ActionScript is the scripting language used to control playback of SWFs at runtime in Flash Player. Use ActionScript to make your applications play dynamically in a nonlinear way, and to add interesting or complicated functionality that cannot be added using the Timeline.

Using ActionScript, you can add complex interactivity, playback control, and data display to your application. You can add ActionScript in the authoring environment by typing code into the Script pane of the Actions panel. ActionScript follows its own rules of syntax, uses reserved keywords, and supports variables (used to store and retrieve information). ActionScript includes a large library of built‑in classes that you can use to create objects that perform many useful tasks, such as generating random numbers.

When you create a new FLA document, you must choose which version of ActionScript to use. You can change this setting if you decide later to write your scripts using a different version of ActionScript by updating the publish settings.

Flash supports several versions of ActionScript to meet the needs of different kinds of developers and playback hardware:

• ActionScript 3 (the most recent version) executes extremely fast and is fully compliant with the ECMAScript specification. It offers better XML processing, an improved event model, and an improved architecture for working with onscreen elements. FLA files that use ActionScript 3 cannot include earlier versions of ActionScript.
• ActionScript 2 is older and simpler to learn than ActionScript 3. Although Flash Player runs compiled ActionScript 2 code slower than compiled ActionScript 3 code, ActionScript 2 is still useful for many kinds of projects that are not computationally intensive—such as design-oriented content.
• ActionScript 1 is the simplest form of ActionScript, and is still used by some versions of the Flash Lite player. ActionScript 1 and 2 can coexist in the same FLA file.
• Flash Lite 2 and Flash Lite 1 are subsets of the corresponding version of ActionScript that are supported on mobile phones and devices.

Different features and file formats may require setting the corresponding version of ActionScript in the publish profile in order to function and render properly.