We can run our helloworld sample (https://github.com/Moddable-OpenSource/moddable/tree/public/...) with the xsbug debugger attached in 6000 bytes of RAM on the ESP8266. 5456 bytes for the stack and heaps, 256 for the dynamic symbol table, and 288 for the alias array.
Yes, absolutely. XS just provides the basic built-in JavaScript objects (Date, RegExp, String, Number, etc.). Everything else is up to you (meaning the person who creates the virtual machine). And memory allocation can be static or dynamic, so you can control how much memory is available.
With the 26KB RAM Duktape needs for "Hello world" we can run apps like the Sensor hub described in this blog post (run on a Thunderboard that only has 32KB of RAM total): http://blog.moddable.tech/blog/moddable-tech-presentation-to...