The only iterating mechanism that lets you access the “`done` value” is `yield*`. All other constructs (`for-of`, spread, destructuring, …) completely ignore it. Its main purpose is to let `yield*` make recursive generator calls with results, without disrupting normal yielding. That is, for normal iteration it is an out-of-band value.

Axel


> On 06 Apr 2015, at 00:43, Mark Volkmann <r.mark.volkmann at gmail.com> wrote:
> 
> Thanks, that helps. I had been thinking that for-of was a model for how iterators should be consumed.
> 
> When implementing any kind of iterator (generator or not), it seems important to recognize that if you choose to return a value with done set to true, you are precluding the use of for-of with that iterator (assuming you want all the values).
> 
> On Sun, Apr 5, 2015 at 11:40 AM, Allen Wirfs-Brock <allen at wirfs-brock.com <mailto:allen at wirfs-brock.com>> wrote:
> 
> On Apr 5, 2015, at 6:04 AM, Mark Volkmann wrote:
> 
>> I thought that when an iterator returns done: true, the value should not be used.
> 
> Why do you think that?  The definition of the IteratorResult interface in the the ES6 spec. doesn't say that.
> 
>> However, if a generator function ends by returning a value, done will be true when that value is returned and the value should be used.
> 
> Could be used.  Whether a generator or any Iterator provides a meaningful value when it reach the the 'done' state depends upon its specific definition.
> 
>> Given this, how can a consumer know the correct way to handle the value when done is true? Clearly consumers shouldn't have to be aware of whether the iterator is actually a generator and whether it ends by returning a value.
> 
> An Iterator does not have to be a generator to provide a 'done' state value.  In general, If you are going to do anything other than basic iteration, such as is performed by for-of, you need to know about the specific Iterator you are using. 
> 
> Allen
> 
> 
> 
> 
> 
> -- 
> R. Mark Volkmann
> Object Computing, Inc.
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-- 
Dr. Axel Rauschmayer
axel at rauschma.de
rauschma.de



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The only iterating mechanism that lets you access the “`done` value” is `yield*`. All other constructs (`for-of`, spread, destructuring, …) completely ignore it. Its main purpose is to let `yield*` make recursive generator calls with results, without disrupting normal yielding. That is, for normal iteration it is an out-of-band value.