JavaScript: undefined !== undefined?

Question

NOTE: As per ECMAScript5.1, section 15.1.1.3, window.undefined is read-only.

• Modern browsers implement this correctly. for example: Safari 5.1, Firefox 7, Chrome 20, etc.
• Undefined is still changeable in: Chrome 14, ...

When I recently integrated Facebook Connect with Tersus, I initially received the error messages Invalid Enumeration Value and Handler already exists when trying to call Facebook API functions.

It turned out that the cause of the problem was

object.x === undefined 

returning false when there is no property 'x' in 'object'.

I worked around the problem by replacing strict equality with regular equality in two Facebook functions:

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

This made things work for me, but seems to hint at some sort of collision between Facebook's JavaScript code and my own.

What could cause this?

Hint: It is well documented that undefined == null while undefined !== null. This is not the issue here. The question is how comes we get undefined !== undefined.

Answer 1

The problem is that undefined compared to null using == gives true. The common check for undefined is therefore done like this:

typeof x == "undefined" 

this ensures the type of the variable is really undefined.