On 5/14/16 3:11 AM, Andrea Giammarchi wrote:
1. why is that?
Why does mutating the proto after an object has been exposed to script
end up deoptimizing things? Because it invalidates assumptions JITs
otherwise make. So the options are to make the
proto-hasn't-been-mutated case slower by not making those assumptions or
to make the proto-has-been-mutated case slower. Guess which one is a
better choice?
There is a
spec'd method that is not even on Annex B and Firefox deliberately
discourage its usage.
Sure. Just because something specced doesn't mean it's a good idea to
actually do it.
This is why in the HTML spec there's all sorts of stuff that's marked as
"not valid HTML" for authoring purposes even though the spec then goes
ahead and defines what a browser should do with that stuff if authors do
it anyway.
Why I don't see warnings every time I
`[].slice.call(arguments)`?
Because that's not as big a performance hit?
I understand it might de-optimize but I
wonder if that's really always necessary (maybe it doesn't have to
deopt if it's a well known operation with a predictable result).
I'm not an expert on the type inference setup (which is what ends up
deoptimizing on proto mutation, iirc), so I can't usefully answer this.
On the other side, I also wish Firefox woudn't show warnings about
modern and recent specifications. Deprecated stuff is OK,
There can totally be things that are both recently added to the spec
(for UA implementation purposes, because everyone has to do it for web
compat) and deprecated for authoring purposes (because they're a bad
idea). Dynamic proto mutation is one of those. ;)
2. where were you when the `__proto__` landed on specs? :P
You mean when every browser on the market implemented it, which was the
relevant bit? The addition to the spec was just acknowledging ugly reality.
Where was I when browsers implemented __proto__? We're talking 20ish
years ago, so probably high school or a few years into college.
-Boris
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