To communicate between Flash/Flex and JS on the page, use the ExternalInterface class. You can't directly pass objects, but you can convert your object into a serialisable/string. Here's how you'd call a function called 'myFunc' and set it two arguments, a string and a number:
ExternalInterface.call('myFunc',1,'aString');
After the function name, which must always be a string, there is a ...rest parameter. Simply enough, this means you can send any number of arguments to the function, separating them with commas (we do two here).
If you used AS2 at any point in the past you may know the 'eval' function, this was inherited from (and is thus still used by) JS - it analyses a string and attempts to parse it into JavaScript, using this you can literally send Javascript code instead of a func/args:
ExternalInterface.call('alert("Hello!")');
If you want two-way communication, use the ExternalInterface.addCallBack function to register a function as callable from JS.
In case of errors when doing this, you may need to adjust your embed code: "In the object tag for the SWF file in the containing HTML page, set the following parameter:
param name="allowScriptAccess" value="always"