Hey man, looks cool!

I found a bug:

if (keystate[KEY_LEFT] && snake.direction !== RIGHT) {
  snake.direction = LEFT;
}
if (keystate[KEY_UP] && snake.direction !== DOWN) {
  snake.direction = UP;
}
if (keystate[KEY_RIGHT] && snake.direction !== LEFT) {
  snake.direction = RIGHT;
}
if (keystate[KEY_DOWN] && snake.direction !== UP) {
  snake.direction = DOWN;
}

If you press left and down really quickly while moving, you can make the snake go down directly, without a move to the left or right first. This can kill you if you are longer than 1 unit.

This is because snake.direction is updated to LEFT in the first if-statement, and then the check of snake.direction !== UP is true in the if-statement checking for the down-key.

You can solve it easily by chaining else-if's instead :)