On Feb 26, 2006, at 1:04 PM, Cameron McCormack wrote:

It's not that simple. Selectors for stylesheets are matched by examining the element when applying style, and testing what rules match. This API would require finding the elements that match a selector. The naiive way to do this is to walk the whole document and test each element for matching. But this is going to be way slower than, say, getElementById which operates in O(1) time in most browsers.

Both live lists and non-live lists have issues with being implementable efficiently. For a static list, you have to compute the whole list right away to make sure it remains static in teh face of changes. That means that even if you want only the first match you have to walk the whole document. This means that probably you want a selectSingleElement() in addition to selectElements().

Live lists have the problem that any document change forces you to recompute the list, so iterating the list while modifying the document often has O(N^2) behavior. For a subset of selectors it may be possible to avoid this through caching, and detecting exactly what changes make a difference, and putting in lots of notifications. But I think for at least some selectors this is not feasible.

I also think live lists are a confusing programming model. If you really are modifying the document while traversing the list, it is likely more annoying than helpful for the list to change. For example:

// severely buggy function removeAllDivs() { var divs = document.getElementsByTagName("divs"); for (var i = 0; i < divs.length; i++) { var div = divs[i]; div.parentNode.removeChild(div); } }

This will not remove all divs. Could you even say without thinking about it which divs it will remove?

It seems to me that, when modifying the document, you generally do not want a live list, and when not modifying the document, it doesn't matter which kind you have. Therefore I would recommend against propagating the "live list" concept in new APIs.

So, in brief: - Having an existing style system does not make it trivial to implement a DOM API to find elements by selector. - Static lists are probably more efficiently implementable than live lists. - Static lists would require a single-element version too, so you don't have to build the whole list just to get one element. - Live lists are confusing as a programming model. - Live lists suck for performance and make it easy to accidentally write O(N^2) code.

Regards, Maciej