MQTT-IN not possible to extract only one value

[autonodered]

hello,

I

want to extract from MQTT IN only one value, it returns me 14 values ​​in the debug of which 13 are undefined, my code does not work.

Function code

var B1 = msg.payload.z.B1;
msg.payload = B1; 
return msg;

[bakman2]

Use a switch node instead:

It will pass the message if B1 exists in payload.z
Ofcourse it is also possible in a function node.

if("B1" in msg.payload?.z){
  return msg
}

[jbudd]

As @bakman2 says, you need to test that msg.payload.z contains the key B1.
Rather than using a function node you can easily achieve the same thing with core Node-red switch and change nodes:

[autonodered]

Thanks for your help, it works.