Null iterable in for-of?

Allen Wirfs-Brock allen at
Thu Jun 12 15:05:33 PDT 2014

On Jun 12, 2014, at 2:36 PM, Erik Arvidsson wrote:

> Somehow I missed when we decided to allow null/undefined as the iterable value in for-of loops.
> The following test passes using the spec algorithms:
> var c = 0;
> for (var x of null) {
>   c++;
> }
> assert.equal(c, 0);
> However, if we get a null value here we are most likely just masking an user bug.
> I assume the justification is that for-in allows null here? However, for-of is new syntax and we have the chance to get this right this time around.

Yup, there was an issue that was reported and fixed fairly recently pointing out that for-of was inconsistent with for-in in this respect. 

I agree that treating null/undefined as an empty collection has a smell.  However, in this case my I agree with who ever it was who reported this. that consistancy between for-in and for-of is what we should have for this condition.

BTW, I believe this behavior for for-in was added in ES5.  My recollection was that Doug Crockford pushed for it.  I don't recall if it was because it matched web reality or simply because he thought it was a good idea. 


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