It's fine to propose something to make maps immutable. Just don't call it Object.freeze.
On Thu, Apr 30, 2015 at 11:56 AM -0700, "C. Scott Ananian" <[email protected]<mailto:[email protected]>> wrote: Can you make an alternative proposal that still preserves the essential property of Object.freeze on collections -- that is to say, preserves object identity while preventing future writes? Here's another strawman: > Add "[[Immutable]]" to the arguments of OrdinaryCreateFromConstructor in step > 2 of 23.1.1.1 (Map) and 23.2.1.1 (Set) > Add "If the [[Immutable]] internal slot of M is true, throw a TypeError > exception" between steps 3 and 4 of 23.1.3.1 (Map.prototype.clear), 23.1.3.3 > (Map.prototype.delete), 23.1.3.9 (Map.prototype.set), 23.2.3.1 > (Set.prototype.add), 23.2.3.2 (Set.prototype.clear), and 23.2.3.4 > (Set.prototype.delete). > Add `Map.prototype.makeReadOnly()` and `Set.prototype.makeReadOnly()` methods > with the definition: > 1. Let M be the this value. > 2. If Type(M) is not Object, throw a TypeError exception > 3. If M does not have a [[Immutable]] internal slot, throw a TypeError > exception > 4. Set the [[Immutable]] internal slot of M to true. This is somewhat awkward and ad-hoc, but it won't break ES6 code. I am concerned about this issue in part because as written is seems to be impossible to prevent writes to Map. If you subclass it, freeze it, override set/delete/etc it is still possible to mutate the map using `Map.set.call(m, 'a', 'b');`. And there is no way for use code to get at the [[MapData]] slot to protect it. --scott On Thu, Apr 30, 2015 at 2:42 PM, C. Scott Ananian <[email protected]<mailto:[email protected]>> wrote: On Thu, Apr 30, 2015 at 2:22 PM, Mark S. Miller <[email protected]<mailto:[email protected]>> wrote: It would also not be compatible with ES6 code. SES will be freezing Map, Set, WeakMap, and WeakSet instances in order to tamper proof their API. I expect many others will as well. Having this freeze then cause a non-mutability in ES7 will break all such ES6 code. This is a non-starter all around. Couldn't SES use Object.seal/Object.preventExtensions/Object.defineProperty to perform tamper-proofing without flipping the "frozen" bit? --scott
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