Node-Red compare new and previous values

I'm new to Node-Red and I don't know if that's possible without some storage (SQL database for example).
So, I have an MQTT endpoint that pushes numeric value when update made.
For example - message.payload = 799.
How to on every new message compare this new value with the previous one (the one that was received right before) without any external storage?kIPZi

Have you read the section on context variables in the docs?

And then read this thread in the FAQ section Store a value in a function

1 Like

There is also the RBE node which is report by exception which should achieve this!

Another solution, without using storage. Using a join node to put the values into an array and then a change node to subtract the members of the array:

[{"id":"af1019af.e762c8","type":"debug","z":"e8e5ccfb.8b54e","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","x":630,"y":140,"wires":[]},{"id":"d6ad51ff.f94b4","type":"inject","z":"e8e5ccfb.8b54e","name":"","topic":"","payload":"2","payloadType":"num","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":110,"y":140,"wires":[["729a39b.4a576c8"]]},{"id":"729a39b.4a576c8","type":"join","z":"e8e5ccfb.8b54e","name":"","mode":"custom","build":"array","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","accumulate":false,"timeout":"","count":"2","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"","reduceFixup":"","x":300,"y":140,"wires":[["871e9009.b424"]]},{"id":"871e9009.b424","type":"change","z":"e8e5ccfb.8b54e","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"payload[0]-payload[1]","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":460,"y":140,"wires":[["af1019af.e762c8"]]},{"id":"3561fcbd.d33044","type":"inject","z":"e8e5ccfb.8b54e","name":"","topic":"","payload":"4","payloadType":"num","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":110,"y":180,"wires":[["729a39b.4a576c8"]]}]