Thats a good heads up (although I would say "not accurate yet"), but for now there is also the option to use a bluetooth sensor (like chest belt) which the plugin also covers
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.
However if you are moving about the heart rate bpm is not accurate.
https://github.com/espruino/BangleApps/issues/1248