Unexpected behavior when storing an array in a flow context variable

General
1

Hello,

I'm trying to create a flow context variable in a Function Node to store an array which will be dynamically filled or emptied according to some logic. Unfortunately, it seems that the context variable does not work as I was expecting.

This is a minimal code to reproduce what I'm trying to do:

//Set the flow var with an empty array
flow.set("test_var", [])

// Update the flow variable
flow.set("test_var", flow.get("test_var").push(5))

// Store flow var in msg.payload
msg.payload = flow.get("test_var")
return msg;

Looking at the code, I was expecting my msg.payload equal to [5] but, the returned result is a number (1).

What I'm doing wrong ?

Thank you in advance for your help.

Filippo

2

You are storing the result of push.

From mdn...

Return value

The new length property of the object upon which the method was called.

1 Like
3

The return value of javascript's push is the new length of the array, and this what you are then setting to test_var again.

If you store an object in flow context, you don't need to keep storing it every time you change it. So your code can be simplified to:

flow.set("test_var", [])

// Update the flow variable
flow.get("test_var").push(5)

// Store flow var in msg.payload
msg.payload = flow.get("test_var")
return msg;
4

That only applies for the default memory based context. Generally it is good practice to store it back. If a switch was made to using persistent storage (or other storage mechanism) then it may not work reliably if it is not written back after updating.

1 Like
5

Things are more clearer now.
Thank you all for your explanations.

6

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