JavaScript: undefined !== undefined?


NOTE: As per ECMAScript5.1, section, 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.


It turns out that you can set window.undefined to whatever you want, and so get object.x !== undefined when object.x is the real undefined. In my case I inadvertently set undefined to null.

The easiest way to see this happen is:

window.undefined = null;
alert(window.xyzw === undefined); // shows false

Of course, this is not likely to happen. In my case the bug was a little more subtle, and was equivalent to the following scenario.

var n = window.someName; // someName expected to be set but is actually undefined
window[n]=null; // I thought I was clearing the old value but was actually changing window.undefined to null
alert(window.xyzw === undefined); // shows false


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