It's only valid as a keyword within the `function*` block, so the parser can treat it as an identifier elsewhere. I haven't read the detailed spec, so I'm not sure that this is the standard behavior, but this would be a way to preserve complete backward compatability.

Yes, that’s how it works. `yield` is only reserved in ES5 strict mode. But due to it only being a keyword inside generators, there are no problems in non-strict mode.