custom cross-realm brand checking

Richard Gibson richard.gibson at
Mon Oct 1 21:12:52 UTC 2018

One of the topics discussed in the final breakouts of last week's TC39
meeting was something like "self-hosted implementation of cross-realm
internal slots". I facetiously joked about using the global symbol registry
(which is shared across realms), but it was pointed out that such a
mechanism would not be private.

I've been thinking about it since, and I believe that the current surface
area of the language is sufficient for this to be possible without
introducing something like private symbols—specifically by using built-in
functions whose identity (regardless of originating realm) is verified with

There's a demonstration of using this technique to build custom
brand-sharing classes of the sort that could trust each other across
realms, and I'd love to get some feedback on it: . If this actually is as robust as it seems to
be, then it can also be a foundation for cross-realm access to custom
analogs of internal slots.
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