Hi Gareth, mostly for historic reasons, for-in is one of the darkest corners of the ES spec. It is intentionally vague, because existing implementations differ widely in their handling of this construct.
To address your question: the spec simply does not prescribe anything coherent for this case. Have you tried different implementations? I assume they will differ, and consequently, there likely is little chance to tighten the spec for this case, nor to agree on a suitable test case for test262. It's long been acknowledged that for-in is beyond repair, which is why ES6 will have for-of as a more well-behaved replacement. Nevertheless, it's probably worth filing a bug against the spec (https://bugs.ecmascript.org/describecomponents.cgi?product=ECMA-262). It could at least make clear that nothing is clear for this case. :) And yes, I can see how that sucks for JSCert. As for your other question, yeah, I think that "property" and "property with name" are used interchangeably in those parts of the spec. /Andreas On 8 May 2013 20:16, Gareth Smith <[email protected]> wrote: > Hello, > > I hope this is the right place to ask this question - please let me know > if not! > > I am trying to understand the specification of for-in, both for ES5 and > for ES6, and in particular the interaction between shadowing and adding > or deleting properties. Here are the parts of the ES5 spec and ES6 draft > that seem most relevant: > > ES5: http://www.ecma-international.org/ecma-262/5.1/#sec-12.6.4 > ES6: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-8.3.12 > > Here are two motivating examples. In both examples, I'll assume that 'y' > is enumerated first. For each example, the question is: does the > specification require 'x' to be enumerated, or can an implementation > choose to skip it? > > Example 1: > > var b = { x : 2 } > var a = { y : 0, __proto__ : b } > > for (i in a){ > console.log(i + ": " + a[i]); > a.x = 1 > } > > In this first example b.x becomes shadowed by a.x before being > enumerated. Thus we have both "If new properties are added to the object > being enumerated during enumeration, the newly added properties are not > guaranteed to be visited in the active enumeration" and "a property of a > prototype is not enumerated if it is “shadowed” because some previous > object in the prototype chain has a property with the same name." > > Does the specification allow us to skip 'x'? > > Example 2: > > var b = { x : 2 } > var a = { y : 0, x : 1, __proto__ : b } > > for (i in a){ > console.log(i + ": " + a[i]); > delete(a.x) > } > > In this second example, a.x is deleted before it is visited. Thus, > following "If a property that has not yet been visited during > enumeration is deleted, then it will not be visited", it should not be > visited. The b.x property is no longer shadowed, but it was not supposed > to be enumerated in the first place. So can it be skipped? > > An alternative reading of the specification is that we are interested in > property /names/, not properties. Given this reading, in both examples a > property named 'x' was always reachable from a, so it must be enumerated > in both examples. Is this second reading closer to the intent of the > specification? I note that the current wording of the spec talks about > "properties" when things are being added and deleted, and "property > names" only when talking about duplicate enumerations. > > Thanks a lot! > > Gareth. > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
