last value from iterator

Domenic Denicola d at
Sun Apr 5 23:24:37 UTC 2015

I don't think it's good to think of the value sent with `done: true` as "one of the values." It's in a different category, and not part of the yielded sequence.
From: Mark Volkmann<mailto:r.mark.volkmann at>
Sent: ‎2015-‎04-‎05 18:43
To: Allen Wirfs-Brock<mailto:allen at>
Cc: es-discuss list<mailto:es-discuss at>
Subject: Re: last value from iterator

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<mailto:allen at>> 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.


R. Mark Volkmann
Object Computing, Inc.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the es-discuss mailing list