请问一下，根据书本敲的，然后在浏览器打开就是空白的，请问哪儿有问题
#代码如下
<html>
<meta charset="utf-8" />
<head>
<canvas id="gameCanvas" width="600" height="600">您的浏览器不支持Canvas</canvas>
<script type="text/JavaScript">
// 获取画布元素
var canvas = document.getElementById("gameCanvas");
var ctx = canvas.getContext("2d");
//定义游戏场景
var bgReady = false;
var bgImage = new Image();
bgImage.src = "background.png";
bgImage.onload = function () {
bgReady = true;
}
//定义mario对象样式
var marioReady = false;
var marioImage = new Image();
marioImage.src = "mario.jpg";
marioImage.onload = function(){
marioReady = true;
}
//定义怪物样式
var monsterReady = false;
var monsterImage = new Image();
monsterImage.src = "monster.jpg";
monsterImage.onload = function(){
monsterReady = true;
}
//定义mario初始参数
var mario = {
speed: 256,
x: canvas.width/2,
y: canvas.height/2
}
//定义怪物初始参数
function monster(){
this.x = Math.random() * canvas.width;
this.y = Math.random() * canvas.height;
this.speed = 100;
this.xDirection = 1;
this.yDirection = 1;
this.move = function (modifier) {
this.x += this.xDirection * this.speed * modifier;
this.y += this.yDirection * this.speed * modifier;
if (this.x >= canvas.width - 32){
this.x = canvas.width -32;
this.xDirection = -1;
}else if (this.x <= 0) {
this.x = 0;
this.xDirection = 1;
}else if (this.y >= canvas.width - 32) {
this.y = canvas.width -32;
this.yDirection = -1;
}else if (this.y <= 0) {
this.y = 0;
this.yDirection = 1;
}
};
}
var monsterSum = 0;
var monsterList = new Array();
monsterList[monsterSum] = new monster();
var keysDown = {};
//添加键盘操作监控事件
addEventListener{
"keydown",
function (e) {
keysDown[e.keyCode] = true;
},
false
};
addEventListener{
"keyup",
function (e) {
delete keysDown[e.keyCode];
}
};
//定义移动事件
var Move = function (modifier) {
if (38 in keysDown){
mario.y -= mario.speed * modifier;
}
if (40 in keysDown){
mario.y += mario.speed * modifier;
}
if (37 in keysDown){
mario.x -= mario.speed * modifier;
}
if (39 in keysDown){
mario.x += mario.speed * modifier;
}
if (mario.x >= canvas.width - 32) {
mario.x = 0;
}else if (mario.x <= 0) {
mario.x = canvas.width - 32;
}
if (mario.y >= canvas.width - 32) {
mario.y = 0;
}else if (mario.y <= 0) {
mario.y = canvas.width -32;
}
for (var i = 0; i <= monsterSum; i++) {
monsterList[i].move(modifier);
}
}
//定义绘图事件
var Draw = function () {
if(bgReady){
ctx.drawImage(bgImage, 0, 0);
if (marioReady) {
ctx.drawImage(marioImage, mario.x, mario.y);
}
if (monsterReady) {
for(var i = 0; i <= monsterSum; i++)
ctx.drawImage(monsterImage, monsterList[i].x, monsterList[i].y);
}
}
ctx.fillStyle = "rgb(250, 250, 250)";
ctx.font = "24px Helvetica";
ctx.textAlign = "left";
ctx.textBaseline = "top";
last = Date.now() - start;
ctx.fillText(last/1000, 12, canvas.height-590);
}
//设置怪物刷新并判断位置
var Check = function () {
if(monsterSum != Math.floor(last / 5000)){
monsterSum ++;
monsterList[monsterSum] = mew monster();
}
for(var i = 0; i <= monsterSum; i++){
if (
(monsterList[i].x - 32) <= mario.x
&& mario.x <= (monsterList[i].x + 32)
&& (monsterList[i].y - 32) <= mario.y
&& mario.y <= (monsterList[i].y + 32)
) {
end = Date.now();
alter("你被怪物抓到了，游戏结束！");
End();
}
}
}
var End = function () {
if(bgReady){
ctx.drawImage(bgImage, 0, 0);
}
window.clearInterval(timer);
return;
}
var main = function () {
var now = Date.now();
var delta = now - then;
Move(delta / 1000);
Draw();
Check();
then = now;
}
var then = Date.now();
var start = then;
timer = setInterval(main, 1);
</script>
</body>
</html>
