Jsonata globalContext() inconsistent handling of string vs variable

General
1

When accessing a map object via jsonata globalContext, it does not work when the get method uses a variable - but using a string works.
v 3.0.2

example:

[{"id":"8885e1d87c774eb3","type":"change","z":"f1e84ae7.4dcf98","name":"get with variable","rules":[{"t":"set","p":"payload","pt":"msg","to":"$globalContext(\"testmap\").get(payload)","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":877,"y":1033,"wires":[["e28f0de5eb5a8d25"]]},{"id":"f07e5a42e3af87ee","type":"inject","z":"f1e84ae7.4dcf98","name":"variable","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"MEQ0838051","payloadType":"str","x":615,"y":1082,"wires":[["8885e1d87c774eb3"]]},{"id":"e28f0de5eb5a8d25","type":"debug","z":"f1e84ae7.4dcf98","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":1138,"y":1012,"wires":[]},{"id":"1e367b4a4fcb63fb","type":"inject","z":"f1e84ae7.4dcf98","name":"config","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":"2","topic":"","payload":"[{\"hm_addr\":\"MEQ0838051\",\"system\":\"hm\",\"type\":\"lock\"},{\"hm_addr\":\"MEQ0092869\",\"system\":\"hm\",\"type\":\"motion\"}]","payloadType":"json","x":615,"y":993.0000610351562,"wires":[["9ba31ab579766bd2"]]},{"id":"9ba31ab579766bd2","type":"function","z":"f1e84ae7.4dcf98","name":"load map","func":"var m = new Map()\n\nmsg.payload.forEach((item) => {\n    m.set(item.hm_addr,item)\n});\n\nglobal.set(\"testmap\",m);\nreturn null\n","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":854.9993896484375,"y":964.982421875,"wires":[[]]},{"id":"5c49ee580fc41d67","type":"change","z":"f1e84ae7.4dcf98","name":"get with string","rules":[{"t":"set","p":"payload","pt":"msg","to":"$globalContext(\"testmap\").get(\"MEQ0838051\")","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":895,"y":1086,"wires":[["e28f0de5eb5a8d25"]]},{"id":"c9776cce0d6852d7","type":"inject","z":"f1e84ae7.4dcf98","name":"string","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"MEQ0838051","payloadType":"str","x":633,"y":1135,"wires":[["5c49ee580fc41d67"]]}]
2

Clue might lie in the output of this function node?

const x = global.get("testmap");
node.warn(x instanceof Map)
node.warn(x instanceof Object)

const y = new Map()
node.warn(y instanceof Map)
node.warn(y instanceof Object)

Where you will find that it outputs FALSE, FALSE, TRUE, TRUE. Which is rather unexpected.

I think this is because the node get/set functions don't necessarily do what people expect. In that they may serialise/unserialise the data though even I'll admit that I was surprised that the got map was not an instance of Object. I've not yet worked out what it IS an instance of.

3

OK, no, I'm wrong. That isn't the problem.

The problem is the opposite I believe. I've checked with both memory and file global variables and both do in fact return Map's.

Which means that it is JSONata that can't cope with a Map most likely. Maps are not standard JavaScript objects, they are, as you probably know, a special type of object and so some things we are used to doing with objects do not work.

For example node.warn(Object.getPrototypeOf( new Map() )) will fail with TypeError: encodeObject Error: [Method Map.prototype.entries called on incompatible receiver #<Map>] Value: Object [Map]. But it works fine with a standard object.

And Object.prototype.toString.call(z) will return [object Map] for a Map but [object Object] for a standard object.

Maps are weird. For example, you can do this with a Map: map1.set({'a':1}, 97) that is perfectly valid! If you do console.log([...map1.keys()]), you get something like this: Array ["a", "b", "c", Object { a: 1 }]. Very weird.

I still don't know why this returns FALSE though, it should return TRUE so something odd is happening.

const x = global.get("testmap");
node.warn(x instanceof Map)

And it is still true that you cannot directly serialise a Map with JSON.stringify. It will not work. So if you are using something like the the REDIS store for variables, I think global.set and get will fail.

1 Like
4

When using the msg variable the map is not in base context so can not see the payload.
use
$$.payload
So JSONata is looking in correct place.

This is an example why i always use $$ when referencing the msg context.

1 Like
5

Thank you all for the quick investigation and help!

6

Ah! I perhaps overlooked something in my response :grin:

I'm glad it works. However I foresee maps creating more problems in the future for Node-RED. I've already fallen foul of them in my own code. Before now, we've mostly assumed we can serialise most things without too many issues and that is less true now.

7

Wondering if the map() is even needed here, would not a simple object do, the use $lookup(object, payload)

8

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