Peachpit, Written by Kris Hadlock
ActionScript 3 offers a great solution to ensuring that your method parameters have values even when no values are passed. The solution is called default parameter values, which allow you to predefine default values for specific method parameters.
Default parameter values are useful for many reasons. Say you have a method that is used numerous times and it usually receives the same value for a specific parameter with one or two exceptions. It can get a little annoying to add the same value to a method call over and over. Setting a default value, if no value is passed, can easily prevent this annoyance.