Detecting "null" flow variables

General
#1

I have this bit of code:

var w = flow.get("who_1") || {};
var p = msg.payload;
if (w.length > 0)
{
    //  do stuff here.

The idea is that at/on first invocation the flow.get("who_1") will not be set.

I was under the impression that the || {} implies that if there isn't a value, set it to null.
(Don't know if there should be a space between the { and }.

The situation is that if the variable is not assigned, it needs to do the do stuff here part.
It isn't.

#2

variable === null is the usual way of testing. A switch node is capable of it directly, flow.variable with rule “is null” or “is not null”

1 Like
#3

Alas because of a multitude of things, I am going with a function node rather than a lot of switch nodes one after the other.

It has been suggested I use the FSM node. But there are two I can see and both are old.
I know nothing about them and reading their descriptions, it is a whole new world to learn if I want to use them.

#4

This is the new code:

var w = flow.get("who_1")|| {};
var p = msg.payload;
if (w === null)
{
    //
    node.warn("1");
    return [msg,null];
} else
if (w == p)
{
    //
    node.warn("1");
    return [msg,null];
} else
node.warn("2");
return [null,msg];

with nothing being set in flow.get('who_1) I would hope that 1 is the output.
Alas it isn't.
I am getting 2 used and I am not seeing why.

1 Like
#5

Not quite - and that is the source of the issue in your Function.

When you have statement like:

var A = B || C;

Then A is assigned the value of B if it has a true-like value. But if B has a 'false-like' value (0, null, undefined, "", false) then A is given the value of C.

So where you have var w = flow.get("who_1")|| {};, if the flow context value who_1 is not defined, then w is given the value {} - which is an empty JavaScript object, not null.

If you have not yet set a value in flow.who_1, then the flow.get() statement will return undefined.

So the proper way to do the check you are doing is:

// don't use the ||{} trick as you want to know if it is undefined
var w = flow.get("who_1");
var p = msg.payload;
// Check if it is undefined, not null
if (w === undefined)
{
    //
    node.warn("1");
    return [msg,null];
} else if (w == p) {
    //
    node.warn("1");
    return [msg,null];
} else {
    node.warn("2");
}
return [null,msg];
4 Likes
#6

For me this worked as as the debug shows ''null'' and the default variable ''null'' does the job. When trying to match if there is msg.payload.length I got this ""TypeError: Cannot read property 'length' of null"".

if (msg.payload=== null) {
msg.payload = 0;
}
return msg;