Thursday, January 10, 2008

Speeding up Javascript in one quick move

If you don't want to read the whole thing and just go to the solution, click here.

I reached a stage in an ASP.Net project where a I needed to make some pages work faster. I used dotTrace to profile the speed of each form and I optimized the C# and SQL code as much as I could. Some pages still were very slow.

Now, I had the idea to look for Javascript profilers. Good idea, bad offer. You either end up with a makeshift implementation that hurts more than it helps, or with something commercial that you don't even like. FireFox has a few free options like FireBug or Venkman, but I didn't even like them and then the pages I was talking about were performing badly in Internet Explorer, not FireFox.

That got me thinking of the time when Firefox managed to quickly select all the items in a <select> element, while on Internet Explorer it scrolled to each item when selecting it, slowing the process tremendously. I then solved that issue by setting the select style.display to none, selecting all the items, then restoring the display. It worked instantly.

Can you guess where I am going with this?

Most ASP.Net applications have a MasterPage now. Even most other types of sites employ a template for all the pages in a web application, with the changing page content set in a div or some other container. My solution is simple and easy to apply to the entire project:

Step 1. Set the style.display for the page content container to "none".
Step 2. Add a function to the window.onload event to restore the style.display.

Now what will happen is that the content will be displayed in the hidden div, all javascript functions that create, move, change elements in the content will work really fast, as Internet Explorer will not refresh the visual content in the middle of the execution, then show the hidden div.

A more elegant solution would have been to disable the visual refresh of the element while the changes are taking place, then enable it again, but I don't think one can do that in Javascript.

This fix can be applied to pages in FireFox as well, although I don't know if it speeds anything significantly. The overall effect will be like the one in Internet Explorer table display. You will see the page appear suddenly, rather than see each row appear while the table is loaded. This might be nice or not nice, depending on personal taste.

Another cool idea would be to hide the div and replace it with a "Page loading" div or image. That would look even cooler.

Here is the code for the restoration of display. In my own project I just set the div to style="display:none", although it might be more elegant to also hide it using Javascript for the off chance that someone might view the site in lynx or has Javascript disabled.
function addEvent(elm, evType, fn, useCapture)
// addEvent and removeEvent
// cross-browser event handling for IE5+, NS6 and Mozilla
// By Scott Andrew
if (elm.addEventListener){
elm.addEventListener(evType, fn, useCapture);
return true;
} else if (elm.attachEvent){
var r = elm.attachEvent("on"+evType, fn);
return r;
} else {
alert("Handler could not be removed");

function initMasterPage() {