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

I am reviewing the code 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, can it key

be undefined (or be evaluated as true

for angular.isUndefined

). This seems unlikely since when I try the following:

var a = { undefined: 'hello' }

      

        
        
        
      

    

It turns out that undefined

actually a String

.