Suggestion: Destructuring object initializer.

Isiah Meadows isiahmeadows at
Thu Feb 15 11:54:41 UTC 2018

There currently are *not*, and I can tell you that the suggested
optimization for `Object.assign` would be substantially more complex.
It's possible they may turn down the `Object.assign` optimization
until later, also - they *don't* perform optimizations based on the
way a function is called beyond argument count/type currently, which
also prevents them from eliding the return value allocation in the
common case of `splice`-to-remove-from-arrays.


Isiah Meadows
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On Tue, Feb 13, 2018 at 10:12 PM, Bob Myers <rtm at> wrote:
> Thanks for engaging.
> This entire little sub-thread, and my question about what nuance I was
> missing, was related to one single item on your useful list of reasons why
> property spread was a good idea, namely your assertion that property spread
> permits engine optimizations which are different, or better, or easier, than
> those possible with `Object.assign`.
> I asked about this specifically because I had been under the impression that
> `{...a, ...b}` is semantically equivalent *in every regard* to
> `Object.assign(a, b)`. Then I was hoping that the issue you posted to the V8
> issue tracker would cast light on the difference, but could not see anything
> there that helped me understand it either. On the contrary, in your post as
> I read it you seem to be assuming/implying that the two cases are equivalent
> in terms of opportunities for optimization.
> I am perfectly willing to consider reasons for bringing in property spreads
> such as "parity" or "brevity". It's a separate discussion as to whether or
> not those other criteria do or do not apply, or to what degree they might
> apply, to property picking. First, I just want to understand the point about
> engine optimization, because if that is actually the case it would seem to
> be a very compelling argument. The simple version of the question is, are
> there or are there not engine optimizations specifically made possibly by
> property spread syntax which would not be possible with `Object.assign`, and
> if so what are they?
> Bob
> On Wed, Feb 14, 2018 at 12:44 AM, Isiah Meadows <isiahmeadows at>
> wrote:
>> The nuance is that it is far more concise, making it more useful for
>> frequent immutable updates, and it brings consistency with array spread (it
>> was briefly considered to make a symbol for customizing object spread, but
>> it was ultimately rejected for reasons I can't remember). Also, the
>> optimizations would be much less speculative (you could perform them at the
>> baseline level for object spread with some effort, unlike with
>> `Object.assign`).
>> On Tue, Feb 13, 2018, 07:26 Bob Myers <rtm at> wrote:
>>> Cool. I don't claim to fully understand this, but as I read your issue,
>>> it seems the optimization could/would apply to either spread properties OR
>>> `Object.assign` case. If that's true, then there's nothing specially
>>> optimizable about spread properties, in which case that particular point
>>> would NOT have been a reason to support its adoption. Or is there some
>>> nuance I'm missing?
>>> On Tue, Feb 13, 2018 at 4:58 PM, Isiah Meadows <isiahmeadows at>
>>> wrote:
>>>> BTW, regarding engine optimizability of those, I've filed a couple V8
>>>> bugs:
>>>> - (object spread
>>>> + `Object.assign`)
>>>> - (array spread +
>>>> `Array.prototype.concat`)
>>>> There are things engines *could* do that they *aren't currently
>>>> doing*. Part of why I proposed V8 take a look at this is because it
>>>> has one of the more flexible IC systems out of all the engines (they
>>>> can lower `` to a simple loop for dense arrays even
>>>> though a simple `delete array[index]` within the loop breaks the
>>>> assumption - this is *exceptionally* difficult to implement with the
>>>> ability to deoptimize).
>>>> -----

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