I guess you're using ESP8266 so I'm not sure if you're using a build that has it disabled or something, but on an official board with up to date firmware then it's just already there as soon as you type new Promise(...)
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.
It's built-in to all the new firmwares: http://www.espruino.com/Reference#PromisĀe
I guess you're using ESP8266 so I'm not sure if you're using a build that has it disabled or something, but on an official board with up to date firmware then it's just already there as soon as you type
new Promise(...)