20行JS代码实现贪吃蛇

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>20行JS代码实现贪吃蛇</title>
</head>
<body>
    <canvas id="can" width="400" height="400" style="background:black;"></canvas>
    <script>
    var sn=[42,41],dz=43,fx=1,n,ctx=document.getElementById("can").getContext("2d");
    function draw(t,c){
        ctx.fillStyle=c;
        ctx.fillRect(t%20*20,~~(t/20)*20,18,18);
    }
    document.onkeydown=function(e){fx=sn[1]-sn[0]==(n=[-1,-20,1,20][(e||event).keyCode-37]||fx)?fx:n};
    !function(){
        sn.unshift(n=sn[0]+fx);
        if(sn.indexOf(n,1)>0 || n<0||n>399||fx==1&&n%20==0||fx==-1&&n%20==19) return alert("GAME OVER");
        draw(n,"Lime");
        if(n==dz){
            while(sn.indexOf(dz=~~(Math.random()*400))>=0);
            draw(dz,"Yellow");
        }else
            draw(sn.pop(),"Black");
        setTimeout(arguments.callee,500);
    }();
    </script>
</body>
</html>