JavaScript :: How To Dynamically Call Function At Runtime Without Using Eval

I'm trying to build an API in JS that will perform some operations and then execute the callback that's registered in AS when it's done. Because it's an API, I am just providing a JS method signature for another developer to call in Flash. Thus, the callback name that's registered in the AS part of the code should be a parameter that's passed in to the JS API in order for JS to communicate back to Flash.

For example:
[AS3 code]
ExternalInterface.addCallback("flashCallbackName", processRequest);
ExternalInterface.call("namespace.jsFnToCall", flashCallbackName);
function processRequest(data:String):void {
//do stuff
[Code] .....

Because the definition of the function is in AS, I can't use the window [function name] approach. The only way I can think of is to build the callback in a string and then use the eval() to execute it.