使用原生js做一个贪吃蛇游戏

<script>
 var timer = null;
 var oMain = document.getElementById("main");
 
 function Map(atom,xnum,ynum){//地图,设置单位大小，及根据单位大小创建地图
  this.atom = atom;
  this.xnum = xnum;
  this.ynum = ynum;
  
  this.create = function(){
  
  this.canvas = document.createElement("div");
  this.canvas.style.cssText = "position: relative;top: 40px;border: 1px solid red;background: #F1F1F1;"
  this.canvas.style.width = this.atom * this.xnum + "px";//画布宽
  this.canvas.style.height = this.atom * this.ynum + "px";//画布高
  main.appendChild(this.canvas);
  }
 }
 
 function Food(map){//食物
  this.width = map.atom;
  this.height = map.atom;
  //实现随机背景色
  this.bgColor = "rgb("+Math.floor(Math.random()*200)+","+Math.floor(Math.random()*200)+","+Math.floor(Math.random()*200)+")";
 
  this.x = Math.floor(Math.random()*map.xnum);
  this.y = Math.floor(Math.random()*map.ynum);
  
  this.flag = document.createElement('div');
  this.flag.style.width = this.width + 'px';
  this.flag.style.height = this.height + 'px';
  this.flag.style.backgroundColor = this.bgColor;
  this.flag.style.borderRadius = '50%';
  this.flag.style.position = "absolute";
  this.flag.style.left = this.x * this.width + 'px';
  this.flag.style.top = this.y * this.height + 'px';
  
  map.canvas.appendChild(this.flag);
 }
 
 //重新开始
 function restart(map,snake){
  for(var i=0; i<snake.body.length; i++){
  map.canvas.removeChild(snake.body[i].flag);
  }
  snake.body = [
  {x : 2,y : 0},//蛇头
  {x : 1,y : 0},//蛇身
  {x : 0,y : 0}//蛇尾
  ]
  snake.direction = "right";
  snake.display();
  
  map.canvas.removeChild(food.flag);
  food = new Food(map);
 }
 
 function Snake(map){
  this.width = map.atom;
  this.height = map.atom;
  
  this.direction = "right";
  
  this.body = [
  {x : 2,y : 0},
  {x : 1,y : 0},
  {x : 0,y : 0}
  ]
  
  this.display = function(){
  for(var i=0; i<this.body.length; i++){
   if(this.body[i].x != null){
   var s = document.createElement('div');
   //将节点保存
   this.body[i].flag = s;
   
   //设置样式
   s.style.width = this.width + 'px';
   s.style.height = this.height + 'px';
   s.style.backgroundColor = "rgb("+Math.floor(Math.random()*200)+","+Math.floor(Math.random()*200)+","+Math.floor(Math.random()*200)+")";
   s.style.borderRadius = '50%';
   
   //设置位置
   s.style.position = "absolute";
   s.style.left = this.body[i].x * this.width + 'px';
   s.style.top = this.body[i].y * this.height + 'px';
   
   //添加到地图
   map.canvas.appendChild(s);
   }
  }
  }
  
  this.run = function(){
  for(var i=this.body.length-1; i>0; i--){
   this.body[i].x = this.body[i-1].x;
   this.body[i].y = this.body[i-1].y
   
  }
  switch(this.direction){
   case "left":this.body[0].x -= 1;break;
   case "right":this.body[0].x += 1;break;
   case "up":this.body[0].y -= 1;break;
   case "down":this.body[0].y += 1;break;
  }
  //吃食物
  if(this.body[0].x == food.x && this.body[0].y == food.y){
   this.body.push({x : null,y : null,flag : null});
   
   map.canvas.removeChild(food.flag);
   food = new Food(map);
  }
  
  //判断游戏结束
  if(this.body[0].x<0 || this.body[0].x>map.xnum-1 || this.body[0].y<0 || this.body[0].y>map.ynum-1){
   clearInterval(timer);
   alert("GAME OVER!");
   restart(map,this);
   return false;
  }
  for(var i=4; i<this.body.length; i++){
   if(this.body[0].x == this.body[i].x && this.body[0].y == this.body[i].y){
   clearInterval(timer);
   alert("GAME OVER!");
   restart(map,this);
   return false;
   }
  }
  
  //删除原来
  for(var i=0; i<this.body.length; i++){
   if(this.body[i].flag != null){
   map.canvas.removeChild(this.body[i].flag);
   }
  }
  this.display();
  }
 }

 
 //创建地图对象
 var map = new Map(20,40,20);
 map.create();
 
 //创建食物对象
 var food = new Food(map);
 
 //创建蛇对象
 var snake = new Snake(map);
 snake.display();
 
 
 //加上键盘事件(改变蛇头方向)
 window.onkeydown = function(e){
  event = e || window.event;
  
  switch(event.keyCode){
  case 38:
   if(snake.direction != "down")//禁止掉头
   snake.direction = "up";
  break;
  case 40:
   if(snake.direction != "up")
   snake.direction = "down";
  break;
  case 37:
   if(snake.direction != "right")
   snake.direction = "left";
  break;
  case 39:
   if(snake.direction != "left")
   snake.direction = "right";
  break;
  }
 }
 
 var speed = 100;
 
 function start(){
  clearInterval(timer);
  timer = setInterval(function(){
  snake.run();
  document.getElementById("score").innerHTML = snake.body.length-3;
  start();
  },speed)
 }
 
 //难度
 document.getElementById("1").onclick = function(){
  speed = 100;
 }
 
 document.getElementById("2").onclick = function(){
  speed = 50;
 }
 
 document.getElementById("3").onclick = function(){
  speed = 20;
 }
 
 document.getElementById("begin").onclick = function(){
  start();
 }
 
 document.getElementById("pause").onclick = function(){
  
  clearInterval(timer);
 }
</script>