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and then reverse direction. To achieve the reversal in direction, multiply the speed
property by -1 . If the aliens are moving to the right ( speed = 2 ), this will make the speed
= -2 , which will subtract from the x position and move the aliens to the left. If the aliens
hit the left side of the canvas, the speed property will again be multiplied by -1 ( -2 *
-1 ), which will equal 2 . The alien will then move to the right because 2 will be added
to the x value for the alien each time drawScreen() is called:
//Move Aliens
for (var i=aliens.length−1; i>= 0;i−−) {
aliens[i].x += aliens[i].speed;
if (aliens[i].x > (theCanvas.width-aliens[i].width) || aliens[i].x < 0) {
aliens[i].speed *= -1;
aliens[i].y += 20;
if (aliens[i].y > theCanvas.height) {
The next step in drawScreen() is to detect collisions between the aliens and the missiles.
This part of the code loops through the missiles array backward while nesting a loop
through the aliens array. It will test every missile against every alien to determine
whether there is a collision. Since we have already covered the hitTest() function, we
only need to discuss what happens if a collision is detected. First, we call the play()
function of the explodeSound . This is the second critical line of code in this iteration of
Space Raiders , as it plays (or attempts to play) the explosion sound every time a collision
is detected. After that, it splices the alien and missile objects out of their respective
arrays, and then breaks out of the nested for:next loop. If there are no aliens left to
shoot, we set the appState to STATE_RESET , which will add more aliens to the canvas so
the player can continue shooting:
missile: for (var i=missiles.length−1; i>= 0;i−−) {
var tempMissile = missiles[i]
for (var j=aliens.length-1; j>= 0;j−−) {
var tempAlien =aliens[j];
if (hitTest(tempMissile,tempAlien)) {;
break missile;
if (aliens.length <=0) {
appState = STATE_RESET;
The last few lines of code in drawScreen() loop through the missiles and aliens arrays,
and draw them onto the canvas. This is done using the drawImage() method of the
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