Well, when I say solved I mean solved for the two most important browsers - Internet Explorer and Mozilla/Firefox. Still that’s good enough isn’t it?

First, let me define the problem. The window.onload event is used by programmers to kick-start their web applications. This could be something trivial like animating a menu or something complex like initialising a mail application. The problem is that the onload event fires after all page content has loaded (including images and other binary content). If your page includes lots of images then you may see a noticeable lag before the page becomes active. What we want is a way to determine when the DOM has fully loaded without waiting for all those pesky images to load also.

Mozilla provides an (undocumented) event tailor-made for this: DOMContentLoaded. The following code will do exactly what we want on Mozilla platforms:

// for Mozilla browsers

if (document.addEventListener) {
	document.addEventListener("DOMContentLoaded", init, null);
}

So what about Internet Explorer?

IE supports a very handy (but non-standard) attribute for the <script> tag: defer. The presence of this attribute will instruct IE to defer the loading of a script until after the DOM has loaded. This only works for external scripts however. Another important thing to note is that this attribute cannot be set using script. That means you cannot create a script using DOM methods and set the defer attribute - it will be ignored.

Using the handy defer attribute we can create a mini-script that calls our onload handler:

<script defer src="ie_onload.js" type="text/javascript"></script>

The contents of this external script would be a single line of code to call our onload event handler:

init();

There is a small problem with this approach. Other browsers will ignore the defer attribute and load the script immediately. There are several ways round this. My preferred method is to use conditional comments to hide the deferred script from other browsers:

<!--[if IE]><script defer src="ie_onload.js"></script><![endif]-->

IE also supports conditional compilation. The following code is the JavaScript equivalent of the above HTML:

// for Internet Explorer

/*@cc_on @*/
/*@if (@_win32)
	document.write("<script defer src=ie_onload.js><"+"/script>");
/*@end @*/

So far so good? We now need to support the remaining browsers. We have only one choice - the standard window.onload event:

// for other browsers

window.onload = init;

There is one remaining problem (who said this would be easy?). Because we are trapping the onload event for the remaining browsers we will be calling the init function twice for IE and Mozilla. To get around this we should flag the function so that it is executed only once. So our init method will look something like this:

function init() {
	// quit if this function has already been called
	if (arguments.callee.done) return;

	// flag this function so we don‘t do the same thing twice
	arguments.callee.done = true;

	// do stuff
};

And that, as they say, is that.

I’ve provided a sample page that demonstrates this technique. Start copying.