Function scope

A function’s scope determines not only where in a program that function can be called, but also what definitions the function can access. The same scope rules that apply to variable identifiers apply to function identifiers. A function declared in the global scope is available throughout your code. For example, ActionScript 3.0 contains global functions, such as isNaN() and parseInt() , that are available anywhere in your code. A nested function—a function declared within another function—can be used anywhere in the function in which it was declared.

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 or Adobe AIR 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.

Function closures

A function closure is an object that contains a snapshot of a function and its lexical environment . A function’s lexical environment includes all the variables, properties, methods, and objects in the function’s scope chain, along with their values. Function closures are created any time a function is executed apart from an object or a class. The fact that function closures retain the scope in which they were defined creates interesting results when a function is passed as an argument or a return value into a different scope.

For example, the following code creates two functions: foo() , which returns a nested function named rectArea() that calculates the area of a rectangle, and bar() , which calls foo() and stores the returned function closure in a variable named myProduct . Even though the bar() function defines its own local variable x (with a value of 2), when the function closure myProduct() is called, it retains the variable x (with a value of 40) defined in function foo(). The bar() function therefore returns the value 160 instead of 8 .

function foo():Function 
    var x:int = 40; 
    function rectArea(y:int):int // function closure defined 
        return x * y 
    return rectArea; 
function bar():void 
    var x:int = 2; 
    var y:int = 4; 
    var myProduct:Function = foo(); 
    trace(myProduct(4)); // function closure called 
bar(); // 160

Methods behave similarly in that they also retain information about the lexical environment in which they were created. This characteristic is most noticeable when a method is extracted from its instance, which creates a bound method. The main difference between a function closure and a bound method is that the value of the this keyword in a bound method always refers to the instance to which it was originally attached, whereas in a function closure the value of the this keyword can change. For more information, see Methods.