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I would like to have a confirmation that cloning the "object properties" on Map and Set is really intentional. I don't think that is particularly useful behavior. In my first (non-specification based) implementation I didn't even consider that possibility. https://bugzilla.mozilla.org/show_bug.cgi?id=1036136
It's also a little surprising because the spec doesn't clone expando properties of Dates or Arrays. Always cloning expandos would at least be consistent.
The current text is more or less a placeholder. I expect eventually the cloning of native JS types to be defined in JS proper.
With my security hat on, I would vastly prefer to not clone expandos (and only do so for Object and Array). I can explain why if needed, but it sounds like nobody's really in disagreement here.
The current spec text is in terms of [[MapData]] and [[SetData]], which looks correct to me. Should this issue be marked as resolved?
Indeed.
The text was indeed wrong and is unfortunately still wrong. This is now https://github.com/whatwg/html/issues/754 and I hope to fix it soon.