Behavior of for-in on null or undefined

[GeorgNeis]

What should happen when trying to enumerate (for-in) null or undefined? If I understand the spec correctly, currently such a for-in statement is equivalent to a break, which I find very strange:

http://tc39.github.io/ecma262/#sec-runtime-semantics-forin-div-ofheadevaluation-tdznames-expr-iterationkind

6. If iterationKind is enumerate, then
  a. If exprValue.[[value]] is null or undefined, then
    i. Return Completion{[[type]]: break, [[value]]: empty, [[target]]: empty}.

(I would have expected that this returns a normal completion record with the undefined value.)

[bterlson]

It does eventually return an undefined value with a normal completion. The break completion value is handled uniformly in 13.1.7. I'm not sure if the break semantics are required for some reason but it does just work with the existing machinery so that's a plus. You could imagine (I think) checking the result of evaluating the head in each block of 13.7.5.11, though I'm not inclined to change this without good reason.

[GeorgNeis]

Thank you, I missed that.