On Sun, Jul 3, 2011 at 12:00 PM, Mark S. Miller <[email protected]> wrote: >> Here is a possible revised semantics for [[DefineOwnProperty]] on trapping >> proxies, and the Object.defineProperty built-in: >> [[DefineOwnProperty]] (P, Desc, Throw) >> When the [[DefineOwnProperty]] internal method of a trapping proxy O is >> called with property name P, property descriptor Desc and Boolean flag >> Throw, the following steps are taken: >> 1. Let descObj be FromGenericPropertyDescriptor(Desc) // see >> harmony:proxies_semantics page >> 2. Return DefineProxyProperty(O, P, descObj, Throw) >> DefineProxyProperty (Proxy, P, DescObj, Throw) >> When the DefineProxyProperty auxiliary function is called with a trapping >> proxy Proxy, a property name P, an Object DescObj and a Boolean flag Throw, >> the following steps are taken: >> 1. Let handler be the value of the [[Handler]] internal property of Proxy. >> 2. Let defineProperty be the result of calling the [[Get]] internal method >> of handler with argument “defineProperty”. >> 3. If defineProperty is undefined, throw a TypeError exception. >> 4. If IsCallable(defineProperty) is false, throw a TypeError exception. >> 5. Let trapResult be the result of calling the [[Call]] internal method of >> defineProperty providing handler as the this value, P as the first argument >> and DescObj as the second argument. > > This pass-through without protective copying is too hazardous. It means each > handler must individually protect itself, not only against accessor > behavior, but against the caller changing the shared DescObj after the fact.
I think if an "Object.toPropertyDescriptor" function were introduced, then this protection burden may be sufficiently small. This would be similar to the FixedHandler approach of providing *optional* builtin invariant maintenance support. The first argument to this function would be the object to convert to a property descriptor, and there could potentially be a second argument which could be a boolean as to whether to keep non-standard attributes. > >> >> 6. Let success be ToBoolean(trapResult) >> 7. If success is false, reject >> 8. Return true >> Object.defineProperty (O, P, Attributes) >> 1. If Type(O) is not Object throw a TypeError exception. >> 2. Let name be ToString(P). >> 3. If O is a trapping proxy >> a. Call DefineProxyProperty(O, name, Attributes, true) >> b. Return O. >> 4. Let desc be the result of ToPropertyDescriptor(Attributes) >> 5. Call the [[DefineOwnProperty]] internal method of O with arguments >> name, desc, and true. >> 6. Return O. Cheers, Sean Eagan _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
