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).

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 

Answer 2

object.hasOwnProperty(name) only returns true for objects that are in the same object, and false for everything else, including properties in the prototype.

function x() {   this.another=undefined; };  x.prototype.something=1; x.prototype.nothing=undefined; y = new x;  y.hasOwnProperty("something"); //false y.hasOwnProperty("nothing"); //false y.hasOwnProperty("another"); //true  "someting" in y; //true "another" in y; //true 

Additionally the only way do delete a property is to use delete. Setting it to undefined do NOT delete it.

The proper way to do it is to use in like roborg said.

Update: undefined is a primitive value, see ECMAScript Language Specification section 4.3.2 and 4.3.9.