Javascript – Uncaught TypeError: Cannot read property ‘msie’ of undefined – jQuery tools

The usual way to check if the value of a property is the special value undefined, is:

if(o.myProperty === undefined) {
  alert("myProperty value is the special value `undefined`");
}

To check if an object does not actually have such a property, and will therefore return undefined by default when you try and access it:

if(!o.hasOwnProperty('myProperty')) {
  alert("myProperty does not exist");
}

To check if the value associated with an identifier is the special value undefined, or if that identifier has not been declared. Note: this method is the only way of referring to an undeclared (note: different from having a value of undefined) identifier without an early error:

if(typeof myVariable === 'undefined') {
  alert('myVariable is either the special value `undefined`, or it has not been declared');
}

In versions of JavaScript prior to ECMAScript 5, the property named "undefined" on the global object was writeable, and therefore a simple check foo === undefined might behave unexpectedly if it had accidentally been redefined. In modern JavaScript, the property is read-only.

However, in modern JavaScript, "undefined" is not a keyword, and so variables inside functions can be named "undefined" and shadow the global property.

If you are worried about this (unlikely) edge case, you can use the void operator to get at the special undefined value itself:

if(myVariable === void 0) {
  alert("myVariable is the special value `undefined`");
}

What's going on!?

"jQuery probably copies those properties into the jQuery object." You're exactly correct, so it sounds like you already know! :)

Hopefully jQuery will update their code to stop touching that, but at the same time WebKit should have known better than to log a deprecation warning on an event (at least in my opinion). One mousemove handler and your console explodes. :)

Here's a recent jQuery ticket: http://bugs.jquery.com/ticket/10531

UPDATE: This is fixed now if you upgrade to jQuery 1.7.

Please note that if upgrading jQuery doesn't fix the issue for you it may have something to do with used extensions / plugins as Jake stated in his answer.