js>>>>范围之内随机数。

var 随机变量  = randFn（20，30）；：产生20-30之间的随机数；；

键盘监听：用上下左右键 控制物体上下左右移动移动

//键盘监听事件
document.onkeydown = function(e){
switch(e.keyCode){
case 37:
//左
hero.left = true;
hero.right = false;
hero.top = false;
hero.bottom = false;
break;
case 38:
// 上
hero.left = false;
hero.right = false;
hero.top = true;
hero.bottom = false;
break;
case 39:
//右
hero.left = false;
hero.right = true;
hero.top = false;
hero.bottom = false;
break;
case 40:
//下
hero.left = false;
hero.right = false;
hero.top = false;
hero.bottom = true;
break;
default:
break;
}
}
document.onkeyup = function(){
hero.left = false;
hero.right = false;
hero.top = false;
hero.bottom = false;
}用在：
move: function() {
// onkeydown
//左
if(this.left) {
this.x -= this.speed;
}
//右
if(this.right) {
this.x += this.speed;
}
// 上
if(this.top) {
this.y -= this.speed;
}
//xia
if(this.bottom) {
this.y += this.speed;
}
}
};

创建游戏等需要的图片方法：
//C创建游戏所用的到所有图片对象
var background1Img = new Image();
background1Img.src = "img/background.png";
var background2Img = new Image();
background2Img.src = "img/background.png";
var heroImg = new Image();
heroImg.src = "img/herofly.png";
var enemy1 = new Image();
enemy1.src = "img/enemy1.png";
var enemy2 = new Image();
enemy2.src = "img/enemy2.png";
var enemy3 = new Image();
enemy3.src = "img/enemy3.png";
var bullet1 = new Image();
bullet1.src = "img/bullet1.png";
var bullet2 = new Image();
bullet2.src = "img/bullet2.png";
var prop = new Image();
prop.src = "img/prop.png";