It's not hard really (especially if you're running or have access to a Linux machine) - details here: https://github.com/espruino/Espruino/blob/master/README_Building.md
There are plenty of threads here on tweaking the build for different things as well.
@Gordon started
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 not hard really (especially if you're running or have access to a Linux machine) - details here: https://github.com/espruino/Espruino/blob/master/README_Building.md
There are plenty of threads here on tweaking the build for different things as well.