Their commercial license is 25¢/unit (no minimum), and their FOSS license is LGPL for the runtime, GPL for the development tools, and Creative Commons for their example code.

For anyone interested in XS specifically or embedded software licensing in general, they do a good job of explaining their rationale here: https://www.moddable.com/license

It's MIT https://github.com/svaarala/duktape/blob/master/LICENSE.txt