The scope chain

Any time a function begins execution, a number of objects and properties are created. First, a special object called an activation object is created that stores the parameters and any local variables or functions declared in the function body. You cannot access the activation object directly, because it is an internal mechanism. Second, a scope chain is created that contains an ordered list of objects that Flash Player checks for identifier declarations. Every function that executes has a scope chain that is stored in an internal property. For a nested function, the scope chain starts with its own activation object, followed by its parent function’s activation object. The chain continues in this manner until it reaches the global object. The global object is created when an ActionScript program begins, and contains all global variables and functions.