Here’s a quick snippet of code for setting up intercommunication between a SWF and JavaScript, using ExternalInterface:

import flash.external.ExternalInterface;

// Calling a JavaScript function from AS3 (with optional parameters)
if (ExternalInterface.available) ExternalInterface.call("javascriptFunction", param1, param2);

// Implementing a JavaScript callback in AS3 (flashFunction is called when JavaScript calls javascriptFunction)
if (ExternalInterface.available) ExternalInterface.addCallback("javascriptFunction", flashFunction);