Are ES Modules garbage collected? If so, do they re-execute on next import?

Guy Bedford guybedford at
Thu Jul 16 13:23:06 UTC 2020

Node.js in the CommonJS loader and dynamic loaders like SystemJS have
supported module unloading for many years by permitting eviction from the
loader registry. Evicting the full parent tree was the traditional
reloading workflow in SystemJS, but live binding pushes are also permitted
in SystemJS as well for this.

I agree there are issues and invariants of course to consider from a
theoretical perspective, but those are decisions to be made in terms of
what invariants are valued, and I don't feel they are necessarily absolute
constraints. These decisions should be made based on what is best for
the JS users and engines. Not that I feel strongly this should be a
requirement but that it should still be open to consideration.

I'm not sure it was Allen's intention to ban any concept of reloading
modules when defining the idempotency requirement for the host resolve
function. Perhaps he could speak to that if he's around.

On Tue, 14 Jul 2020 at 23:05, Mark S. Miller <erights at> wrote:

> Only a module registry as a whole may be GCed. During the lifetime of any
> one module registry, it can only grow. No other solution is possible.
> Btw, I remember being surprised ages ago when the same issue came up for
> the Java ClassLoader. A classLoader holds on to all the classes it ever
> loaded. Each class holds onto its classLoader. Each instance holds on to
> its class. During the lifetime of a classLoader or any of its classes, the
> graph of that classLoader and its classes can only grow new classes. Not
> until the classLoader and all of its classes are unreachable at the same
> time can any of them be collected. This was equally unfortunate,
> surprising, and inescapable.
> On Tue, Jul 14, 2020 at 10:16 PM #!/JoePea <joe at> wrote:
>> How can we ensure that long-running applications (even if theoretical),
>> that may load and unload an unlimited number of new modules over time
>> (f.e. routes in a web page specified by 3rd parties as time
>> progresses), not leak memory?
>> Even if it is theoretical, I don't like the thought of something that
>> only ever allocates memory that will never be freed.
>> Is someone working on a solution for this?
>> #!/JoePea
>> On Wed, Jul 1, 2020 at 6:16 AM Mark S. Miller <erights at> wrote:
>> >
>> > No, definitely not. The table from specifiers to module instances is
>> indexed by specifiers. Specifiers are strings, so this table is not weak.
>> It is not a "cache" in the sense that it is allowed to drop things. Rather
>> it is a registry of module instances. Only a registry as a whole can be
>> gc'ed, and which point that context is no longer around for instantiating
>> or reinstantiating modules.
>> >
>> > As you suggest, if it could drop things because of GC that it would
>> then need to regenerate, that would expose the non-determinism of gc. That
>> would be a big deal. We carefully designed WeakMaps so that gc was
>> non-observable. WeakMaps introduce no observable non-determinism. WeakRefs
>> alone expose the non-determinism of gc, and are kept well quarantined from
>> the rest of the language.
>> >
>> >
>> > On Tue, Jun 30, 2020 at 5:42 PM #!/JoePea <joe at> wrote:
>> >>
>> >> I am curious: can modules be garbage collected if the exports are not
>> >> references by anything anymore? And if so, will the module be
>> >> re-evaluated the next time it is imported?
>> >>
>> >> I haven't tried an experiment to answer this yet. I'll be back to post
>> >> findings if someone doesn't post an official answer first.
>> >>
>> >> I'm thinking about code longevity. For example, if we make
>> >> long-running web-based applications with many routes and features (for
>> >> sake of example imagine a desktop environment, or a MMORPG game, with
>> >> apps or components that are loaded within the same context). Over
>> >> time, if imports are not collected, then it means we have a memory
>> >> leak.
>> >>
>> >> Imagine, for example, an infinite-universe MMORPG where you can land
>> >> on different planets where the code for features of a planet are
>> >> provided by third parties as ES Modules. I know, this might not be a
>> >> safe idea to import any code into an app, but just imagine it for sake
>> >> of example (imagine we have a continuous integration system to test
>> >> and verify code security, or something, before that code is allowed to
>> >> be consumed in the app). Imagine you play this app for many many days,
>> >> and visit many places, and you leave the app running the whole time
>> >> (because farming for resources is disabled if the app is not running,
>> >> or something).
>> >>
>> >> I would imagine that we want unused modules (when we leave a planet,
>> >> for example) to be (destroyed) garbage collected so that we don't
>> >> waste memory.
>> >>
>> >> #!/JoePea
>> >> _______________________________________________
>> >> es-discuss mailing list
>> >> es-discuss at
>> >>
>> >
>> >
>> >
>> > --
>> >   Cheers,
>> >   --MarkM
> --
>   Cheers,
>   --MarkM
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