actionscript rayo enemigo (juego flash)

The EnemyMissile class is linked to an EnemyMissile movie clip in the library that will get attached to stage when enemy ships fire.
*/


class EnemyMissile extends MovieClip
{

//We want to create a couple of varibles to keep track of important EnemyMissile information.
//We must define them here

//This will store how fast the enemy missile travels
var speed;

//Minibosses and the Boss can shoot diagonally. This variable keeps track of positive and negative vertical movement of te missile
var yDirection;


//This onLoad function is a built-in function of every movie clip.
//When an enemy missile is first loaded onto stage (it will get attached to the stage from the library) we want to:
function onLoad()
{
//Set it's speed to -15 (we want it to move 15 pixels in the negative x direction (left) at frame rate)
speed = -15;
}


//This onEnterFrame function is a built-in function of every movie clip.
//All the code that we need to continuously execute at 30 frames a second goes inside this function
function onEnterFrame()
{
// --- HORIZONTAL MOVEMENT LOGIC---
//move the enemy missile left 15 pixels (we set it's speed to -15 in the onLoad() function)
_x += speed;

// --- VERTICAL MOVEMENT LOGIC---
//If this missile has a y Direction (set in the MiniBoss and Boss classes when a missile is fired)
if(yDirection)
{
//move the missile up or down (multiply this value to get various degreesof travel. 4 looked good in this game)
_y += yDirection * 4;
}

// --- COLLISION LOGIC---
//If this missile is hitting the hero ship AND ( && ) the hero ship's shield is NOT activated
if(this.hitTest( _root.ship) && _root.ship.shield._visible == false)
{
//remove this missile from stage
this.removeMovieClip();
//subtract 10 from the ship's health
_root.ship.updateHealth(-10);
//start the ship shaking to reflect that it's taking damage
_root.ship.initShake();
//create a small explosion to reflect that the ship is taking damage
var explosion = _root.attachMovie("SmallExplosion","SmallExplosion" + _root.getNextHighestDepth(),_root.getNextHighestDepth());
//position the explosion at the ship's location on stage
explosion._x = _root.ship._x;
explosion._y = _root.ship._y;
}

//If this enemy missile has reached the left edge of the screen, just remove it.
if(_x < 0)
{
this.removeMovieClip();
}
}