Suggestion: Proxy.[[GetOwnProperty]] caching non-configurable, non-writable data descriptors?

Caridy Patiño caridy at
Thu Oct 5 16:11:18 UTC 2017

We had some preliminary discussions around proxies, and how to potentially fix some mistakes from the past, some details here: <>

But this is very tricky, specially because Proxies were designed with the idea that the handler could also be a Proxy, that’s why we do all the gymnastics in the spec.

V8 has started some effort to improve proxies: <>

My suggestion is to wait and see how far they can go, and we can attempt to change the spec if there is anything that prevent them from achieving a great deal of improvements based on the current spec.


> On Oct 5, 2017, at 5:50 AM, Alex Vincent <ajvincent at> wrote:
> Proxies are a little slow right now, and maybe the rules of ECMAScript can safely allow for caching a non-configurable, non-writable data descriptor.
> Specifically, in ValidateAndApplyPropertyDescriptor from ES8 (section, step 7a-iii), we've already shown current and desc both have the following traits:
> isDataDescriptor(current) and isDataDescriptor(desc) return true
> [[Configurable]] is false
> [[Writable]] is false
> SameValue(Desc.[[Value]], current.[[Value]]) is true
> The [[GetOwnProperty]] or [[DefineOwnProperty]] internal method is executing, indirectly calling ValidateAndApplyPropertyDescriptor for this case.
> The [[Get]] or [[Set]] internal methods is probably executing, calling [[GetOwnProperty]] or [[DefineOwnProperty]] for this case.
> That is a very specific set of traits.  As I understand it, it means the proxy target's property named P in Proxy.[[GetOwnProperty]] is permanently locked and can never change - and so neither can the proxy ever report a different value.  Therefore, calling on the proxy handler for that property a second time is, at least in my view, either redundant or unnecessarily expensive.
> I would suggest that implementers in section 9.5 (Proxy internals) optionally have a private Map where values that are non-configurable and non-writable are stored.  Then [[GetOwnProperty]] could insert a couple of new steps into its algorithm.  Between steps 4 and 5 of the current algorithm, I would add:  "If the optional [[Map]] object exists and [[Map]].has(P) returns true, return [[Map]].get(P)."  In step 17 of the current algorithm, I would add a substep:  "If isDataDescriptor(resultDesc) and resultDesc.[[Writable]] is false and the optional [[Map]] object exists, call [[Map]].set(P, resultDesc).  (If the [[Map]] internal slot exists but does not contain a Map, the [[Map]] may be filled with a Map at this time.)"
> I would also suggest similar changes for [[DefineOwnProperty]]. (Section 9.5.6)
> Alternatively, if storing a specific descriptor in the Map is unpalatable, the specification could request storing resultDesc.[[Value]] in step 17, and create a new non-configurable, non-writable descriptor before the current step 5.
> Also, if a Map is too expensive, the spec could allow for an equivalent native map<string, JSValue> or whatever other appropriate data structure fills the need.
> Counter-point (1):  if the intent of the proxy is to treat all property look-ups equally, then caching the non-writable, non-configurable descriptors goes against that intent - because now the trap is invoked only once for properties returning this type of descriptor.  But there's a lot of steps, including invoking a custom proxy handler trap with an unknown number of steps, and a lot of complexity in that trap to ensure what it returns will pass all the assertions in the spec.
> Counter-point (2):  I don't know how common it is to have a property descriptor that meets all six criteria at the beginning of this post, in particular that both [[Configurable]] and [[Writable]] are false.  So this does add another pointer, at least to null initially, for the [[Map]] slot.  This is why I am proposing the [[Map]] slot as optional for implementers.  Let the engines decide whether it's warranted or not as an optimization.
> Thoughts?
> Alex Vincent
> Hayward, CA, USA
> -- 
> "The first step in confirming there is a bug in someone else's work is confirming there are no bugs in your own."
> -- Alexander J. Vincent, June 30, 2001
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