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

Mark S. Miller erights at
Wed Jul 1 13:15:51 UTC 2020

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
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