Yes I checked multiple times. And I don't get any exception so I guess the Puck is able to write the value. If I specify a non-existing characteristic, I do get an exception.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
I'm not sure what to suggest there - what you've got looks fine.
Are you sure you've got the right characteristic there?