What is an .axd file?

What kind of purpose do .axd files serve?

I know that it is used in the ASP.Net AJAX Toolkit and its controls. I'd like to know more about it.

I tried Googling for it, but could not find getting basic information.

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An .axd file is a HTTP Handler file. There are two types of .axd files.

1. ScriptResource.axd
2. WebResource.axd

These are files which are generated at runtime whenever you use ScriptManager in your Web app. This is being generated only once when you deploy it on the server.

Simply put the ScriptResource.AXD contains all of the clientside javascript routines for Ajax. Just because you include a scriptmanager that loads a script file it will never appear as a ScriptResource.AXD - instead it will be merely passed as the .js file you send if you reference a external script file. If you embed it in code then it may merely appear as part of the html as a tag and code but depending if you code according to how the ToolKit handles it - may or may not appear as as a ScriptResource.axd. ScriptResource.axd is only introduced with AJAX and you will never see it elsewhere

And ofcourse it is necessary