javascript woes with substr() on msg.payload

[Phil Grant]

Hi,

I'm struggling to get a function working and not sure why, I'm sure it'll be an easy answer but I just don't understand enough yet to get it.

I'm trying to get a section of the data sent into a function using substr() but I get an error in the debug window.

code in the function

var mac =  msg.payload.substr(14, 12);
msg = mac;
return msg;

The payload is a UDP message set as a Buffer in the node, is this why it's failing? maybe it's not a string?? do I need to covert it?

As usual any help will be greatly appreciated, thanks

Phil

[Dave C-J]

Yup - if it's a Buffer... then it's a Buffer :-)

there is an option in the udp node to output a string...

(or you can  msg.payload.toString()   it in a function if you want to... )

[Phil Grant]

Thanks Dave,

So this in a function should work ?

var msg =  msg.payload.toString();
return msg;

All I get is undefined in the debug.

[Dave C-J]

oops no...

as Mark points out you are re-assigning msg not msg.payload... so by default debug won't show it..

try

var mac =  msg.payload.toString().substr(14, 12);
msg.payload = mac;
return msg;

[Phil Grant]

Thanks for the help, with your clues I got to this, which works a treat.

var mac =  msg.payload.toString('hex').substr(12, 12);

msg.payload = mac;
return msg;

I just didn't understand what a buffer was and how to convert between base64 and hex.

Thanks alot.

[Dave C-J]

Phew - all good.