2012/12/13 David Bruant <[email protected]> > Le 13/12/2012 20:47, Jason Orendorff a écrit : > > > David: https://gist.github.com/4279162 > > I think this is what Kevin has in mind. Note in particular that the target > of the Proxy is just a dummy object, and the handler ignores it entirely. > The proxy uses it for invariant checks, but the intent is that those would > always pass. > > but they do not; try: > > var [p, setTarget] = retargetableProxy({}); // I love destructuring > sooo much! > Object.defineProperty(p, 'a', {configurable: false, value:31}); > setTarget({}); > Object.getOwnPropertyDescriptor(p, 'a'); // invariant check throws here > > Any variant that can be written will have the same issue. Even trickeries > with the defineProperty trap. > The proxy is enforcing invariants against the dummy [[target]]. The same > is to be expected from WindowProxy instances even if their underlying > window changes. It doesn't matter if the invariant is enforced on the dummy > target on an actual window instance. It is enforced and that's the > "problem" (with WindowProxy implemented as they are now not being emulable > with proxies) >
To clarify, there won't be any invariant violations if you ensure all three of the following conditions hold: a) the dummy target object never acquires any invariants (letting it be a dummy empty object and otherwise ignoring the target completely achieves that) b) handler traps that *query* the proxy never reveal any invariants, even if the "real" target currently pointed-to has invariants (i.e. getOwnPropertyDescriptor always changes the returned property descriptor's configurable attribute to true, isExtensible always returns false, etc.) c) handler traps that *update* the proxy refuse to commit (preventExtensions throws, defineProperty returns false when dealing with configurable:false properties, ...) It's a heavyweight way of going about things, but as Brandon mentioned, it's the price to pay for wanting to do weird things no normal ES5 object could ever do. The more a proxy's behavior deviates from that of an ES5 object, the uglier its implementation will be to "circumvent" the invariant checks.
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