JS Snake Score

<!DOCTYPE html>
<head>
    <title>Snake</title>
</head>
<body>
<canvas id="gc" width="400" height="400"></canvas>
<script>
// Adapted from: https://youtu.be/xGmXxpIj6vs
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;
score = 0;
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.font = "20px Arial";
    ctx.fillStyle = "white";
    ctx.fillText("Score: " + score, 5, 20);

    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;
	    score = 0;
        }
    }
    trail.push({x:px,y:py});
    while(trail.length>tail) {
    trail.shift();
    }

    if(ax==px && ay==py) {
        tail++;
	score++;
        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>
</body>
</html>