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Use Fiddler to validate version problems with Javascript libraries

I’m a big fan of Fiddler for a long time. Last week I had to test some jQuery compatibility issues with a new web application we were building. At first I was replacing all the Javascript files with newer versions, updating all the references and so on… Although this approach worked it was cumbersome and error-prone. Isn’t there a better alternative?
Fiddler to the rescue!
I found the following post by Eric Law where he uses Fiddler to load a different version of jQuery without updating any reference:http://blogs.msdn.com/b/ieinternals/archive/2011/08/19/using-fiddler-to-verify-a-jquery-update-will-fix-a-compatibility-problem.aspx.  Exactly where I was looking for!
So how can you do this?
  • First, download the (newer) version of the library you want to test.
  • Start Fiddler and go to the AutoResponder tab. Use the Add… button to create a new rule to map requests for your Javascript library to the newly downloaded file.image
  • Also set the Unmatched requests passthrough option to ensure that Fiddler doesn’t automatically generate 404s for requests that don’t match any of the rules.
  • Reload your web application you’ll see that  Fiddler intercepts the request for the older library and returns the newer one instead.
If I only found out this feature sooner…

Remark: Don't forget to remove this rule afterwards.
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