The `undefined` value is represented in exactly the same way as `true`, `false`, and `null` in V8. They're so called "oddballs" internally, global values that are neither allocated nor freed.
Either way, the key/values of a (regular) map do not keep anything alive about the map. Adding or removing entries from a set or map can of course cause allocation/deallocation, regardless of the keys/values themselves. (Similarly if you are using regular objects as maps, btw.) There are also lots of other things that cause allocation/deallocation under the hood of a JavaScript engine, e.g. compilation, optimisation, deoptimisation, etc. Some of that predominantly happens at start-up. If you want to reduce random noise from your experiments, try to warm up the code first. But even then, don't read too much into micro-benchmarks -- JS VMs are far too complicated, dynamic, and heuristics-based to draw useful conclusions from tiny tests (that's e.g. why JSPerf tests are often bollocks). In general, there is very little you can do in JavaScript that does not potentially cause allocation and thus GC. Certainly nothing involving dynamic data structures. /Andreas On 5 May 2016 at 07:48, /#!/JoePea <j...@trusktr.io> wrote: > > The only v8 shell I have lying around is too old (3.14.5.10) to have > Set, so I can't tell you what it would do. > > On my first attempt, I noticed 8 Major GCs: > https://cloud.githubusercontent.com/assets/297678/15036715/f41ea4d4-1247-11e6-8823-f153c3c1b7bb.png > > On second attempt, no Major GCs: > https://cloud.githubusercontent.com/assets/297678/15036788/c066483a-1248-11e6-970b-3f9d20710bbc.png > > I wonder why. I'm in Chrome 50. > > So, the second attempt looks good. I'm not sure why the first is so > different. I tried it a few times, but I only got that Major GC zig-zag the > first time. > > Thanks for pointing out Set! > > On Wed, May 4, 2016 at 7:30 PM, Boris Zbarsky <bzbar...@mit.edu> wrote: > >> On 5/4/16 5:03 PM, Steve Fink wrote: >> >>> The only v8 shell I have lying around is too old (3.14.5.10) to have >>> Set, so I can't tell you what it would do. >>> >> >> I have v8 "4.8.0 (candidate)" (meaning whatever rev I checked out), and >> it does 1163 minor ("Scavenge") GCs on your testcase. It also does 1163 >> minor GCs if I take out the add() calls. It does none if I remove the >> clear() calls, no matter whether the add() calls are there or not. >> >> -Boris >> _______________________________________________ >> es-discuss mailing list >> es-discuss@mozilla.org >> https://mail.mozilla.org/listinfo/es-discuss >> > > > _______________________________________________ > es-discuss mailing list > es-discuss@mozilla.org > https://mail.mozilla.org/listinfo/es-discuss > >
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