OK, so I like many of us that us NR, see JSONata as this mystical experience that works at times well other times just not quite right way to do something. I just going to write a small function node to extract a string from a MQTT subscription string... and thought, hey, can I do this in a change node expression? Maybe even with JSONata?
Where msg.topic is 'command/target/status', and I want to pull 'target' from the topic? In a function node, easy, like msg.target = msg.topic.split(/). But a function node for a one-liner? How could you implement this in a change node? Of course the following does not work...
But hey, someone that know change nodes, expressions, or JSONata better than me, know a trick? Or am I just too far outside the box on this on?
OK, I get that the functions are referenced by $, but the $$ is giving me a head scratching. It reminds me a bit of a powershell or bash reference, like $? (bash) or $_ (powershell). How would one know or learn of such as $$ use and syntax? Clearly $.topic is a type of substitution? Shorthand reference?
it's just my convention, you can omit them if you wish. It comes into play with more complicated expressions. It reminds me what scope the property is in.
$$ is the msg/base scope, while $ is the curent scope.
Well when i tested there was no difference, running 1000 operations the average for both was 160ms. Again I do it as a matter of consistency and with more complicated expressions it makes sure i do not call a property from the wrong scope, since doing so I find less expressions fail. It works for me and cost me nothing.
topic.$split("/") tells JSONata to map over topic as if an array and split each element and output index 1. $split($$.topic, "/") tells JSOnata to split topic and output index 1.
Both work just fine, as JSONata treats single string as a 1 element array.