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From ES6 POW nothing missing. But from the embedding POW missed part is a real use cases of interop between native code and JS. In case of SpiderMonkey we can look into FireFox codebase, in case of V8 - into chromium codebase. In case of ChakraCore - EDGE is a closed source. And I even not sure EDGE use version of Chakra what published on github.
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I am not sure than ChakraCore is faster than SpiderMonkey.
In one my project I used a lot of float operation (3D rendering + design) in browser
Calculation in FireFox and chrome took the same time, but calculation in Edge took 2-3 times more time
I cannot say anything about object and properties(test needed)
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