Passing in arguments from js --> as3 via ExternalInterface

You'd use ExternalInterface.addCallback to register a function call from JS:

// AS
// register call from Javascript
ExternalInterface.addCallback("callFromJS", callFromJS);
 
// display the message from Javascript
function callFromJS(s:String):void
{
    textfield.text = s;
}

Then call from JS:

// JS
flashObject.callFromJS("Hello from JS");

You may need to set allowScriptAccess to true in the HTML embed code.