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In TYPO3's history there have been a lot of ups and downs, in any regard. This blog post tells you something about JavaScript in TYPO3: how was it back then, what's happening now and what will maybe happen.
Since TYPO3 CMS 7 it is possible to use RequireJS in the backend. A short explanation: RequireJS enables developers to have some kind of dependency handling for JavaScript. The JavaScript is written as so-called "Asynchronous Module Definition" (AMD). Some libraries delivered with TYPO3 are written as modules. This tutorial explains how you can write such modules on your own.