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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
Are you talking about the MDBT42 breakout rather than a bare module? https://www.espruino.com/MDBT42Q
Just checking since the breakout will be fine off 5v as it has a regulator (as long as you use the Vin pin, not 3.3v!) https://www.espruino.com/MDBT42Q#pinout, but the module won't be, since it'll take only 3.6v maximum.
But as @allObjects says you just need some diodes in there. I'm not sure he was totally explicit about this but Schottky diodes are best for what you want since they lose smaller amounts of voltage (~0.3v vs ~0.7v for normal diodes) - while that doesn't matter for USB, the less voltage you can lose from the coin cell the better as it means more battery life.