Some Sitepoint Article: How not to Critique a Javascript Library

for (var i = fromIndex; i < arr.length; i++) {

This for loop looks up the .length property of the array (arr)
each time through the loop. Simply by setting a variable to
store this number at the start of the loop, you can make the
loop run much faster:

From six lines above, in array.js:

if (arr.indexOf) {
return arr.indexOf(obj, opt_fromIndex);
}
if (Array.indexOf) {
return Array.indexOf(arr, obj, opt_fromIndex);
}

It only falls through to there if Array already has no indexOf prototype method on it at all. In that case, performance on "for" loop is the least of your concerns, because you're using a terribly slow browser where it won't matter anyway. They used a cached length property later on in the very same file, in places where it might actually matter.

Google’s developers seem to have figured this trick out
later on in the same file. From array.js, line 153:
var l = arr.length; // must be fixed during loop... see docs
...
for (var i = l - 1; i >= 0; --i) {

This loop is better in that it avoids a property lookup each
time through the loop, but this particular for loop is so
simple that it could be further simplified into a while loop,
which will run much faster again:
var i = arr.length;
...
while (i--) {

On V8 and SquirrelFish Extreme, these two loops are exactly the same speed (and indeed, probably generate near-identical native code.) On FireFox and older Chrome/WebKit, the average difference on an array of 10,000 elements is something like the difference between 0.068ms and 0.072 ms to iterate (I don't remember the actual figure on my machine, it's negligible.) In V8, Google's own native iterators are implemented in JavaScript using that incrementing 'for' loop.

But not all of Closure Library’s performance woes are due
to poorly optimized loops. From dom.js, line 797:
switch (node.tagName) {
case goog.dom.TagName.APPLET:
case goog.dom.TagName.AREA:
case goog.dom.TagName.BR:
case goog.dom.TagName.COL:
case goog.dom.TagName.FRAME:
case goog.dom.TagName.HR:
case goog.dom.TagName.IMG:
case goog.dom.TagName.INPUT:
case goog.dom.TagName.IFRAME:
case goog.dom.TagName.ISINDEX:
case goog.dom.TagName.LINK:
case goog.dom.TagName.NOFRAMES:
case goog.dom.TagName.NOSCRIPT:
case goog.dom.TagName.META:
case goog.dom.TagName.OBJECT:
case goog.dom.TagName.PARAM:
case goog.dom.TagName.SCRIPT:
case goog.dom.TagName.STYLE:
return false;
}
return true;

This kind of code is actually pretty common in Java, and
will perform just fine there. In JavaScript, however,
this switch statement will perform like a dog each and
every time a developer checks if a particular HTML element
is allowed to have children.

Experienced JavaScript developers know that it’s much
quicker to create an object to encapsulate this logic:

var takesChildren = {}
takesChildren[goog.dom.TagName.APPLET] = 1;
takesChildren[goog.dom.TagName.AREA] = 1;
...
This code can be further bulletproofed against outside
interference using hasOwnProperty (see below for a full
explanation of this).

return !takesChildren.hasOwnProperty(node.tagName);

Really? You tested it? Because a 'switch' does not necessarily generate the same code as a dynamically constructed set of object properties. In Google's original example, if none of the cases match, it immediately returns 'true'. In Sitepoint's example, if the property does not match on the highest level, it must crawl the property chain on the object until it eventually reaches 'undefined'. Not to mention you are making two inner function calls and a fully dynamic property lookup (!(), hasOwnProperty, and node.tagName), none of which I believe can be calculated statically.

Actually, I'm going to end this rant early, because I just read more of the article, and it looks like the author just keeps pulling random shit that he remembers reading from "Javascript: The Good Parts" or whatever and vomiting it back up onto his keyboard.