false,"outputs":1,"x":770,"y":3420,"wires":[[]]},{"id":"4b1ea411.1ceea4","type":"inject","z":"b74f6bef.ade128","name":"","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":140,"y":3380,"wires":[["e1685970.6417b8","4f0551bf.e9dc7"]]},{"id":"e1685970.6417b8","type":"trigger","z":"b74f6bef.ade128","op1":"8","op2":"[]","op1type":"num","op2type":"str","duration":"3","extend":false,"units":"s","reset":"","bytopic":"all","name":"","x":430,"y":3380,"wires":[["d894ce7c.7fca88","d6d252ac.e4a67"]]},{"id":"653949a7.d218a8","type":"trigger","z":"b74f6bef.ade128","op1":"8","op2":"[]","op1type":"num","op2type":"str","duration":"4","extend":false,"units":"s","reset":"","bytopic":"all","name":"","x":430,"y":3340,"wires":[["411548bb.9fff18"]]},{"id":"411548bb.9fff18","type":"function","z":"b74f6bef.ade128","name":"","func":"msg.payload = [];\nmsg.topic = \"something\";\nreturn msg;","outputs":1,"noerr":0,"x":560,"y":3340,"wires":[["c20aa324.08e85","d6d252ac.e4a67","d894ce7c.7fca88"]]},{"id":"c20aa324.08e85","type":"function","z":"b74f6bef.ade128","name":"","func":"if (msg.payload.length === 0 )\n{\n    node.warn(\"Null payload detected\");\n    msg.payload = \"N/C\";\n}\nreturn msg;","outputs":1,"noerr":0,"x":690,"y":3340,"wires":[["d894ce7c.7fca88"]]},{"id":"4f0551bf.e9dc7","type":"delay","z":"b74f6bef.ade128","name":"","pauseType":"delay","timeout":"4","timeoutUnits":"seconds","rate":"1","nbRateUnits":"1","rateUnits":"second","randomFirst":"1","randomLast":"5","randomUnits":"seconds","drop":false,"x":280,"y":3340,"wires":[["653949a7.d218a8"]]},{"id":"a3add0d4.f3ce7","type":"ui_group","z":"","name":"Mark","tab":"e5699c6d.5d7e8","order":1,"disp":false,"width":"12","collapse":false},{"id":"e5699c6d.5d7e8","type":"ui_tab","z":"","name":"Events","icon":"dashboard","order":4,"disabled":false,"hidden":false}]

There should be an easier way.

I think the space between the [ and the ] threw me. Progress has been made. But not to conclusion.

Still stuck with the flow.

Got it!

This works.

[{"id":"d894ce7c.7fca88","type":"ui_gauge","z":"b74f6bef.ade128","name":"","group":"a3add0d4.f3ce7","order":4,"width":"4","height":"3","gtype":"gage","title":"gauge","label":"units","format":"{{value}}","min":0,"max":10,"colors":["#00b500","#e6e600","#ca3838"],"seg1":"","seg2":"","x":570,"y":3400,"wires":[]},{"id":"d6d252ac.e4a67","type":"ui_chart","z":"b74f6bef.ade128","name":"","group":"a3add0d4.f3ce7","order":5,"width":"4","height":"3","label":"chart","chartType":"line","legend":"false","xformat":"HH:mm:ss","interpolate":"linear","nodata":"No comms","dot":false,"ymin":"","ymax":"","removeOlder":1,"removeOlderPoints":"","removeOlderUnit":"3600","cutout":0,"useOneColor":false,"colors":["#1f77b4","#aec7e8","#ff7f0e","#2ca02c","#98df8a","#d62728","#ff9896","#9467bd","#c5b0d5"],"useOldStyle":false,"outputs":1,"x":570,"y":3460,"wires":[[]]},{"id":"4837c5a8.e6de54","type":"function","z":"b74f6bef.ade128","name":"","func":"if (msg.payload.length === 0 )\n{\n    node.warn(\"Null payload detected\");\n    msg.payload = \"N/C\";\n}\nreturn msg;","outputs":1,"noerr":0,"x":340,"y":3400,"wires":[["d894ce7c.7fca88","d6d252ac.e4a67"]]},{"id":"c985eea9.9264d","type":"inject","z":"b74f6bef.ade128","name":"","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":180,"y":3460,"wires":[["5bb5f108.85885"]]},{"id":"5bb5f108.85885","type":"trigger","z":"b74f6bef.ade128","op1":"8","op2":"[ ]","op1type":"num","op2type":"json","duration":"3","extend":false,"units":"s","reset":"","bytopic":"all","name":"","x":330,"y":3460,"wires":[["4837c5a8.e6de54"]]},{"id":"a3add0d4.f3ce7","type":"ui_group","z":"","name":"Mark","tab":"e5699c6d.5d7e8","order":1,"disp":false,"width":"12","collapse":false},{"id":"e5699c6d.5d7e8","type":"ui_tab","z":"","name":"Events","icon":"dashboard","order":4,"disabled":false,"hidden":false}]