Best way to store/initialize a persistant variable in Node-Red block?

Question

I am using node-red and I would be interested on making a persistant varible ( atcually an array), that is not deleted each time that data reached a node. I have read about context option but I cannot manage to make it work.

The point is that I have a block where I woould like to store some incoming data, so later I can compare that stored data with the new incoming one. However, if I define a variable in such a block, that variable will be overwritten each time that something arrives to the block itself, so I cannot effectively compare.

How can I do that then? And, what/where is the best way to initialize that variable?

Thanks in advance,

Best regards

Answer 1

The context functionality is exactly what you want.

There are many ways you can work with context properties. You can get/set them with the Change node or do it with the Function node.

For example:

var myArray = flow.get('myArray');
if (!myArray) {
    myArray = [];
}

// do something with myArray

// save it back to context
flow.set('myArray',myArray);

The documentation has some more details: http://nodered.org/docs/writing-functions#storing-data