for in and for of do completely different things and there is no reason to expect consistency between them.

This was discussed already; search esdiscuss.org.

On Dec 22, 2014 8:37 AM, Glen Huang <curvedmark at gmail.com> wrote:
In that case we have a gotcha.

Is there any interest to change that behavior? Since es6 isn’t finial yet.

On Dec 22, 2014, at 9:29 PM, Gary Guo <nbdd0121 at hotmail.com<mailto:nbdd0121 at hotmail.com>> wrote:

On Mon, 22 Dec 2014 21:06:18 +0800, Glen Huang <curvedmark at gmail.com<mailto:curvedmark at gmail.com>> wrote:
>Ideally it shouldn’t, because its twin `for (var a in null) {}` won’t.
>
>But looking at step 8 in https://people.mozilla.org/~jorendorff/es6-draft.html#sec-runtime-semantics-forin-div-ofexpressionevaluation-abstract-operation, when passing `null` to `GetIterator()`, it will throw a type error when the result of step 2 `CheckIterable(null)`, which is `undefind`, is called in step 3 in the algorithm for `GetIterator()`.
>
>Did I miss something or the current spec does throw an error?

To me since `IsCallable(undefined)` is `false`, so it throws an `TypeError`. Traceur and V8 will both throw a `TypeError` saying that cannot read property @@iterator from undefined or null, and Firefox throws a `TypeError` saying that null have no property. So it should be an error.

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for in and for of do completely different things and there is no reason to expect consistency between them.

This was discussed already; search esdiscuss.org.