Question

Connecting to http://www.manage-us.com on an iPad under 3G [used to] result in a JavaScript error which can be seen if the developer console has been enabled. If the same page is accessed using the same iPad under a WiFi connection no error is displayed. [The error has now gone because I applied the fix below!].

Why is this?

I’ve tried simulating low bandwidth (using dummynet) on Safari on Mac and on the iPad simulator on Mac. This does not reproduce the problem.

I am currently suspecting this is a problem being introduced by my mobile operator in the UK (O2) which is known to modify some content through a proxy cache such as downgrading image files. If you can confirm that you don’t experience this problem when connecting by 3G on iPad or iPhone through another mobile operator that would be helpful.

Answer 1

I’ve investigated this further and discovered that the problem is that the UK mobile operator O2 (the original exclusive iPhone operator for Apple), modifies web content before sending it to iPhones and iPads. Probably before sending it to any device running a mobile browser.

They non-deterministically inline some of the CSS and JavaScript into the main source files of the web pages. This can create errors either because of mistakes in their algorithm or the consequence of stripping white space from source files with syntactic mistakes in the source files which were otherwise benign.

These modifications also strip copyright messages from copyrighted javascript libraries and css libraries and play havoc with delivery optimisations.

For example, imagine if a user is visiting a sequence of pages on your site which all link to jQuery libraries. Instead of letting your mobile browser locally cache the library, O2 inline the library on every page, forcing your phone to load the entire library over and over again for every page.

I’ve written a blog about the issue here in the hope if drawing a bit more attention to this: http://stuartroebuck.blogspot.com/2010/07/mobile-proxy-cache-content-modification.html

My workaround is to use document.write() to insert the JavaScript library dependencies at load time and prevent O2 from inlining them. This seems to work quite well. e.g.:

<script type="text/javascript">
// <![CDATA[
// Using document.write to load JavaScript dependencies to bypass O2 network inlining of JavaScript.
function loadJS(file){document.write("<" + "script type='text/javascript' src='" + file + "'></" + "script>")}
loadJS("/js/jquery-1.4.2.min.js");
loadJS("/js/myJSLibrary.js");
// ]]>
</script>

Note that, as ever, document.write will not work if the page is served as XHTML.