Thanks, Please file a bug on this at bug.ecmascript.org
AllenOn Jan 30, 2015 8:41 PM, Leon Arnott <[email protected]> wrote: > > Lately I've been puzzled about what the Proxy "enumerate" trap is supposed to > accomplish w/r/t enumeration over prototype properties. Consider the > following: > ``` > let obj = {"actual-property":1}; > > let proxy = new Proxy(obj, {*enumerate(){ > yield "falsified-property"; > }}); > ``` > The implication of the above is that all for-in loops run on the proxy will > always produce "falsified-property" and never "actual-property": > ``` > for (i in proxy) { > console.log(i); // Logs "falsified-property" > } > ``` > However, this proxy behaviour can be easily bypassed by just adding > Object.create(): > ``` > for (i in Object.create(proxy)) { > console.log(i); // Logs "actual-property" > } > ``` > When for-in is performed on an inheritor of a proxy, the proxy's "enumerate" > trap is never hit at all. This seems undesirable - I think it's intuitive > that since enumeration is a prototype-traversing operation, it should also > respect prototypes' traps. And, of course, that adding an empty ordinary > object to the head of the prototype chain shouldn't dramatically alter the > results of a for-in performed on that chain. > > As far as I know, this enumerate trap prototype behaviour is unique among > traps: the other three traps that explicitly deal with prototype properties - > "get", "set" and "has" - are specced to have their traps triggered by > operations performed on ordinary object inheritors of them. > > The reason for this is, of course, due to how Ordinary Object [[Enumerate]] > is currently specced, compared to Ordinary Object [[Get]], [[Set]] and > [[Has]]. [[Enumerate]] is required to process properties from prototypes of > the object, but is *not* required to call the prototype's [[Enumerate]] as > the means of accessing them. (In fact, the informative algorithm provided > calls the prototype's [[OwnPropertyKeys]] instead.) > > That being said, I want to ask: > * Is there a reason or requirement that enumerate traps only trigger when > for-in operations are performed on the proxy directly? > * Could Ordinary Object [[Enumerate]] be changed to, at the least, require > prototypes' [[Enumerate]] be used to retrieve their properties, rather than > any method that would ignore it? > > Thanks. > _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
