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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.
@Gordon
The MDBT42Q breakout?
Unfortunately the problem is it's connected to a voltage regulator, so it only 'sees' the regulated voltage. You'll be able to use code like that to see when the voltage starts to drop from 3.3v (meaning the LiPo voltage is near or below that), but by that time the battery is only 10% full or so.
Best bet is to add two 1MOhm resistors to the battery voltage to create a 'potential divider', then feed the new (halved) voltage into an analog pin - and then you can use some code a bit like you posted (even in JS) to work out how much battery is left