Lazy evaluation

Andrea Giammarchi andrea.giammarchi at
Mon Sep 11 21:48:00 UTC 2017

Steve it's not solved in any other way. Even if you use a WeakMap with an
object, you gonna lazy attach properties to that object.

I honestly would like to see alternatives, if any, 'cause so far there is a
benchmark and it proves already lazy property assignment is around 4x

So, it's easy to say "it's not the best approach" but apparently hard to
prove that's the case?

Looking forward to see better alternatives.

On Mon, Sep 11, 2017 at 8:15 PM, Steve Fink <sphink at> wrote:

> On 9/11/17 5:36 AM, Matthew Robb wrote:
> > I think it's irrelevant if internally VMs are not too happy. VMs are
> there to solve our problems, not vice-versa ;-)
>> This ^​ is very important for everyone to get on board with. Regardless
> the cost should be negligible as the shape is only changing at the point of
> delayed init. This will cause, for example V8, to deop the object and have
> to build a new hidden class but only the one time. I guess it would
> potentially be interesting to support an own property that when undefined
> would delegate up the proto chain.
> (I don't know, but) I would expect it to be worse than this. The shape is
> changing at the point of delayed init, which means that if an engine is
> associating the possible set of shapes with the constructor (or some other
> form of allocation site + mandatory initialization), then that site will
> produce multiple shapes. All code using such objects, if they ever see both
> shapes, will have to handle them both. Even worse, if you have several of
> these delayed init properties and you end up lazily initializing them in
> different orders (which seems relatively easy to do), then the internal
> slot offsets will vary.
> You don't need to bend over backwards to make things easy for the VMs, but
> you don't want to be mean to them either. :-)
> Not to mention that the observable property iteration order will vary.
> On Mon, Sep 11, 2017 at 7:09 AM, Andrea Giammarchi <
> andrea.giammarchi at> wrote:
> Hi Peter.
>> Unless you have a faster way to do lazy property assignment, I think it's
>> irrelevant if internally VMs are not too happy. VMs are there to solve our
>> problems, not vice-versa ;-)
>> Regards
>> On Mon, Sep 11, 2017 at 11:54 AM, peter miller <fuchsia.groan at>
>> wrote:
>>> Hi Andrea,
>>> ```
>>>> class CaseLazy {
>>>>   get bar() {
>>>>     var value = Math.random();
>>>>     Object.defineProperty(this, 'bar', {value});
>>>>     return value;
>>>>   }
>>>> }
>>>> ```
>>> Doesn't this count as redefining the shape of the object? Or are the
>>> compilers fine with it?
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