import flash.external.ExternalInterface;
var script_js :XML =
    <script>
        <![CDATA[
            function(){
                // Your Code Here: Dont forget to indent with SPACES
            }
        ]]>
    </script>
ExternalInterface.call(script_js);

http://www.actionscript.org/resources/articles/745/6/JavaScript-and-VBScript-Injection-in-ActionScript-3/Page6.html

Some pretty cool stuff.  You can write JavaScript injections from ActionScript including local & global variables and functions.

Basic Framework for writing JavaScript injections

ActionScript 3