Where is &_= Coming From? (…not a typo…)

I recently was playing with Prototype, the JavaScript framework that implements an AJAX object to send Request to do a cross-browser XMLHttpRequest.

My server was reporting problems with the messages being sent from AJAX, and after a quick debugging session, I found that everything AJAX was sending had a “&_=” appended onto the end of it.

This clearly looks like a bogus parameter, say appended to a GET sequence, designed to pacify something. A little bit of digging on Google, and it appears it was introduced to resolve an old problem in Apple’s Safari.

Problem is, Prototype is still sending it, and when I sent an XML message to my server, the SAX parser didn’t take too kindly to the extra cruft at the end of the document.

If you open up prototype-1.4.0.js, and jump to line 631, you’ll see a line that looks like this:
if (parameters.length > 0) parameters += '&_=';

…removing it solves the problem. I found this more elegant than making my server pre-process an XML message.