Help with context variables and their treatment

General
1

Hello everyone, sorry for my English, I only speak Spanish.
I am new to Node-red, and I have come across this doubt.
I want to save a data in a context variable, to later recover it without modifying its saved value.
example:
In a function block I store a value obtained from a process:

flow.set("variable.data",100);code here

but if later in another block I retrieve this value and do:

msg.payload = flow.get("variable");
msg.payload.data = 200;
return msg;

the value stored in flow.variable.data (context) is also modified (it becomes 200)
I'm not understanding, shouldn't it be kept?
Is it possible that an object cannot be saved to a context variable or do I need to do it as JSON?
Sorry if it's a very stupid question.
Greetings to everybody and thanks.

2

Javascript stores arrays and objects by reference (google it you will get a full explanation).

So you say msg.payload = flow.get, now both context and payload point to the same memory reference.

You would need to copy/clone the value. Node-red provides RED.util.cloneMessage() for this reason
https://nodered.org/docs/api/modules/v/0.20.0/@node-red_util_util.html
e.g

msg.payload = RED.util.cloneMessage( flow.get("variable") );
2 Likes
3

Hello E1cid, thank you very much for your prompt response.
Now it's clear to me what's going on, I missed that detail...
If I apply the change node before the function block, does it get a "copy" of the context variable or am I wrong?
slds

4

The change node has an option to make a deep copy of a msg or context, as you can get the same issue when setting msg/context to another in some cases.

5

Ok, that's more than clear to me.
Once again, thank you very much for your help!
slds
Richard

6

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