Using node variables

General
1

Trying to do something that should be straight forward but missing something... basic, and the tutorials I am looking at I feel they are not using variables the way I am doing it in a node

For simplicity I have node where I need to define a starting value once and then increment it. I have it working this way:

On Start
this.testVar = 100

On Message
testVar = testVar + 1
msg = testVar
return msg;

This works but is global and I only want it to exist in the node, so I read, and it says do this:

On Start
(nothing)

On Message
var testVar = context.get('testVar') || 0
testVar = testVar + 1
msg = testVar
return msg;

But this doesn't work as it resets testVar to 0 on each pass (even though it says it shouldn't)

So how would I define a node variable on start, and then use it correctly as it runs?

2

Hi @mem16421

You are most there, you just need to write back the value (if its not already an object that is setup)

On Message

let testVar = context.get('testVar') || 0
testVar++
msg.payload = testVar
context.set('testVar', testVar) / * <--- Here */
return msg;
2 Likes
3

This is a function node then, and the scope of the context variable is just that node.
You are never saving the incremented testvar back to context.

var testVar = context.get('testVar') || 0  // Retrieve the con text variable, or else default to 0
testVar = testVar + 1                      // Increment
context.set('testVar', testVar)            // Save to context
msg = testVar                              // This is wrong. Should probably be msg.payload = testVar
return msg;

Note that you might have seen an example where a context variable holds an object

let car = context.get('car') || {"make": "ford", "model": "fiesta"}
car.model = "focus"

In this case the change will be saved if the variable exists, without an explicit context.set, though it's probably bad form to rely on that and not call context.set.

2 Likes
4

All up in my face @jbudd :rofl:

5

Once I starts to type I cannot be distracted...

2 Likes
6

@jbudd @marcus-j-davies

Thank you both!

So if I needed to set an initial value of 100 on testVar I could use On Start for that with a context.set(ā€˜testVarā€™,100)?

or should On Start just not be used?

7

Yes, though I tend not to do that for simple cases like this. I tend to forget that On Start is there and when I look at the code in On Message it looks as if the context variable is not initialised, so I usually just use testVar let testVar = context.get('testVar') ?? 0

Note that nowadays let and const are preferred over var, google will tell you why if you are interested. Also the newer operator ?? is to be preferred over || in this situation as it specifically checks for null or undefined. Keep || for boolean expressions.

2 Likes
8

As far as I can recall I never use On Start for that or anything else.

It makes sense to me for the declaration of a variable to be visible in the same editor window as it's use.

Easy enough for you to try it and report back!

Edit - If you put context.set(ā€˜testVarā€™,100) into On Start, every time a message arrives at the node, testVar will be set to 100, probably defeating the purpose of using a context variable - to save values from one message to the next.

I suppose you could do (not tested)

const foo = context.get('testVar') ?? 100
context.set('testVar', foo)

But I don't see the point.

1 Like
9

I think the whole point of On Start is that it is only executed once, when the node is initially started.

On Message runs every time a message arrives.

1 Like
10

Of course you are right.
I checked my test flow and I seem to have omitted the increment line.

Where's the dunce's hat emoji?

2 Likes
11

Thank you!