Split of string using JSONata in a change node?

General
1

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(/)[1]. But a function node for a one-liner? How could you implement this in a change node? Of course the following does not work...

image

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?

2 
msg.topic.split(/)[1]

is not what you specified in the expression.
but split is a function within jsonata, every function starts with $:

topic.$split("/")[1]

(omit msg)

3

That would be

$split($$.topic, "/")[1]

//or

($$.topic ~> $split("/"))[1]
4

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?

5

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.

6

Yeah, now I get it... like bash $? or powershell $_ is the 'current' context scope.

7

Well, "it works", but it is inherently slow, would not recommend it on large objects.

Just traverse down the path and start from there.
and this works perfectly fine topic.$split("/")[1]

8

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("/")[1] tells JSONata to map over topic as if an array and split each element and output index 1.
$split($$.topic, "/")[1] tells JSOnata to split topic and output index 1.
Both work just fine, as JSONata treats single string as a 1 element array.

9

It is no secret that JSONata can be 100's of times slower than native javascript.

Ofcourse, this does not become a dealbreaker with small(er) objects like this example, but if the path is known I would choose to traverse from there.

10

It is normally not 100's of times slower, bit of an exaggeration to say that. It is slower 1 to 4 ti.es normally, unless you have written a bad expression

11

So, the bottom line is... abusing a function node, if you get my meaning? Is really leaner and meaner, even though it does hide some of the elegance of a visual language. Just an observation.

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