Question on global context & function behaviour

General
1

Hi,
I have a problem using context. Consider this flow:

[{"id":"c966d72c.b3b428","type":"function","z":"d4de0d13.1abea8","name":"get global dummy","func":"var dummy = global.get(\"dummy\")||10;\nmsg.payload = dummy;\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","x":770,"y":300,"wires":[["375b567e.93a7d2"]]},{"id":"c069bc8e.88a7b8","type":"change","z":"d4de0d13.1abea8","name":"set global.dummy","rules":[{"t":"set","p":"dummy","pt":"global","to":"payload","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":530,"y":300,"wires":[["c966d72c.b3b428"]]},{"id":"cdadfa02.84ae4","type":"inject","z":"d4de0d13.1abea8","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"0","payloadType":"num","x":350,"y":280,"wires":[["c069bc8e.88a7b8"]]},{"id":"4b751bf8.bf39e4","type":"inject","z":"d4de0d13.1abea8","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"2","payloadType":"num","x":350,"y":320,"wires":[["c069bc8e.88a7b8"]]},{"id":"375b567e.93a7d2","type":"debug","z":"d4de0d13.1abea8","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":990,"y":300,"wires":[]},{"id":"235495da.cc389a","type":"inject","z":"d4de0d13.1abea8","name":"get global dummy","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":560,"y":180,"wires":[["c966d72c.b3b428"]]}]

I start with no global context "dummy".
When I try to get it, the functions defaults the value to 10:
var dummy = global.get("dummy")||10;

When I inject 2, global context is set to 2, the subsequent function yields 2. Ok.

But when I inject 0, the global value is set to 0, however, the function yields 10.

Why is this the case? I am not a JS expert and all I read so far was that "||10" is useful when the part to the left of it is undefined. Is that wrong?
How can I fix that?

Thanks in advance, Uwe

2

Not quite. It is useful when the left is a false-like value, which includes undefined. But it also includes the number 0, the empty string "" , null and actual Boolean false.

If 0 is a valid value, but you want the default to be non 0, then you'll need to do a more explicit test:

var dummy = global.get("dummy");
if (dummy === undefined) {
   dummy = 10;
}
3

Thanks for that super quick answer!

I bet this is something others trapped over, too, or will, particularly those discovering NR without proper JS background.
Ciao, Uwe

4

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