Question

What’s the different between
var myObject : Sprite = new Sprite();
and
var myObject : Sprite = new MovieClip();

Answer 1

MovieClips and Sprite are separate classes, albeit related to each other because MovieClip extends Sprite. As such, MovieClip has all the same capabilities that Sprite has, and it also adds more, mostly related to timeline animation (play(), stop(), gotoAndPlay(), addFrameScript() et c.)

Because MovieClip has all the same capabilities as Sprite, you can assign a MovieClip object to a variable typed as Sprite.

var myObject : Sprite = new MovieClip();

Sprite, however, does not share all of MovieClip’s functionalities, so this does not work the other way around:

// Will not work
var myObject : MovieClip = new Sprite();

If you want to know the inheritance chain for a particular class, check out the documentation, e.g. for MovieClip: . You can see that it extends Sprite, which in turn extends InteractiveObject, and so on.

In language-agnostic terms, this is called inheritance, one of the strengths of which is polymorphism.