Leon, are you working on this? Otherwise I might have some free cycles.

OK, Thanks Johan a lot for the help. Actually I tried to investigate around but have no clear idea yet.. Let me study how you would do, thanks ;-)

It would be great to have some examples of where this might be needed. I only find

https://cs.chromium.org/chromium/src/components/autofill/content/public/interfaces/autofill_types.mojom?l=196

and from a casual look at the code that uses this map a vector of pairs would be fine, because there's no lookup. I also suspect this example might not be the best one for using a map in Mojo as the key type is *huge* and probably really inefficient to hash, compare for equality, and copy.

Here is an example:
https://codesearch.chromium.org/chromium/src/ui/events/mojo/latency_info.mojom?rcl=0&l=106

This is now done for C++ and Java. Since JS maps uses object identity for keys the best we can do is support sending data as maps, but the only thing users can do with those maps is iterate over them.

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JS support is not practical, and C++/Java support was implemented.