object - JavaScript - Identify whether a property is defined and set to 'undefined', or undefined

Question

Say I have the following code:

function One() {}
One.prototype.x = undefined;

function Two() {}

var o = new One();
var t = new Two();

o.x and t.x will both evaluate to undefined. o.hasOwnProperty('x') and t.hasOwnProperty('x') will both return false; the same goes for propertyIsEnumerable. Two questions:

• Is there any way to tell that o.x is defined and set to undefined?
• Is there ever any reason to? (should the two be semantically equivalent?)

A small caveat: doing (for propName in o) loop will yield 'x' as one of the strings, while doing it in t will not - so there IS a difference in how they're represented internally (at least in Chrome).

question from:https://stackoverflow.com/questions/385960/javascript-identify-whether-a-property-is-defined-and-set-to-undefined-or-u

Answer 1

A slightly simpler way than your method is to use the Javascript in operator

alert('x' in o); // true
alert('x' in t); // false