deleting window properties

Gašper — Sun, 08 Nov 2009 14:58:48 +0000

Seems like there’s really something special about window and document objects in IE. Just as with iterating over window properties, there seems to be some weirdness about deleting the properties in window DOM object.

In IE, deleting a custom window property doesn’t work. The following code is fully ECMAScript compliant.

window.p = 'VAL';
delete window.p;

But it throws an exception in IE: Object doesn’t support this action. The same action works on custom objects:

var o = {};
o.p = 'VAL';
delete o.p;

Just like with iterating over properties, it seems as if IE treats window like a special object, and it’s custom properties as something out of reach for certain actions. So I’ve come up with a solution to delete properties that works on all tested browsers:

try {
delete obj.prop;
}
catch (e) {
obj.prop = undefined;
}

You can test this here. This issue is present even in IE8, which is supposed to be much better with standards, but this example clearly shows that some very basic behavior is still missing from their Javascript engine.

Iterating over properties in window object

Gašper — Sun, 06 Sep 2009 12:20:03 +0000

I’ve been playing around a bit with JavaScript, and found out an interesting thing concerning property iteration in the window object in IE. In some cases, properties that are set on the window object don’t show up in the iteration.

As you probably know, setting a variable outside a function (in the so called global scope) actually writes to the window object.

window.X = 123;
window['X'] = 123;
X = 123;
var X = 123;

All of these do exactly the same: set a property X on object window.

But, when it comes to iterating window properties in IE (surprise), there seems to be a difference between the first two assignments and the last two. The code for iterating is simple; it iterates through all of the window’s properties and alerts if X is found.

for (var v in window) {
if (v == 'X') {
alert(v + '=' + window[v]);
}
}

With the first two assignments (the ones that write explicitly to window), the property shows up, but with the last two (where properties are implicitly assigned to window), it doesn’t. It works in Firefox, Opera, Chrome, Safari, but not in IE (not even in IE8). You can try it out here.

Why this is so, I can only guess. The property obviously exists, because window['X'] and window.X always contain the correct value. One possibility is that iterating over object properties in IE doesn’t iterate over the properties that object actually owns, but rather over a separate collection of property names. This separate collection isn’t updated properly when variables are assigned to the window object implicitly. Another possibility is that such variables exist in a global closure, and as such aren’t true members of the window object. When you access window.X and if it doesn’t exist, it’s looked up in that closure.

Interesting if not life-changing stuff. Maybe I’ll do some more research on this.

wide thoughts » javascript

deleting window properties

Iterating over properties in window object