Not sure this is a good idea or not, for one it'd be awesome for doing performance oriented and threaded code in JS/runtimes, the idea seems related to how C# s

Not sure this is a good idea or not, for one it'd be awesome for doing performance oriented and threaded code in JS/runtimes, the idea seems related to how C# struct's already work (and tuples under the hood). Interop with WASM code might also be simplified if struct-like access was a built-in. The bad is that people wouldn't necessarily be prepared for their semantics (are they value or reference based?), how to shared prototypes between environments (mentioned as problem in the proposal itself), not entirely sure if this proposal would add to the complexity vs security for spectre like attacks. It'd be useful, but worth it is another question? (And would all major players see interest in it? esp considering that it'd need to be "JSzero" level propsal if they go in that direction. (There was a post here a few days ago about layering runtimes with JS0 being the core with everything else being syntax transforms on top).