Simon Willison’s Weblog

Curious Javascript in .NET

I’ve never had the opportunity nor the inclination to do anything with .NET; at work we use open source tools for all of our web development, and I prefer open source tools for my own personal experiments as well. At any rate, the javascript:__doPostBack links I’ve seen on .NET powered sites such as Channel 9 and Orkut plain give me the willies.

Anyway, I decided to view source and see what __doPostBack actually does. Here’s the function:

function __doPostBack(eventTarget, eventArgument) {
  var theform;
  if (window.navigator.appName.toLowerCase().indexOf("microsoft") > -1) {
    theform = document._ctl0;
  }
  else {
    theform = document.forms["_ctl0"];
  }
  theform.__EVENTTARGET.value = eventTarget.split("$").join(":");
  theform.__EVENTARGUMENT.value = eventArgument;
  theform.submit();
}

Note the use of a dubious user-agent based browser detection method. Now what I just don’t understand is why that browser detection is there in the first place. There are numerous ways of accessing a form by name using Javascript. document.forms['form-name'] is part of DOM level 0, and is supported by virtually every browser since Netscape 2. document.form-name is a Microsoft invention. The thing is, document.forms['form-name'] is supported by MS browsers as well. So why do a browser detect and use the MS only method when just using the other method with no detect would work equally as well?

This is Curious Javascript in .NET by Simon Willison, posted on 26th April 2004.

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Previously hosted at http://simon.incutio.com/archive/2004/04/26/dodgy