ActionScript 3 - Error #1010: A term is undefined and has no properties - actionscript-3

Question

When testing my project, I got this error :
TypeError: Error #1010: A term is undefined and has no properties. firegame.as:115]
at firegame/checkhitammo()[..\Desktop\Flash\firegame.as:115
at firegame/mainloop()[..Desktop\Flash\firegame.as:77
I don't understand why and if I change the if statement to something simple it's all work fine.
This is my code :
package {
import flash.display.*;
import flash.events.KeyboardEvent;
import flash.events.MouseEvent;
import flash.utils.Timer;
import flash.events.TimerEvent;
import flash.events.Event;
public class firegame extends MovieClip {
var tiger:Tiger = new Tiger();
var enemys:Array = new Array();
var scorea:Number = 0;
var ammoleft:Number = 0;
var ammo:Array = new Array();
var setint:Timer = new Timer(110);
var setenemy:Timer = new Timer(980);
var newenemy:Number;
var hitcheck:Array = new Array();
var totallength:Number;
public function firegame() {
startgame();
}
public function startgame() {
addplayer();
stage.addEventListener(MouseEvent.MOUSE_MOVE, moveplayer);
stage.addEventListener(MouseEvent.MOUSE_DOWN, shotfire);
stage.addEventListener(MouseEvent.MOUSE_UP, shotfirestop);
setint.addEventListener(TimerEvent.TIMER, shotfirestart);
setenemy.addEventListener(TimerEvent.TIMER, addenemy);
stage.addEventListener(Event.ENTER_FRAME, mainloop);
setenemy.start();
}
public function addplayer():void {
tiger.y = 200;
tiger.x = 507;
addChild(tiger);
}
public function moveplayer(e:MouseEvent):void {
tiger.y = mouseY;
tiger.x = 507;
}
public function shotfire(e:MouseEvent):void {
setint.start();
}
public function shotfirestop(e:MouseEvent):void {
setint.stop();
}
public function shotfirestart(e:TimerEvent):void {
var fire:Fire = new Fire();
fire.x = 460;
fire.y = mouseY;
addChild(fire);
ammoleft -= 1;
ammo.push(fire);
}
public function addenemy(e:TimerEvent):void {
var enemy:Enemy = new Enemy();
enemy.x = 0;
enemy.y = Math.floor(Math.random() * (370 - 30) + 30);
addChild(enemy);
enemys.push(enemy);
}
public function mainloop(e:Event):void {
setscoreandammo();
moveammo();
moveenemy();
checkhitammo();
}
public function moveammo():void {
for (var i:int = 0; i < ammo.length; i++) {
ammo[i].x -= 15;
if (ammo[i].x < -30) {
removeChild(ammo[i]);
ammo[i] = null;
ammo.splice(i, 1);
}
}
}
public function moveenemy():void {
for (var b:int = 0; b < enemys.length; b++) {
enemys[b].x += 5;
if (enemys[b].x > 590) {
removeChild(enemys[b]);
enemys[b] = null;
enemys.splice(b, 1);
}
}
}
public function setscoreandammo():void {
score.text = String(scorea);
leftammo.text = String(ammoleft);
}
public function checkhitammo():void {
for (var i:int = ammo.length; i >= 0; i--) {
for (var b:int = enemys.length; b >= 0; b--) {
if (ammo[i].hitTestObject(enemys[b])) { // <--- this is the line where the error is fired
removeChild(ammo[i]);
ammo[i] = null;
ammo.splice(i, 1);
removeChild(enemys[b]);
enemys[b] = null;
enemys.splice(b, 1);
scorea += 50;
break;
}
}
}
}
}
}

Answer 1

Your specific problem is right here...
public function checkhitammo():void {
for (var i:int = ammo.length; i >= 0; i--) {
for (var b:int = enemys.length; b >= 0; b--) {
...you are starting the counters i and b with the length of the array, instead of the position of the last index. It should read...
public function checkhitammo():void {
for (var i:int = ammo.length - 1; i >= 0; i--) {
for (var b:int = enemys.length - 1; b >= 0; b--) {
That is, the length is 1-based, and positions are 0-based

Answer 2

first of all, all your for loops only has increment by 1 you may want to use like this
for (var i in ammo) {
for (var b in enemys) {
you just have to loop through array until something happens and break out of loop to make sure it doesn't check twice.
in this line
if (ammo[i].hitTestObject(enemys[b])) {
the error says program cannot find object in the array list, since it's just array it does not throw error like null pointer exception
if you swap ammo with enemys you will see that hitTestObject is trying to access a null object
and you don't really need to call this
ammo[i] = null;
since its already being removed