is an iterator allowed to reuse the same "state" object?

John Lenz concavelenz at
Tue Apr 28 23:21:07 UTC 2015

You would hope that the engines might be able to create these objects on
the stack but I don't think anyone does that yet and the result is a flood
of eden objects.

I would like to know I'm wrong about this.
On Apr 27, 2015 4:59 PM, "Allen Wirfs-Brock" <allen at> wrote:

> On Apr 27, 2015, at 3:29 PM, Tab Atkins Jr. wrote:
> On Mon, Apr 27, 2015 at 3:11 PM, John Lenz <concavelenz at> wrote:
> By which I mean the object that returns the current value and "done" state?
> IIRC, it's not supposed to.  The built-in iterators will return fresh
> objects each time, so there's no mutation hazard.  Userland iterators
> can of course violate this, but at their peril.
> Well, that's not exactly what the ES2015 spec. says.  The specification of
> the Iterator interface (
> )
> does not require that the `next` method return a fresh object each time it
> it called.  So a userland iterator would not be violating anything by
> reusing a result object.
> However,  the specifications for all ES2015  built-in iterators require
> that they return fresh objects.
> None of the built-in consumers of the Iterator interface (for-of,
> Array.from, etc.) retain references to IteratorResult objects after testing
> for `done` and accessing the `value`, so semantically they don't care
> whether the ResultObject is reused. However, such reuse might preclude some
> otherwise plausible engine level optimizations.
> Allen
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