That's a good suggestion to use the Garmin as a reference. I don't have the "compatible chest strap" yet. I'm going to hunt for a 2nd hand Polar H10, as it seems it's going to provide the needed data, but if someone has other suggestions, I'm all ears.
Most Garmin watches can be plugged with a cable (usb-c or else) and then accessed like a mounted drive. I grabbed .fit files from the Fenix5 and also from a forerunner 235, really easy, no cloud needed, no softwares to install.
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.
@Mi