index.js

class Game {
  constructor(grid, snake) {
    ...
    this.score = 0;
  }

  async run() {
   ...
    while (true) {
      ...
      if(this.food) {
        ...
        if(this.snake.collision(this.snake.head, this.food)) {
          ...
          this.score++;
          document.getElementById("score").innerHTML = `Points: ${game.score}`;
        }
      }
    }
  }
}
...
document.getElementById("score").innerHTML = `Points: ${game.score}`;
game.run();