2013/9/21 Allen Wirfs-Brock <al...@wirfs-brock.com> > > On Sep 20, 2013, at 4:27 PM, Brandon Benvie wrote: > > On 9/20/2013 12:19 PM, Jason Orendorff wrote: > > I think .hasOwn() should be removed. > > > Actually, taking the precedent of removing "getPropertyDescriptor", it's > "has" that would be removed. The implementation of [[Has]] can easily walk > the [[GetPrototype]] chain calling "hasOwn" on each until true or > [[GetPrototype]] returns null. In fact, some agreed for a short time to > *remove* the "has" trap specifically for this reason, but it was added back. > > https://mail.mozilla.org/pipermail/es-discuss/2012-November/026274.html > > > [[Get]] and [[Set]] enable an exotic object to define its own property > lookup semantics that isn't necessary the same as a simple "follow the > [[Prototype]] chain until a matching own property is found". For example, > they could use some sort of multiple inheritance resolution algorithm. > [[HasProperty]] is needed to allow an external client to check for the > existence of a property using the same algorithm as [[Get]] and [[Set]]. >
Indeed, we decided in favor of keeping has() for the reasons Allen stated: so that proxies can virtualize the proto-chain-walk consistently for property access (get/set) as well as property lookup (in-operator), and enumeration (for-in loop). Put another way: with only the fundamental traps, all proto-chain-walking happens external to the proxy, so the proxy cannot virtualize it.
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