Is it possible to have undefined as a key when foreaching over a JavaScript object?

Question

I have been doing a code review and I see code like this:

var a = { ... }; // in reality, an actual object filled with key-value pairs
for (var key in a) {
    if (!angular.isUndefined(key)) { // does this ever fail?
        do.stuff();
    }
}

My question is whether key can ever be undefined (or evaluated to true per angular.isUndefined). It seems unlikely, as when I try to do the following:

var a = { undefined: 'hello' }

It turns out that undefined is actually a String.

Answer 1

My question is whether key can ever be undefined

No. All keys (property names) of an object are converted to strings.