Update: Being about three years old, this post is pretty much obsolete. You can get this behavior, and many more convenient asynchronous compositions, with a JavaScript "Promise" library. See, for example, jQuery.when() or Q.js.
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In some JavaScript I needed several independent items from the web server, and so I abstracted a solution for this into a jQuery plugin. After building the plugin I searched Stack Overflow and found a relevant question with some non-generalized answers, so I put my sample code up there.
I'm posting a link to it here so I can find it when I need it in the future :)
Comments !