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

Allen Wirfs-Brock allen at
Mon Apr 27 23:59:14 UTC 2015

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.  


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