JavaScript undefined undefined

Question

When I first tried to access Facebook API operations after integrating Facebook Connect with Tersus, I received the error messages Invalid Enumeration Value and Handler already exists.

It was discovered that the source of the problem was

object.x === undefined

When there is no property 'x' in 'object,' returns false.

I solved the problem by substituting normal equality for rigorous equality in two Facebook functions:

FB.Sys.isUndefined = function(o) { return o == undefined;};
FB.Sys.containsKey = function(d, key) { return d[key] != undefined;};

This got things to work for me, however, it appears to point to a clash between Facebook's JavaScript code and my own.

What may be causing this?

It is commonly established that undefined == null, whereas undefined!

== null. This isn't the problem at hand. The issue is, how did we become   undefined !  == undefinable