ActionScript: The Definitive Guide

Functions as Objects

In ActionScript, functions are technically a special type of built-in object. Let’s see what that means and how it affects what you can do with functions.

Passing Functions to Functions

Perhaps surprisingly, we can use any function as an argument to another function like this:

function1(function2);

Note that if there are no parentheses following function2, the interpreter doesn’t execute function2( ) but instead just passes its “object reference” as an argument to function1( ). That is, function1( ) receives function2 itself, not the return value of function2( ). Because objects are passed by reference, we can pass a function identifier to another function and it will arrive unscathed. The passed function can be executed like this:

function doCommand(command) {
  command( );      // Executes the passed function
}

// Some examples:
doCommand(stop);  // Pass the internal stop( ) function (stops the current movie)
doCommand(play);  // Pass the internal play( ) function (plays the current movie)

Because functions are a type of object, we may treat them like any other data. In the following example, we assign the internal gotoAndPlay function to the variable gp, which gives us a shorter way to refer to the function:

gp = gotoAndPlay;  // Create a shortcut reference to gotoAndPlay( )
gp(25);            // Invoke gotoAndPlay( ) using our reference

In addition to passing and storing functions as objects, we can exploit the “objectness” of functions by attaching properties to them, like this: ...