Ah, interesting to know. Is such an optimization guaranteed? */#!/*JoePea
On Tue, Aug 2, 2016 at 1:59 PM, Jason Orendorff <[email protected]> wrote: > On Tue, Aug 2, 2016 at 3:09 PM, Bergi <[email protected]> wrote: > >> Why would `tmp` be stored as the [[HomeObject]] when the function doesn't >> use `super`? In that case a [[HomeObject]] is not needed at all. >> > > To clarify this: of course the JS engine knows whether the code uses > `super` or not and can decide to retain the [[HomeObject]] only when > there's some danger of its actually being needed. But do implementations > actually do this optimization in practice? > > SpiderMonkey has a findPath() primitive that searches the GC heap for > paths from one object to another, handy for answering questions like this > one: > > js> function SomeFactory(name) { > let tmp = { > [name]() { /* this doesn't use `super` */ } > } > return findPath(tmp[name], tmp); > } > js> SomeFactory("foo") === undefined > true > > That is, there is no GC-path from the method back to the object; > apparently [[HomeObject]] is not being retained. What if we change the > method to use `super`? > > js> function SomeFactory2(name) { > let tmp = { > [name]() { return super[name](); } > }; > return findPath(tmp[name], tmp); > } > js> typeof SomeFactory2("foo") > "object" > js> SomeFactory2("foo").length > 1 > > Now there is a direct GC-reference from the method to its home object, as > expected. > > -j > > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss > >
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