for-of loops, IteratorClose and the rest of the iterations in the spec

Boris Zbarsky bzbarsky at
Wed Sep 10 10:58:09 PDT 2014

On 9/10/14, 1:47 PM, Allen Wirfs-Brock wrote:
>> 1. Let method be the result of CheckIterable(V).
>> 2. ReturnIfAbrupt(method).
>> 3. If IsCallable(method) is false, go off and do something else,
>>    since V is not iterable.
>> 4. Let iter be GetIterator(V, method).
>> 5. ReturnIfAbrupt(iter).
>> and then IteratorStep my way through "iter".
> So, try expressing this in JS. That's what a JS help-hosting compliant implementation is going to have to do.


   var method = V[Symbol.iterator];
   if (typeof method != "function") {
     // Not iterable; do something else
   var iter =;

and then go through calling .next() and so on.

Now in this self-hosted case, how do I express IteratorClose?  Seems to 
me like it's just a matter of wrapping the relevant bits (which ones, 
though?) in a try/catch and doing this in the catch clause:

   catch (e) {
     if ("return" in iter) {
     throw e;

That ought to match the semantics of IteratorClose, yes?


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