Update - As of October 2011, some browsers (Safari 5.1, Firefox 7) no longer seem to let you change window.undefined, but some still do (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 and my own.
Any ideas 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
Source: Tips4all
The problem is that undefined compared to null using == gives true.
ReplyDeleteThe common check for undefined is therefore done like this:
typeof x == "undefined"
this ensures the type of the variable is really 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.
ReplyDeleteThe 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
That's a bad practice to use the == equality operator instead of ===.
ReplyDeleteundefined === undefined // true
null == undefined // true
null === undefined // false
The object.x === undefined should return true if x is unknown property.
In chapter Bad Parts of JavaScript: The Good Parts, Crockford writes the following:
If you attempt to extract a value from
an object, and if the object does not
have a member with that name, it
returns the undefined value instead.
In addition to undefined, JavaScript
has a similar value called null. They
are so similar that == thinks they are
equal. That confuses some programmers
into thinking that they are
interchangeable, leading to code like
value = myObject[name];
if (value == null) {
alert(name + ' not found.');
}
It is comparing the wrong value with
the wrong operator. This code works
because it contains two errors that
cancel each other out. That is a crazy
way to program. It is better written
like this:
value = myObject[name];
if (value === undefined) {
alert(name + ' not found.');
}
I'd like to note something here for beginners that want to learn about undefined
ReplyDeleteIf you run the following code
if( a !== undefined )
alert(a)
in situation where variable a HAS NEVER BEEN DEFINED (I mean there was no line var a; before),
you will get an ERROR EXCEPTION and your script will STOP RUNNING.
The only true way not to be thrown out of your script is to write
if( typeof a != 'undefined' )
alert(a)
I hope I was clear enough
From - JQuery_Core_Style_Guidelines
ReplyDeleteGlobal Variables:
typeof variable === "undefined"
Local Variables:
variable === undefined
Properties:
object.prop === undefined
A). I never have and never will trust any tool which purports to produce code without the user coding, which goes double where it's a graphical tool.
ReplyDeleteB). I've never had any problem with this with FB connect. It's all still plain old javascript running in a browser and undefined===undefined wherever you are.
In short, you need to provide evidence that your object.x really really was undefined and not null or otherwise, because I believe it is impossible for what you're describing to actually be the case - no offence :) - I'd put money on the problem existing in the Tersus code.
var a;
ReplyDeletetypeof a === 'undefined'; // true
a === undefined; // true
typeof a === typeof undefined; // true
typeof a === typeof sdfuwehflj; // true