ActionScript 3.0 Versus ActionScript 2.0 in Flash CS5
Since its introduction in Flash Player 9, ActionScript version 3.0 has brought many major changes and improvements to the way ActionScript performs and to the way it’s created within Flash movies.
Previous versions of ActionScript (1.0+, 2.0) are still in use among some Flash projects, so you still have options for creating or saving movies with older ActionScript version settings, if necessary.
If you have worked with older versions of ActionScript, you should be aware of some key differences:
ActionScript 3.0 can only be placed on keyframes in the timeline or external (.as) files. You cannot place ActionScript 3 statements directly on symbol instances.
ActionScript 3.0 is stricter about a number of coding practices. Actions that Flash Player would normally let slide in ActionScript 2 may cause errors in ActionScript 3.
ActionScript 3.0 projects can be published only to Flash Player 9 or later. This consideration is important if your project or company limits the use of Flash Player to version 8 or earlier.
If you’re a first-time coder or have never worked with scripting or programming languages, some concepts in ActionScript 3.0 may present a significant learning curve.
When you choose to create a new Flash file, you can select an ActionScript 2.0 or 3.0 version file. If you’re adding ActionScript to an existing Flash file, you should verify and adjust the ActionScript version in your Publish Settings to match the version you’ve chosen to work in.
ActionScript versions are matched to specific versions of Flash Player, and 3.0 scripts don’t work in a version 2.0 movie (and vice versa).
To verify and select the appropriate ActionScript version, choose File→Publish Settings; click the Flash tab and choose ActionScript Version 2.0 or ActionScript Version 3.0 from the Script drop-down list.
The Flash Player and ActionScript version shows in the Property inspector panel when no tools or objects are selected.