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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
As @allObjects says, it'll be the amount of power you can draw from coin cells. To be honest sometimes they can even struggle to power an Espruino board (when it isn't sleeping) let alone a relay as well.
You could look at using 4 AA cells - while a bit bigger, they'll last for years with deep sleep enabled, and will easily provide enough power for the relay.