if(key == "esc"){//exit
  // ...
}
if(key == "left" || key == "up" || key == "down" || key == "right"){//newt move
  // ...
}

if(key == "left")px-=step;
if(key == "right")px+=step;
if(key == "up")py-=step;
if(key == "down")py+=step;

if(px<0)px = canvas.width-step;//new head position
if(py<0)py = canvas.height-step;
if(px>canvas.width-step)px = 0;
if(py>canvas.height-step)py = 0;

for(var i = 1; i < tail; i++){
  // ...
}

while(mealy%step){
  mealy+=1;
}

while (mealy % step) {
    mealy += 1;
}

if(bigHead == 0){//drawing head
  ctx.fillStyle = "lime";
  ctx.fillRect(pos[0][0], pos[0][1], step-brd, step-brd);
  ctx.fillStyle = "DarkGreen";
  ctx.fillRect(pos[0][0]+step/7, pos[0][1]+step/7, step/1.8, step/1.8 );
}else{
  ctx.fillStyle = "yellow";
  ctx.fillRect(pos[0][0], pos[0][1], step-brd, step-brd);
  ctx.fillStyle = "red";
  ctx.fillRect(pos[0][0]+step/7, pos[0][1]+step/7, step/1.8, step/1.8 );
  // ...
}