miscoded

Here are the issues with United's waiting screen code:

Misunderstanding try..catch
If we use IE on Windows, they do

try {
  imgLdr[0].src = "http://www.united.com/ual/asset/united2005.gif";
}
catch(err) {
 imgLdr[0].src = "/ube/images/icns/united1005.gif";
}

The try..catch is superfluous. Setting imgLdr[0].src will never throw an error. (It looks like they expect a JavaScript error if the image can't be loaded but that's a complete misunderstanding. The IMG.onerror event is an event, it doesn't stop the main script thread.).

Silly browser sniffing
Then comes the else clause, if you are not using IE on windows - which does exactly the same thing as the IE-Win clause! Cut-and-paste. So why the sniffing?? That's 17 lines of code to pre-load a single gif.

I also like the comment

//Kaunish change the src below to be http:// version of line 48, http://www.united.com/ual/asset/united2005.gif

Can we assume that Kaunish isn't using version control to document his changes..?

IMG.onload
Now, at the bottom of the code we find evidence of thorough testing and research into how browsers handle IMG.onload. If you are Win-IE or Safari, it seems you won't send onload to an image that was already pre-loaded with JavaScript (um, why did they write those 17 lines to pre-load it in the first place then??), so they set onload on an invisible GIF instead.

We also see what they actually meant by the try..catch stuff here:

onerror='this.src = imgLdr[1].src'

. If image A doesn't load, show image B instead. Ugly and inefficient but possibly the smallest problem with this code..


Look at the considerable amount of testing that went into those sniffing statements. Think about the consequences when the IMG.onload approach fails (a HTTP request gone wrong, a server hiccup for example) - the site will never show you the carefully prepared list of possible tickets. The whole booking simply becomes unusable. Did it really never occur to the person writing this how much simpler and more robust it would be to just use window.onload?