snake javascript

<canvas id="gc" width="400" height="400"></canvas>
<script>
    window.onload=function(){
        canv=document.getElementById("gc");
        ctx=canv.getContext("2d");
        document.addEventListener("keydown",keyPush);
        setInterval(game,1000/15);
    }
    px=py=10;
    gs=tc=20;
    ax=ay=15;
    xv=yv=0;
    trail=[];
    tail = 5;
    function game(){
        px+=xv;
        py+=yv;
        if (px<0) {
            px=tc-1;
        }
        if (px>tc-1) {
            px=0;
        }
        if (py<0) {
            py=tc-1;
        }
        if (py>tc-1) {
            py=0;
        }
        ctx.fillStyle="black";
        ctx.fillRect(0,0,canv.width,canv.height);

        ctx.fillStyle="lime";
        for(var i=0;i<trail.length;i++){
            ctx.fillRect(trail[i].x*gs,trail[i].y*gs,gs-2,gs-2);
            if(trail[i].x==px && trail[i].y==py){
                tail = 5;
            }
        }
        trail.push({x:px,y:py});
        while(trail.length>tail){
            trail.shift();
        }

        if(ax==px && ay==py){
                tail++;
                ax=Math.floor(Math.random()*tc);
                ay=Math.floor(Math.random()*tc);

            }

        ctx.fillStyle="red";
        ctx.fillRect(ax*gs,ay*gs,gs-2,gs-2);


    }
    function keyPush(evt){
        switch(evt.keyCode){
            case 37:
                xv=-1;yv=0;
                break;
            case 38:
                xv=0;yv=-1;
                break;
            case 39:
                xv=1;yv=0;
                break;
            case 40:
                xv=0;yv=1;
                break;

        }
    }
</script>