ActionScript is the internal programming language that Flash designers and developers use to add interactivity to projects. Sometimes a linear progression through the Timeline with animations that never vary is not enough. ActionScript can add variety, randomness, and user input and control to the mix.

Introduced in Flash 2, interactive control of Flash has been around for a long time. Flash 4 included support for written scripts. Flash 5, unveiled in 2000, contained the first reasonably full-featured version of a scripting language. This language was called ActionScript and was retroactively named ActionScript 1.0 (AS1) later on.

Since that time, there have been two major architectural changes to the language. Flash MX 2004 (actually released in September of 2003) included ActionScript 2.0 (AS2), a more robust iteration of ActionScript and the first to introduce formal object-oriented programming capabilities to Flash. Later, in 2007, Flash CS3 rebuilt ActionScript from scratch when it let ActionScript 3.0 (AS3) out of the cage.

Rather than enhancing the codebase of AS1 and AS2, and continuing with any baggage or flaws ActionScript adopted through its early existence, the code was reinvented for AS3. The prior code base was just too entrenched to accommodate sweeping improvements without breaking backward compatibility.

Instead, an entirely new codebase was developed and added to Flash Player alongside the legacy player code. The split codebases ...

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