Thanks. Found that discussion, and after thinking about it again, throwing errors on null/undefined actually makes sense, because it’s impossible to sliently loop over null/undefined with regular for loop (if you check length directly) or forEach directly. So throwing an error is actually consistent with how things work in es3/5.
> On Dec 22, 2014, at 9:54 PM, Domenic Denicola <[email protected]> wrote: > > 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 <[email protected]> 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 <[email protected] > <mailto:[email protected]>> wrote: > > On Mon, 22 Dec 2014 21:06:18 +0800, Glen Huang <[email protected] > <mailto:[email protected]>> 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 > > > ><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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