A Change node JSONata question

[jbudd]

I have two global variables:
global.mac = "E4:5F:01:FE:DC:BA"
and
global.macvendors = {
"E8:48:B8":"TP-Link",
"E4:5F:01":"Raspberry Pi",
"5C:CF:7F":"Espressif",
"D8:49:2F":"Canon"
}

So the vendor bit of mac is $substring($globalContext("mac"), 0, 8) or "E4:5F:01"
And the vendor is $globalContext("macvendors")."E4:5F:01" or "Raspberry Pi"

But I can't work out how to combine the two into a single command to get "Raspberry Pi"
$globalContext("macvendors").$substring($globalContext("mac"), 0, 8) is valid JSONata but confusingly it returns the mac address substring "E4:5F:01"

1. How should I do this?
2. Is macvendors structured properly or should it be an array?

[marcus-j-davies]

Hi @jbudd

I believe this will work

• Look up a value based on its key

I don't do much JSONata - so forgive me, if incorrect

$lookup($globalContext("macvendors"), $substring($globalContext("mac"), 0, 8))

[TotallyInformation]

Try this:

(
    $x := $substring($globalContext("mac"), 0, 8);

    $globalContext("macvendors").$x
)

[TotallyInformation]

@marcus-j-davies

$lookup($globalContext("macvendors"), $substring($globalContext("mac"), 0, 8))

Oops, mine returns the vendor mac not the name! Yours is, of course, much better.

[marcus-j-davies]

I use JSONata in my contributed node - I need to question my self if it doesn't work

I say 'use' - I allow it to be

[TotallyInformation]

Yes, many of mine now allow its use as well - though it is a pain to accommodate since it is async. I must admit that I've started using it a bit more since then as it is a great way to quickly add some dynamic processing to a field. Annoying though when you realise that you imagined seeing $sin and $tan functions in the list!

[jbudd]

Thanks both.

Yup, Marcus' $lookup works.
But why can I use
$globalContext("macvendors").string but not
$globalContext("macvendors").$function?

I think Julian's is a multi-line equivalent of my attempt.
I had attempted := (Pascal!) and ; but they were rejected.
Seems like the brackets round the whole lot are important.

I'm really not warming to JSONata's syntax

[marcus-j-davies]

I would agree - its a little odd.
its a love/hate relationship - I do like to use it in the right situation, but its a little odd in behaviour at times

[TotallyInformation]

@jbudd

But why can I use

I suspect it is due to the overloaded use of the dot separator. It is happy with a variable but not a function. I've been at this game far too long!

Yes the round brackets are critical.

[E1cid]

@TotallyInformation

I suspect it is due to the overloaded use of the dot separator. It is happy with a variable but not a function.

It doesn't like variables either, that's why your expression returned the substring of the mac. The issue is the dot has two meanings, one the path notation, and two the mapping function. I think that is why they have lookup() rather than [ ].

Anyway here is an alternative way just for info.

$globalContext("macvendors." & $substring($globalContext("mac"), 0, 8))

Alternatively if the mac was in a msg property you could do it in the change node with no JSONata
set msg. payload
to global macvendors[msg.mac]

[jbudd]

@E1cid's versions work too.

@E1cid

an alternative why just for info.

$globalContext("macvendors." & $substring($globalContext("mac"), 0, 8))

OK "macvenddors." & $substring($globalContext("mac"), 0, 8) is concatenation so this is
$globalContext("macvendors.E4:5F:01")
And indeed similar (javascript) syntax in a function node:
msg.payload = global.get("macvendors.E4:5F:01") works (who knew?)

@E1cid

Alternatively if the mac was in a msg property you could do it in the change node with no JSONata
set msg. payload
to global macvendors[msg.mac]

"with no JSONata" wow, surprised to see that

Thanks for the suggestions everyone, lots of food for thought there.

Three different solutions gathered together:

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