For reasons relating to dynamic loading of javascript dependencies, I wanted to find a way to tell:

1. When a script tag finishes loading a file
2. Whether that file was successfully loaded

For various reasons, I didn’t want to add cruft into the files being loaded — no appending a function call to the end of every file, or anything.

Now, the finishes-loading case turns out to be pretty easy, albeit with some quirky cross-browser ramifications:

var eax, load;
eax = document.createElement('script');
eax.setAttribute('type', 'text/javascript');
eax.setAttribute('src', src);
load = function(e) {
    // FF/Safari get the `load` handler, IE gets the `readystatechange` handler.
    // IE doesn't fire 'load' for JS (but does for CSS...)
    if(!eax.readyState || eax.readyState == 'complete' || eax.readyState == 'loaded') {
        // IE6 stalls at 'loaded' sometimes
        alert('loaded!');
        // remove these listeners for everyone, because it's
        // easier than testing everything to find out whether it's needed.
        removeEventListener(eax, 'readystatechange', arguments.callee);
        removeEventListener(eax, 'load', arguments.callee);
    }
};
addEventListener(eax, 'load', load); // FF/Safari get this
addEventListener(eax, 'readystatechange', load); // IE gets this

It’s a horrible mish-mash of events, obviously, but it works. Insofar as it goes.

Working out whether the load was successful turns out to be the hard part.

In Firefox it’s very easy. The load event doesn’t fire if there are problems loading the JS file, and a error event fires instead. This is lovely.

In Safari the load event doesn’t fire if there are problems, but there’s no other sign given. So I could probably fake this with a setTimeout set to a reasonable length — not perfect, but good enough for most cases.

In IE the readystatechange event fires away regardless. It’s here that I’m stuck — I can’t see any way to tell, in the readystatechange handler whether the script tag was really loaded without problems.

Since IE represents an unfortunately large component of deviantART’s users, half-working failure detection isn’t going to cut it. Especially since all the developers mainly use Firefox/Safari, and wouldn’t expect IE to behave differently.

So for now I’m going with verifying that the script tag loaded something, and says it’s complete. I’ll keep my eyes out for a way to work around IE…