There are actually a bunch of 'small' (for various definitions of small) JS interpreters that are just the interpreter if you wanted to use them: Duktape, JerryScript, TinyJS (which isn't really 'proper' JS), v7, and probably others.
What makes Espruino more useful is that it isn't just the interpreter though - you get the whole API to access the hardware, and for that to really work it does have to have access to the whole device.
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.
There are actually a bunch of 'small' (for various definitions of small) JS interpreters that are just the interpreter if you wanted to use them: Duktape, JerryScript, TinyJS (which isn't really 'proper' JS), v7, and probably others.
What makes Espruino more useful is that it isn't just the interpreter though - you get the whole API to access the hardware, and for that to really work it does have to have access to the whole device.