I guess for this to work perfectly one would need to do the impossible, because whether an arbitrary function will read or not a value from its closure could depend on an arbitrary computation, I guess it makes sense to just not try to be clever here 🤔 Interesting finding.

That isn't possible. Some of the examples in the post show this is happening.

So... basically it's all or nothing. Scope with all referenced variables is retained and shared by all callbacks even though particular function doesn't use it. And it is GC'ed only after references to all functions from that scope are lost.

Nice explanation, specifically about something in scope being callable. Did you know about these? github.com/naugtur/js-traini… github.com/naugtur/js-traini…

I didn't. Is there an explanation somewhere of why this happens? Has an issue been filed?

Thanks for sharing. That also breaks my intuition. Seems like an avoidable oversight in V8. Do you see it in other engines?

I think it's the same in other engines, but it's worth checking

Isn't this expected tho? Whenever there is a closure, the variables they might need from parent function's scope is retained. Returned callback in this case needs "id" variable, which is returned from a timeout that references to the large array buffer.

The setTimeout call cannot be statically analyzed, so it needs to stay in case setTimeout will call that function any number of times. It is possible that someone could override the setTimeout global too, and if they did, it may call the callback any time.

That's solved at runtime. setTimeout releases its reference to the function when it no longer wants to call it. This isn't browser magic, userland code can do exactly the same thing.

Would it be possible to detect this risk of leakage with static analysis? I wonder if there is a eslint rule somewhere for that?

What tooling do you use to track collection? That’s always been a weird thing for me to measure

youtu.be/6Ixyltr8_R0?si=Rxk7… looks like a good explanation of why

is there a way to capture a variable by value so we don’t need to capture the closure context ?

What does "by value" mean for an object?

Are you demonstrating that even though the timeout has ran, the buffer isn't collected? And the only way to collect it is to explicitly clearTimeout()?

No. See the final example of the article.