Ajax Performance

Going native

Looking back on everything I’ve written here in the past 18 months, one trend is clear: every performance optimization technique I’ve discussed has involved network tweaks or client side scripting. After all, a web developer can typically only control those aspects of the delivered application, so frequently we are looking to trick the browser into behaving a bit more intelligently than it would left to its own devices.

That’s why it’s refreshing to see what the browser vendors working to provide native, and dramatically faster, implementations of some foundational functionality in the Ajax space. Way back in March of 2007, John Resig posted about the native getElementsByClassName in Firefox 3, followed in December by native getElementsByClassName in WebKit. In both cases, the performance improvements over the best JS/DOM implementations were staggering.

More recent is the Safari implementation of the W3C Selectors API and its querySelector and querySelectorAll functions. Expect a similar implementation from Firefox in the near future, post 3.0. Conspicuously absent from the discussion is Microsoft, but perhaps we’ll see similar work in IE8.

In general, I think this is great progress for the community, and I like the idea of a formalized process for taking the most successful library functionality and moving it native. There is, however, one point of risk: these native implementations of complex functions will doubtless have the occasional bug, so we may end up in a situation where library vendors need to write messy code to fallback to the JS implementation. For example, if Safari 4.0.1 has a bug involving querying a specific type of CSS selector, every library would potentially need to check the browser version and analyze the input to querySelector. Enough of those checks, and the performance gains from taking the functions native begin to slip away.

Note: I regrettably missed the Firefox 3 getElementsByClassName in the first version of this post and have attempted to correct the story.