Include mathematical functions

General
1

Hi, i have a special node from a project called NibePi, that reads out modbus data:

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I want to make the difference from two values (spread) and multiply this value by a third, multiply the result by a constant and output the result in the dashboard as the thermal power (W).
but I don't know how to get the values of the 3 registers as msg.payload for a function.
Any idea ?

Greetings

2

See this article in the cookbook for an example of how to join messages into one object.
Then you can access them in msg.payload.x and msg.payload.y to do your math.

3

thank you, join works.
how should i name the formula data:

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4

Add a debug node to the output of the join & you can copy the path to the variables.

If you dont understandm then this and this should help you access the properies

5

I got my values: {"40012":26.7,"40017":26,"40072":0} in debug
but how can I link them mathematically and output the result ?
("40012"-"40017") * "40072" * "69,45" = output

6

the general format is...

Add a function node after the join

do your math

return the msg.

Example...

var temp = (msg.payload.a - msg.payload.b) * msg.payload.c * msg.payload.d
msg.originalPayload = msg.payload; //save for later debugging / verifying results
msg.payload = temp; 
return msg; //this causes the msg to be sent out the output & on to the next node

As for what msg.payload.a and msg.payload.b should really be - use the copy path from the debug output

1 Like
7

I´dont work:

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8

So it seems to work:

var temp = (msg.payload[40012] - msg.payload[40017])
msg.originalPayload = msg.payload; //save for later debugging / verifying results
msg.payload = temp;
return msg;

9

For accessing properties with strange variable names (like yours) you should use strings

var temp = (msg.payload["40012"] - msg.payload["40017"]);
msg.originalPayload = msg.payload; //save for later debugging / verifying results
msg.payload = temp;
return msg;
1 Like
10

The general syntax for accessing JavaScript object (like msg) properties is that any property can be accessed quoted in brackets with a double or singe quote (" or '), e.g.
msg.payload['123'], msg.payload['temperature'], or msg.payload["my property"].

Alternatively if the property name consists of only alphanumerical characters or underscores, it can be accessed with just a period as a separator, e.g. msg.payload.temp1 or msg.payload.my_property. The property cannot start with a number though.

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11

but it works fine without " " or ´´ !?

12

In this case yes. The property names (object keys) are always strings (not numbers) internally in JavaScript. In the case of numbers the numeric keys are translated to strings, e.g. msg.payload[123] becomes msg.payload['123'].

The same does not apply if the key is alphanumerical though. This is why I said "the general syntax". msg.payload[temperature] for example would mean that variable named temperature would contain the key. I hope this makes sense.

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