Okay, so it's a V8 bug. Filed it here: https://bugs.chromium.org/p/v8/issues/detail?id=5834
On Thu, Jan 12, 2017, 03:03 T.J. Crowder <[email protected]> wrote: > So to sum up, then, and circle back to Francisco Tolmasky's original > question: > > * For ordinary objects, `Object.getOwnPropertyDescriptor` shouldn't > have side-effects because none of the ordinary operations it uses has > side effects. > * For exotic objects, it may well have side effects as a result of an > exotic version of [[GetOwnProperty]]; for instance, Adam Klein's > `Proxy` example. > * `Error` objects are specified as ordinary objects. > * V8's `Error` object has a `stack` property that claims to be a value > property (not an accessor). > * V8's `Error` object is a exotic object, it has exotic behavior for > [[GetOwnProperty]], because it triggers filling in the string for the > captured stack trace if you call it for `stack` (it has to, in order > to provide the `value` property of the descriptor, since `stack` > claims to be a value property). > * This aspect of V8's `Error` could be in-spec by making `stack` an > accessor instead (or by building the string earlier, but it's deferred > for performance reasons). > > Is that a reasonable summary? > > Additionally, I believe the only exotic object defined by the > specification that has a [[GetOwnProperty]] with potential side > effects is `Proxy`. > > Provided that's all correct, Francisco's answer is: Per spec, you > can't rely on `Object.getOwnPropertyDescriptor` not having side > effects unless you can guarantee you're not dealing with a `Proxy`. > Per spec, you could for non-`Proxy` objects defined by the > specification, but that's not currently the case with V8 (at least). > And there's always the possibility of host objects having exotic > [[GetOwnProperty]] behavior. > > -- T.J. > _______________________________________________ > 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
